A microphone, often referred to colloquially as a mic () or mike, functions as a transducer, converting acoustic energy into an electrical signal. These devices are integral to various fields, including telecommunication, sound recording, broadcasting, and consumer electronics, encompassing applications such as telephones, hearing aids, and mobile devices.
A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in telecommunication, sound recording, broadcasting, and consumer electronics, including telephones, hearing aids, and mobile devices.
Contemporary microphone technology encompasses several types, each employing distinct methodologies to transform the air pressure fluctuations of a sound wave into an electrical signal. Prominent examples include the dynamic microphone, which utilizes a wire coil suspended within a magnetic field; the condenser microphone, characterized by a vibrating diaphragm acting as a capacitor plate; and the contact microphone, which incorporates a piezoelectric crystal. Prior to signal recording or reproduction, microphones generally necessitate connection to a preamplifier.
History
The necessity of amplifying the human voice for larger audiences spurred the development of early acoustic megaphones. Notable historical instances include fifth-century-BC Greek theater masks, which featured horn-shaped mouth openings designed to acoustically enhance actors' voices in amphitheaters. Subsequently, between 1664 and 1685, English physicist Robert Hooke pioneered experiments with non-air transmission mediums, inventing an rudimentary telephone constructed from stretched wire with a cup affixed to each end, a device now recognized as a tin-can telephone.
In 1856, Italian inventor Antonio Meucci devised a dynamic microphone, which operated on the principle of generating electric current through the movement of a wire coil at varying depths within a magnetic field. This modulation technique proved to be a foundational and enduring method for telephone technology. Describing his invention in 1857, Meucci articulated: "It consists of a vibrating diaphragm and an electrified magnet with a spiral wire that wraps around it. The vibrating diaphragm alters the current of the magnet. These alterations of current, transmitted to the other end of the wire, create analogous vibrations of the receiving diaphragm and reproduce the word."
German inventor Johann Philipp Reis constructed an early sound transmitter, known as the "Reis telephone," in 1861. This device employed a metallic strip affixed to a vibrating membrane to generate intermittent current. Subsequent advancements in 1876 saw the introduction of the "liquid transmitter" design by Alexander Graham Bell and Elisha Gray for early telephones, where a diaphragm was connected to a conductive rod immersed in an acid solution. Nevertheless, these early systems yielded notably inferior sound quality.
The (loose-contact) carbon microphone marked the advent of effective voice telephony. This innovation was independently conceived by David Hughes in England, and by Emile Berliner and Thomas Edison in the United States. Despite Edison receiving the initial patent in mid-1877 following extensive legal contention, Hughes had publicly demonstrated his functional apparatus several years prior, leading most historians to attribute the invention to him. The Berliner microphone achieved commercial prominence through its integration into Alexander Graham Bell's telephone systems, resulting in Berliner's employment by Bell. The carbon microphone proved instrumental in advancing telephony, broadcasting, and the recording sectors. Thomas Edison further refined this technology, introducing his carbon-button transmitter in 1886. Notably, this microphone was utilized for the inaugural radio broadcast, a performance at the New York Metropolitan Opera House in 1910.
A subsequent significant advancement occurred in 1916 when E.C. Wente of Western Electric developed the inaugural condenser microphone. By 1923, the first functional moving coil microphone was constructed. The Marconi-Sykes magnetophone, engineered by Captain H. J. Round, subsequently became the established standard for BBC studios in London. This design underwent further enhancement in 1930 by Alan Blumlein and Herbert Holman, who introduced the HB1A, which represented the leading standard of its era.
Concurrently in 1923, the ribbon microphone, another electromagnetic variant, was introduced. Its development is largely attributed to Harry F. Olson, who adapted the principles of ribbon speaker technology to microphone design. Over time, numerous companies refined these microphones, with RCA notably contributing significant advancements in pattern control, thereby imparting directionality to the devices.
The Neumann U 47, introduced in 1949, marked a significant advancement in microphone technology. As the inaugural studio condenser microphone featuring a large dual-diaphragm capsule, switchable pickup patterns, and a vacuum tube amplifier, it established a new benchmark for high-fidelity, warm, and detailed vocal and instrumental recordings. The burgeoning television and film industries subsequently spurred a demand for high-fidelity microphones with enhanced directionality, prompting Electro-Voice to develop its Academy Award-winning shotgun microphone in 1963.
In 1965, the Shure SM57 significantly transformed the recording of musical instruments and amplified live performances. Utilizing the Unidyne III capsule, the SM57 produced clear, distortion-free audio. Its robust and compact construction facilitated close placement to drums and amplifiers, contributing to its status as one of the best-selling microphones ever produced.
Microphone Classifications
Microphones are primarily classified based on their transducer principle, such as condenser or dynamic, and their directional characteristics, including omnidirectional or cardioid patterns. Additional descriptive criteria may encompass diaphragm size, intended application, or the orientation of the primary sound input relative to the microphone's principal axis (e.g., end-address or side-address).
Condenser Microphones
The condenser microphone, developed by E. C. Wente at Western Electric in 1916, is also known as a capacitor microphone or electrostatic microphone, reflecting the historical term "condenser" for capacitors. In this design, the diaphragm functions as one plate of a capacitor. Audio vibrations induce variations in the distance between these plates, which, due to the inverse proportionality between plate capacitance and distance, result in capacitance changes. These capacitance fluctuations are then utilized to detect the audio signal. The entire assembly of fixed and movable plates is referred to as an element or capsule.
Condenser microphones encompass a broad spectrum of applications, ranging from telephone mouthpieces and affordable karaoke microphones to professional high-fidelity recording equipment. These microphones typically generate a superior audio signal and are widely favored in laboratory and recording studio environments. This technology's intrinsic advantage stems from the minimal mass that the incident sound wave must displace, in contrast to other microphone types that demand greater energy input from the sound wave.
Condenser microphones necessitate an external power source, which can be supplied either as phantom power through equipment microphone inputs or by a small battery. This power is essential for establishing the capacitor plate voltage and energizing the microphone's internal electronics. Certain condenser microphones feature dual diaphragms that can be electrically configured to produce various polar patterns, including cardioid, omnidirectional, and figure-eight. Furthermore, some models, such as the Røde NT2000 or CAD M179, allow for continuous adjustment of the polar pattern.
Condenser microphones are primarily divided into two categories based on the method employed to extract the audio signal from the transducer: DC-biased microphones, and radio frequency (RF) or high-frequency (HF) condenser microphones.
DC-Biased Condenser Microphones
In a DC-biased condenser microphone, the capacitor plates are maintained with a constant charge (Q). Air vibrations cause variations in the voltage across these plates, as described by the capacitance equation (C = Q⁄V), where Q represents charge in coulombs, C denotes capacitance in farads, and V signifies potential difference in volts. While the charge across the capacitor remains nearly constant, it undergoes minor fluctuations with capacitance changes; however, at audible frequencies, it is considered effectively constant. The combination of the capsule's capacitance (typically 5 to 100 pF) and the bias resistor's value (ranging from 100 MΩ to tens of GΩ) creates a filter that acts as a high-pass for the audio signal and a low-pass for the bias voltage. It is important to note that the time constant of an RC circuit is determined by the product of its resistance and capacitance.
During the temporal window of capacitance variation, which can extend up to 50 milliseconds for a 20 Hz audio signal, the electrical charge remains nearly constant, and the voltage across the capacitor adjusts instantaneously, mirroring the capacitance modification. This capacitor voltage fluctuates symmetrically around the established bias voltage. The potential difference between the bias and the capacitor is observed across the series resistor. This resistor voltage is subsequently amplified for operational efficacy or data acquisition. Typically, the microphone's internal electronics do not provide voltage gain, given that the inherent voltage differential can be substantial, reaching several volts under conditions of high sound pressure.
Radio Frequency Condenser Microphones
RF condenser microphones employ a relatively low radio frequency (RF) voltage, which is produced by a low-noise oscillator circuit. The oscillator's output signal can be either amplitude-modulated by capacitance variations induced by sound waves displacing the capsule diaphragm, or the capsule itself can integrate into a resonant circuit, thereby modulating the oscillator's frequency. Subsequent demodulation produces a low-noise audio frequency signal characterized by exceptionally low source impedance. The elimination of a high bias voltage facilitates the deployment of a diaphragm with reduced tension, potentially leading to a broader frequency response owing to enhanced compliance. This RF biasing methodology yields a capsule with lower electrical impedance; a beneficial consequence is the operational resilience of RF condenser microphones in humid environments, which typically pose challenges for DC-biased microphones due to potential contamination of insulating surfaces. For instance, the Sennheiser MKH series of microphones utilizes this RF biasing technique. A notable covert application of this identical physical principle was 'The Thing,' a remotely energized device conceived by Soviet Russian inventor Leon Theremin, which was employed to surveil the US Ambassador's residence in Moscow from 1945 to 1952.
Electret Condenser Microphones
The electret microphone, a variant of the condenser microphone, was developed by Gerhard Sessler and James West at Bell Laboratories in 1962. Unlike conventional condenser microphones that rely on an externally applied charge, electret microphones incorporate a permanent charge within an electret material. An electret is defined as a ferroelectric material that has undergone permanent electrical charging or polarization. Its nomenclature derives from the terms electrostatic and magnet; a static charge is intrinsically embedded within an electret through the alignment of static charges within its material structure, analogous to how a permanent magnet is created by aligning magnetic domains in ferrous material.
Owing to their commendable performance, manufacturing simplicity, and consequent low production cost, electret microphones constitute the predominant type of microphone produced currently. One semiconductor manufacturer projects annual production to exceed one billion units. Their applications span a wide range, encompassing high-fidelity recording, lavalier (lapel) usage, and integration into compact sound recording devices and telecommunication instruments. Before the widespread adoption of MEMS microphones, virtually all microphones found in cellular phones, computers, personal digital assistants (PDAs), and headsets were of the electret variety.
Distinguishing them from other capacitor microphones, electrets do not necessitate an external polarizing voltage; however, they commonly incorporate an integrated preamplifier that mandates power. In sound reinforcement and studio environments, this preamplifier is often supplied with phantom power. Monophonic microphones intended for personal computers (PCs), occasionally referred to as multimedia microphones, typically utilize a 3.5 mm plug, similar to those employed for stereo connections; in this configuration, the ring contact, rather than conveying a signal for a second audio channel, transmits power.
Valve Microphones
A valve microphone is a type of condenser microphone that incorporates a vacuum tube (or valve) amplifier. These microphones retain significant popularity among audiophiles who appreciate the distinct characteristics of 'tube sound'.
Dynamic Microphones
The dynamic microphone, alternatively termed the moving-coil microphone, operates on the principle of electromagnetic induction. These microphones are characterized by their robust construction, comparative affordability, and inherent resistance to moisture. These attributes, combined with their capacity for high gain prior to feedback, render them a favored choice for live stage performances.
Dynamic microphones employ the identical dynamic principle found in loudspeakers, albeit in a reversed operational configuration. A diminutive, movable induction coil, situated within the magnetic field generated by a permanent magnet, is affixed to the diaphragm. Upon the ingress of sound through the microphone's windscreen, the incident sound wave displaces the diaphragm, which in turn moves the coil within the magnetic field, thereby generating a fluctuating voltage across the coil via electromagnetic induction.
Ribbon Microphones
Ribbon microphones employ a slender, typically corrugated metallic ribbon suspended within a magnetic field. This ribbon is electrically coupled to the microphone's output, and its oscillation within the magnetic field produces the electrical signal. Both ribbon and moving-coil microphones operate on the principle of magnetic induction to generate sound signals. Standard ribbon microphones exhibit a bi-directional, or figure-eight, pickup pattern due to the ribbon's exposure on both sides. Furthermore, owing to its significantly lower mass, the ribbon responds to air velocity rather than sound pressure. While the symmetrical front and rear pickup can pose challenges in conventional stereo recording, its substantial side rejection can be leveraged by orienting a ribbon microphone horizontally, such as above cymbals, to ensure the rear lobe captures sound exclusively from the cymbals. The figure-eight polar pattern inherent to ribbon microphones is optimally suited for Blumlein pair stereo recording. Alternative directional patterns are achieved by enclosing one side of the ribbon within an acoustic trap or baffle, thereby restricting sound access to a single side. The iconic RCA Type 77-DX microphone features multiple externally adjustable internal baffle positions, enabling the selection of various response patterns, from figure-eight to unidirectional.
Achieving an optimal low-frequency response in earlier ribbon microphones necessitated a very loose ribbon suspension, rendering them inherently delicate. Contemporary ribbon materials, including novel nanomaterials, have been developed to mitigate these vulnerabilities and enhance the effective dynamic range of ribbon microphones at low frequencies. Protective windscreens can diminish the risk of damage to vintage ribbons and concurrently reduce plosive artifacts during recording.
Similar to other dynamic microphone categories, ribbon microphones do not necessitate phantom power; indeed, this voltage can potentially harm certain older ribbon microphone models. Conversely, some contemporary ribbon microphone designs integrate a preamplifier, thereby requiring phantom power. Furthermore, the circuitry of modern passive ribbon microphones (i.e., those lacking an integrated preamplifier) is specifically engineered to withstand damage to the ribbon and transformer from phantom power.
Carbon
The carbon microphone represents the earliest form of microphone technology. The carbon button microphone, also identified as the Berliner or Edison microphone, employs a capsule or button filled with carbon granules compressed between two metallic plates. A voltage is applied across these plates, inducing a minor current to traverse the carbon material. One plate, functioning as a diaphragm, oscillates in response to incident sound waves, exerting variable pressure on the carbon. This fluctuating pressure deforms the granules, altering the contact area between adjacent pairs and consequently modifying the electrical resistance of the granule mass. These resistance variations induce a proportional change in the current flowing through the microphone, thereby generating the electrical signal. Historically, carbon microphones were prevalent in telephone systems; despite their extremely low-fidelity sound reproduction and restricted frequency response range, they are notably robust devices. The Boudet microphone, characterized by its use of comparatively large carbon balls, shared operational similarities with granule carbon button microphones.
Distinct from other microphone categories, the carbon microphone possesses the unique capability to function as an amplifier, utilizing a minimal amount of acoustic energy to modulate a greater quantity of electrical energy. Carbon microphones were employed as early telephone repeaters, facilitating long-distance telephony during the pre-vacuum tube era. Known as a Brown's relay, these repeaters operated by mechanically linking a magnetic telephone receiver to a carbon microphone: the attenuated signal from the receiver was transmitted to the microphone, where it modulated a more potent electric current, thereby generating a stronger electrical signal for transmission along the line.
Piezoelectric
A crystal microphone or piezo microphone operates on the principle of piezoelectricity, a phenomenon where certain materials generate an electrical voltage when subjected to mechanical pressure, thereby converting acoustic vibrations into an electrical signal. Historically, crystal microphones were frequently integrated with vacuum tube (valve) equipment, including consumer-grade tape recorders. Their substantial output impedance was well-suited to the high input impedance (typically around 10 MΩ) characteristic of vacuum tube input stages. However, their compatibility with early transistorized equipment proved challenging, leading to their eventual replacement by dynamic microphones and, subsequently, compact electret condenser units. The inherent high impedance of crystal microphones rendered them highly vulnerable to handling noise, originating from both the device itself and its connecting cable.
Piezoelectric transducers frequently function as contact microphones, enabling the amplification of sound from acoustic musical instruments, the detection of drum impacts for triggering electronic samples, and the acquisition of audio in demanding environments, such as high-pressure underwater settings. Specifically, saddle-mounted pickups found in acoustic guitars are generally piezoelectric devices designed to make direct contact with the strings as they traverse the saddle. This mechanism distinguishes them from the magnetic coil pickups prevalent in electric guitars, which utilize magnetic induction rather than mechanical coupling to capture string vibrations.
Fiber-Optic
A fiber-optic microphone transforms acoustic waves into electrical signals through the detection of variations in light intensity, a method distinct from the capacitance or magnetic field changes employed by conventional microphone technologies.
In operation, a laser source emits light that propagates through an optical fiber to illuminate the surface of a reflective diaphragm. Acoustic vibrations of this diaphragm subsequently modulate the intensity of the light reflected from its surface in a predetermined direction. This modulated light is then conveyed via a second optical fiber to a photodetector, which converts the intensity-modulated optical signal into an analog or digital audio format suitable for transmission or recording. Fiber-optic microphones exhibit a broad dynamic and frequency range, comparable to that of premium high-fidelity conventional microphones.
Fiber-optic microphones are impervious to and do not interact with electrical, magnetic, electrostatic, or radioactive fields, a characteristic referred to as EMI/RFI immunity. Consequently, the design of fiber-optic microphones renders them exceptionally suitable for deployment in environments where conventional microphones are either impractical or hazardous, including the interior of industrial turbines or within magnetic resonance imaging (MRI) equipment suites.
Fiber-optic microphones are characterized by their robustness, resilience to environmental fluctuations in temperature and humidity, and adaptability to various directionality patterns or impedance matching requirements. Furthermore, the separation between the microphone's light source and its photodetector can extend up to several kilometers without necessitating a preamplifier or any other electrical component, thereby positioning fiber-optic microphones as advantageous for industrial and surveillance acoustic monitoring applications.
Fiber-optic microphones find application in highly specialized domains, including infrasound monitoring and noise cancellation. They have demonstrated particular utility in medical contexts, for instance, enabling radiologists, medical staff, and patients to communicate effectively within the intense and noisy magnetic fields of MRI suites, as well as in associated remote control rooms. Additional applications encompass industrial equipment surveillance, audio calibration and measurement, high-fidelity sound recording, and law enforcement operations.
Laser
A laser beam is directed towards a planar surface, such as a window, which is susceptible to acoustic vibrations. Vibrations of this surface alter the reflection angle of the beam, and the subsequent displacement of the returning laser spot is detected and transformed into an audio signal. A more sophisticated and costly implementation involves splitting the returned light and directing it into an interferometer, which ascertains surface movement by measuring variations in the optical path length of the reflected beam. While the former method is typically a laboratory-scale demonstration, the latter necessitates an exceptionally stable laser and high-precision optical components. Research into laser microphones has focused on their capacity to detect sound-induced vibrations on remote surfaces.
An experimental laser microphone employs a laser beam in conjunction with smoke or vapor to detect sound vibrations in free air. On August 25, 2009, U.S. patent 7,580,533 was granted for a Particulate Flow Detection Microphone, which operates based on a laser-photocell pair interacting with a moving stream of smoke or vapor within the laser beam's path. Sound pressure waves induce disturbances in the smoke, which, in turn, cause variations in the amount of laser light reaching the photodetector. A prototype of this device was showcased at the 127th Audio Engineering Society convention in New York City, held from September 9 to October 12, 2009.
Liquid
Early microphones struggled to reproduce intelligible speech until Alexander Graham Bell introduced significant improvements, including a variable-resistance water microphone and transmitter. Bell's water transmitter comprised a metal cup filled with water, to which a small quantity of sulfuric acid was added. A sound wave caused the diaphragm to move, compelling a needle to oscillate vertically within the water. The electrical resistance between the wire and the cup was then inversely proportional to the size of the water meniscus surrounding the submerged needle. Elisha Gray subsequently filed a patent caveat for a variant that utilized a brass rod instead of a needle. Other minor modifications and enhancements to the water microphone were made by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, with Reginald Fessenden patenting one version in 1903. While these represented the first functional microphones, they proved impractical for widespread commercial applications. Notably, the inaugural telephone conversation between Bell and Watson was conducted using a water microphone.
MEMS
The MEMS microphone, also known as a microphone chip or silicon microphone, features a pressure-sensitive diaphragm directly etched into a silicon wafer using MEMS processing techniques, typically integrated with a preamplifier. Most MEMS microphones are variations of the condenser microphone design. Digital MEMS microphones incorporate built-in analog-to-digital converter (ADC) circuits on the same CMOS chip, thereby creating a digital microphone that is more readily integrated into contemporary digital products. Major manufacturers of MEMS silicon microphones include Cirrus Logic, InvenSense (whose product line was acquired by Analog Devices), Akustica, Infineon, Knowles Electronics, Memstech, Sonion MEMS, Vesper, AAC Acoustic Technologies, and Omron.
During the 2010s, piezoelectric MEMS microphones were developed, representing a substantial architectural and material departure from existing condenser-style MEMS designs.
Plasma
In a plasma microphone, an experimental microphone type, a plasma arc of ionized gas is employed. Sound waves induce variations in the pressure surrounding the plasma, which subsequently cause temperature fluctuations. These temperature changes alter the plasma's conductance, and these conductance variations can be detected as modulations superimposed on the electrical supply to the plasma.
Speakers as microphones
A loudspeaker, which functions as a transducer converting an electrical signal into sound waves, is the functional inverse of a microphone. Given that a conventional speaker shares structural similarities with a dynamic microphone (featuring a diaphragm, coil, and magnet), speakers can indeed operate in reverse as microphones. The principle of reciprocity applies, meaning the resulting microphone exhibits the same limitations as a single-driver loudspeaker: restricted low- and high-end frequency response, inadequately controlled directivity, and low sensitivity. In practical scenarios, speakers are occasionally utilized as microphones in applications where high bandwidth and sensitivity are not critical, such as intercom systems, walkie-talkies, or video game voice chat peripherals.
Nevertheless, at least one practical application specifically exploits these inherent weaknesses: the deployment of a medium-sized woofer positioned closely in front of a bass drum within a drum set to function as a microphone. A commercial example is the Yamaha Subkick, which consists of a 6.5-inch (170 mm) woofer shock-mounted within a 10-inch drum shell, designed for placement in front of kick drums. Because a relatively massive membrane is incapable of transducing high frequencies while simultaneously tolerating strong low-frequency transients, the speaker often proves ideal for capturing the kick drum sound while minimizing bleed from adjacent cymbals and the snare drum.
Capsule design and directivity
The internal components of a microphone are the fundamental determinants of directivity variations. A pressure microphone incorporates a diaphragm positioned between a fixed internal air volume and the ambient environment, exhibiting uniform sensitivity to pressure originating from all directions, thereby classifying it as omnidirectional. Conversely, a pressure-gradient microphone employs a diaphragm that is at least partially exposed on both surfaces. The differential pressure across these two sides generates its distinctive directional properties. A pure pressure-gradient microphone demonstrates equivalent sensitivity to sounds incident from the front or rear, yet remains insensitive to lateral sound incidence, as simultaneous sound arrival at both front and rear surfaces precludes the formation of a pressure gradient. The characteristic polar pattern for a pure pressure-gradient microphone is a figure-8 configuration. Alternative polar patterns are achieved through the design of capsules that integrate these two principles in varied configurations. For example, the cardioid pattern results from a partially enclosed rear, yielding a response that synthesizes both pressure and pressure-gradient characteristics. Additional elements, including the microphone's external geometry and auxiliary components like interference tubes, can further modify its directional response.
Polar Patterns
A microphone's directionality, or polar pattern, quantifies its sensitivity to sound incidence from various angles relative to its central axis. These polar patterns delineate the geometric locus of points in polar coordinates from which a consistent signal output level is produced by the microphone, assuming a uniform sound pressure level (SPL) at each point. The orientation of the microphone's physical body concerning these diagrams is contingent upon its specific design. In large-diaphragm microphones, such as the Oktava, the upward vector within the polar diagram typically aligns perpendicularly to the microphone body, a configuration commonly termed side fire or side address. Conversely, for small-diaphragm microphones, such as the Shure, this vector generally extends along the microphone's axis, a configuration referred to as end fire, top, or end address.
Polar patterns are influenced by the housing's inherent shielding properties (encompassing diffraction, dissipation, or absorption) and by the electronic combination of dual membranes. Certain microphone designs integrate multiple principles to achieve a specific polar pattern.
Omnidirectional Microphones
The response of an omnidirectional (or nondirectional) microphone is theoretically modeled as a perfect sphere in three dimensions. However, practical applications reveal deviations from this ideal. Similar to directional microphones, the polar pattern of an "omnidirectional" microphone is frequency-dependent. Given that the microphone body possesses finite dimensions, it inherently obstructs sounds arriving from the rear, leading to a minor flattening of the polar response. This flattening effect intensifies proportionally as the microphone's diameter (assuming a cylindrical form) approaches the wavelength of the incident frequency. Consequently, microphones with the smallest diameters exhibit superior omnidirectional characteristics at higher frequencies.
At 10 kHz, the wavelength of sound measures 1.4 inches (3.5 cm). The smallest measurement microphones, typically 1/4 inch (6 mm) in diameter, effectively mitigate directionality even at the highest frequencies. Unlike cardioid microphones, omnidirectional microphones do not utilize resonant cavities for delay, rendering them among the "purest" microphones due to their minimal sound coloration and negligible alteration of the original audio. Their pressure-sensitive design also enables a remarkably flat low-frequency response, extending to 20 Hz or even lower. Furthermore, pressure-sensitive microphones demonstrate significantly reduced susceptibility to wind noise and plosives compared to directional (velocity-sensitive) microphones.
Areas of application include studios, historical churches, theaters, and on-site television interviews.
A notable example of a nondirectional microphone is the spherical black eight ball.
Unidirectional Microphones
A unidirectional microphone exhibits primary sensitivity to sound originating from a single direction. In each diagram, the microphone is depicted as oriented upwards. Sound intensity for a specific frequency is graphically represented across a radial range from 0 to 360°. Professional diagrams typically incorporate these scales and feature multiple plots corresponding to various frequencies.
Cardioid, Hypercardioid, Supercardioid, and Subcardioid Microphone Patterns
A cardioid microphone, the most prevalent type of unidirectional microphone, derives its name from its distinctive heart-shaped sensitivity pattern. These microphones are frequently employed for vocal and speech applications due to their efficacy in attenuating off-axis sounds. Spatially, the cardioid pattern resembles an apple, with the microphone positioned at the "stem." This directional characteristic minimizes sound acquisition from the sides and rear, thereby mitigating acoustic feedback, particularly from stage monitors. As these directional transducers operate based on pressure gradient sensing, positioning them in close proximity (within a few centimeters) to a sound source induces a low-frequency enhancement, termed the proximity effect, attributed to the amplified gradient. The SM58 has been the most commonly used microphone for live vocals for more than 50 years, underscoring the significance and widespread adoption of cardioid microphones.
The cardioid pattern can be conceptualized as a superposition of an omnidirectional (pressure-sensitive) and a figure-8 (pressure-gradient-sensitive) microphone. When sound waves originate from the rear, the negative signal from the figure-8 component cancels the positive signal from the omnidirectional element; conversely, sound waves from the front result in additive signals from both components. Nevertheless, at lower frequencies, a cardioid microphone exhibits characteristics akin to an omnidirectional microphone.
Varying the ratios of these two components allows for the creation of any polar pattern between omnidirectional and figure-8, collectively forming the first-order cardioid family. Notable configurations within this family are:
- A hypercardioid microphone shares similarities with the cardioid pattern but incorporates a greater contribution from the figure-8 component, resulting in a narrower frontal sensitivity region and a reduced rear pickup lobe. This pattern is achieved by combining the omnidirectional and figure-8 elements in a 3:1 ratio, which generates null points at 109.5°. This specific ratio optimizes the directivity factor (or directivity index).
- The supercardioid microphone resembles the hypercardioid but features enhanced frontal sensitivity and diminished rear pickup. Its pattern is typically formed using an approximate 5:3 ratio of the constituent elements, yielding nulls at 126.9°. This particular ratio maximizes the front-back ratio, which quantifies the energy differential between sound received from the front and rear.
- The subcardioid microphone is characterized by the absence of null points. It is generated by an approximate 7:3 ratio of the components, resulting in a 3–10 dB difference in sensitivity between the front and rear pickup.
A collocated triad comprising three orthogonally oriented cardioid microphones or hydrophones can enhance gain and facilitate the creation of a steerable beam pattern.
Bi-directional Microphone Patterns
Figure-8, also known as bi-directional, microphones exhibit equal sensitivity to sound originating from both the front and rear of their transducer element. This pattern is characteristic of most ribbon microphones. Fundamentally, these microphones do not respond to absolute sound pressure but rather to the change in pressure between the front and rear surfaces. Consequently, sound waves impinging from the sides arrive simultaneously at both surfaces, resulting in no pressure differential and thus no sensitivity to lateral sound. From a mathematical perspective, omnidirectional microphones function as scalar transducers, responding to pressure irrespective of direction, whereas bi-directional microphones operate as vector transducers, sensitive to the pressure gradient along an axis perpendicular to the diaphragm's plane. A further consequence of this design is the inversion of output polarity for sounds originating from the rear.
Shotgun Microphone Patterns
Shotgun microphones are highly effective at capturing sound from a precisely defined direction. Their polar pattern is exceptionally narrow, akin to highly directional hypercardioid or supercardioid microphones. This focused directivity is achieved through the incorporation of a long interference tube featuring multiple slots along its length. Sound waves arriving from off-axis directions enter these slots at varying points, leading to phase cancellation and a consequent reduction in extraneous noise.
However, this design inherently retains some sensitivity to sounds originating from the rear of the microphone. The characteristics of these rear pickup zones are frequency-dependent, which can occasionally introduce undesirable coloration into the recorded audio, thereby impacting overall sound quality.
Boundary Microphones
Multiple methodologies have been developed to optimize microphone performance in acoustically challenging environments, which frequently exhibit excessive sound reflections from one or more bounding surfaces. When a microphone is positioned within or in close proximity to such a boundary, reflections from that surface synchronize with the direct sound, thereby imparting a hemispherical polar pattern to the microphone and enhancing intelligibility. Initially, this was achieved by placing a conventional microphone adjacent to the surface, sometimes encased in acoustically transparent foam. Subsequently, sound engineers Ed Long and Ron Wickersham advanced this concept by orienting the microphone's diaphragm parallel to and facing the boundary.
While the initial implementations of boundary microphones utilized omnidirectional elements, it is also feasible to mount a directional microphone sufficiently close to a surface to leverage some of the benefits of this technique while preserving the inherent directional characteristics of the element.
Specialized Microphone Designs
A lavalier microphone is engineered for hands-free operation, comprising a compact unit worn on the body. Historically secured by a lanyard around the neck, these microphones are now more commonly affixed to clothing using a clip, pin, tape, or magnet. The lavalier cable can be concealed beneath clothing and routed either to an RF transmitter carried in a pocket or clipped to a belt for mobile applications, or directly to a mixer for stationary setups.
A wireless microphone transmits audio as a radio or optical signal, circumventing the need for a physical cable. Most professional wireless microphones employ a small radio transmitter to send their signal to a nearby receiver integrated into the sound system.
A contact microphone captures vibrations directly from a solid surface or object, in contrast to sound vibrations propagated through air. This capability is particularly useful for detecting very low-level sounds, such as those originating from minute objects or insects. Typically, the microphone consists of a magnetic (moving coil) transducer, a contact plate, and a contact pin. The contact plate is positioned directly on the vibrating component of a musical instrument or other surface, with the contact pin transferring these vibrations to the coil. Contact microphones have been successfully employed to record phenomena like a snail's heartbeat and the footsteps of ants. A portable iteration of this microphone has recently been introduced.
A throat microphone represents a variant of the contact microphone, designed to capture speech directly from a person's throat, to which it is strapped. This design facilitates its use in environments with significant ambient noise, which would otherwise render the speaker inaudible.
A parabolic microphone utilizes a parabolic reflector to collect and focus sound waves onto a microphone receiver, mirroring the function of a parabolic antenna in concentrating radio waves. Characterized by exceptionally focused front sensitivity and the ability to capture sounds from considerable distances, its typical applications include nature recording, outdoor sporting events, surveillance, law enforcement, and espionage. However, parabolic microphones are generally not employed for standard recording purposes due to an inherent design limitation that results in a suboptimal low-frequency response.
A stereo microphone integrates two microphone elements within a single unit to generate a stereophonic signal. These microphones are frequently utilized in broadcast applications or field recording scenarios where configuring two separate condenser microphones in a classic X-Y arrangement for stereophonic capture would be impractical. Some models feature an adjustable angle of coverage between their two channels.
A noise-canceling microphone is specifically engineered for highly directional sound capture in acoustically challenging environments. These devices are commonly integrated into boom microphones on headsets within aircraft cockpits. Additionally, they are indispensable for vocalists performing on loud concert stages, facilitating clear sound reinforcement. The operational principle of many noise-canceling microphones involves the integration of signals from two diaphragms, which are either configured with opposing electrical polarities or subjected to electronic processing. In configurations utilizing dual diaphragms, the primary diaphragm is positioned proximally to the desired sound source, while the secondary diaphragm is situated more distally to capture ambient environmental sounds, which are subsequently subtracted from the primary diaphragm's signal. This signal combination process significantly attenuates extraneous sounds, thereby enhancing the intelligibility of the intended audio. Alternative noise-canceling methodologies employ a single diaphragm influenced by strategically placed ports on the microphone's sides and rear, achieving a cumulative 16 dB attenuation of distant sounds. A notable single-diaphragm noise-canceling headset design by Crown has been prominently adopted by vocal artists, including Garth Brooks, Britney Spears, and Janet Jackson.
Stereophonic Microphone Techniques
Standardized methodologies are employed for microphone utilization in diverse applications, including live sound reinforcement, studio recording, and motion picture production. Through the strategic placement and configuration of single or multiple microphones, it is possible to preserve desired sonic characteristics while effectively mitigating undesirable ambient noise.
Microphone Powering Mechanisms
Microphones incorporating active electronic circuitry, predominantly condenser microphones, necessitate external power for their operational components. Early iterations utilized vacuum-tube circuits, requiring a distinct power supply unit connected via a multi-pin cable and connector. The subsequent development of solid-state amplification significantly reduced power demands, enabling the integration of both audio signal transmission and power delivery through a single cable and connector. Throughout the 1960s, several powering methodologies emerged, primarily originating in Europe. The two principal methods, Tonaderspeisung (T-power) and phantom power, were initially codified in German standards DIN 45595 and DIN 45596, respectively. Since the 1980s, phantom power has gained widespread adoption due to its compatibility with both powered and unpowered microphones through a common input. For consumer electronic devices, including DSLRs and camcorders, "plug-in power" is the prevalent method for microphones employing a 3.5 mm phone plug connector. These powering standards—phantom, T-power, and plug-in power—are comprehensively detailed in the international standard IEC 61938.
Microphone Connectors and Interfacing
The predominant connector types utilized for microphones include:
- Male XLR connectors, typically found on professional-grade microphones.
- The 1⁄4 inch (6.35 mm) phone connector, often an unbalanced §78§⁄§910§ inch (6.35 mm) TS (tip and sleeve) variant, is prevalent on more economical musician's microphones. Harmonica microphones frequently employ a high-impedance §1314§⁄§1516§ inch TS connection, designed for direct interfacing with guitar amplifiers.
- The 3.5 mm (or 1⁄8 inch mini) TRS (tip, ring, and sleeve) stereo mini phone plug, also offered in a TS mono configuration, is commonly found on prosumer-grade camera, recorder, and computer microphones.
- USB connectivity facilitates direct interfacing with personal computers. Microphones utilizing USB derive power from the connection, with integrated electronics performing preamplification and analog-to-digital conversion (ADC) prior to the transmission of digital audio data through the USB interface.
Certain microphones incorporate alternative connectors, including 5-pin XLR or mini XLR variants, for compatibility with portable audio equipment. Lavalier microphones, historically known as 'lapel' microphones due to their attachment to clothing, often feature proprietary connectors for interfacing with wireless transmitters, such as radio packs. Commencing in 2005, professional-grade microphones equipped with USB interfaces have emerged, specifically engineered for direct digital audio recording into computer-based software applications.
Impedance Bridging
The selection of an appropriate pre-amplifier for a microphone necessitates knowledge of the microphone's impedance. Impedance, an electrical characteristic measured in ohms (), quantifies the relationship between voltage and current and varies with frequency. In scenarios where power transfer is not the primary concern, signals, including those from microphones, are typically transmitted as fluctuating voltages. Maximizing signal amplitude is achieved through a technique known as impedance bridging. This configuration mandates that the microphone's output impedance be significantly lower than the pre-amplifier's input impedance; a practical guideline suggests the pre-amplifier's impedance should be at least ten times greater than that of the microphone. This approach ensures minimal signal attenuation and negligible power consumption.
Impedance matching represents the primary alternative to impedance bridging, designed to optimize power transfer from a specific source impedance. Nevertheless, its relevance has diminished since the early 20th century, a period characterized by costly and heat-generating amplifiers. During that era, to minimize power loss and consequently reduce the required number of amplifiers in telephone lines, source and load impedances were precisely matched. A significant disadvantage of impedance matching is the resultant 6 signal loss, as only half of the voltage level reaches the pre-amplifier's input. However, specific ribbon and dynamic microphones constitute exceptions, primarily because their designers integrate a particular load impedance into the microphone's internal electro-acoustical damping circuit.
Microphones exhibit considerable variations in impedance, a characteristic directly influenced by their design. For passive microphones, this impedance value is closely correlated with the impedance of the internal coil or analogous mechanism. Conversely, in active microphones, it represents the output impedance of the integrated amplifier circuitry.
Impedances below 600 are categorized as low impedance. Medium impedance ranges from 600 to 10 . High impedance classifications apply to values exceeding 10 . Due to their integrated amplification, condenser microphones typically feature an output impedance ranging from 50 to 200 .
Digital Microphone Interface
The AES42 standard, promulgated by the Audio Engineering Society, establishes the specifications for a digital microphone interface. Microphones adhering to this standard transmit a digital audio stream directly via an XLR or XLD male connector, thereby bypassing analog output. Such digital microphones can be integrated with contemporary equipment featuring compatible AES42-compliant input connections, or alternatively, through an appropriate interface box. Presently, several microphone manufacturers offer studio-grade microphones that operate in accordance with the AES42 standard.
Measurements and Specifications
Variations in microphone construction lead to distinct characteristic responses to sound. These response disparities result in non-uniform phase and frequency characteristics. Furthermore, microphones exhibit non-uniform sensitivity to sound pressure and can accommodate varying input levels without introducing distortion. While a more uniform response is preferred for scientific applications, this preference often diverges in music recording, where a microphone's non-uniform response can impart a desirable sonic coloration. An international standard exists for microphone specifications; however, adherence among manufacturers remains limited. Consequently, comparing published data across various manufacturers proves challenging due to the employment of diverse measurement methodologies. Therefore, caution is warranted when deriving definitive conclusions from this or any other published data, unless the manufacturer explicitly confirms compliance with IEC specifications.
A frequency response diagram illustrates microphone sensitivity, expressed in decibels, across a specified frequency spectrum (typically 20 Hz to 20 kHz), usually under conditions of perfectly on-axis sound incidence (0° relative to the capsule). While frequency response can be textually represented, for instance, as "30 Hz–16 kHz ±3 dB," this format provides limited information. Such a statement implies a largely flat, linear response within the specified frequencies, with amplitude deviations not exceeding ±3 dB. Nevertheless, this textual representation fails to convey the smoothness of these variations or their specific locations within the frequency spectrum. It is crucial to note that broad statements like "20 Hz–20 kHz" lack meaning without an accompanying decibel tolerance. For directional microphones, frequency response exhibits significant variability based on both the distance from the sound source and the source's geometric characteristics. Although IEC 60268-4 mandates frequency response measurements under plane progressive wave conditions (implying considerable distance from the source), this methodology is rarely feasible in practice. Measurements for close talking microphones may employ diverse sound sources and distances; however, the absence of a standardized protocol precludes direct comparison of data across different models unless the specific measurement technique is fully documented.
The self-noise, or equivalent input noise level, quantifies the sound level that generates the same output voltage from a microphone in the absence of any external sound. This metric defines the lower limit of a microphone's dynamic range and is particularly critical for recording subtle audio. Measurements are frequently expressed in dB(A), which denotes the equivalent loudness of the noise on a decibel scale, adjusted by a frequency-weighting curve (A-weighting) to approximate human hearing. For instance, "15 dBA SPL" signifies a sound pressure level relative to 20 micropascals. A lower numerical value indicates superior performance. Some manufacturers employ ITU-R 468 noise weighting, which offers a more accurate representation of perceived noise but yields values approximately 11–14 dB higher. A microphone considered quiet typically exhibits a self-noise of 20 dBA SPL or 32 dB SPL when 468-weighted. Historically, exceptionally quiet microphones, such as the Brüel & Kjaer 4179 with a noise level near 0 dB SPL, have been developed for specialized applications. More recently, the studio and entertainment markets have seen the introduction of low-noise microphones, including models from Neumann and Røde, which claim noise levels between 5–7 dBA. This reduction is often achieved by modifying the frequency response of the capsule and associated electronics to minimize noise within the A-weighting curve, potentially at the expense of increased broadband noise.
The clipping level serves as a crucial indicator of a microphone's maximum usable input, distinguishing it from the 1% total harmonic distortion (THD) figure often cited under maximum sound pressure level (SPL). While 1% THD represents a relatively minor distortion, frequently imperceptible, particularly during transient high peaks, clipping introduces a significantly more audible form of distortion. Notably, for certain microphones, the actual clipping level can substantially exceed the stated maximum SPL.
A microphone's dynamic range is defined as the difference in sound pressure level (SPL) between its noise floor and its maximum SPL. When presented as a singular value, such as "120 dB," this metric provides considerably less comprehensive information compared to the individual specification of both self-noise and maximum SPL.
Microphone sensitivity quantifies its efficiency in converting acoustic pressure into an electrical output voltage. A microphone exhibiting high sensitivity generates a greater voltage, thereby necessitating less amplification from a mixer or recording device. While this characteristic offers practical advantages, it does not directly correlate with overall microphone quality. Indeed, the term "sensitivity" is somewhat imprecise; "transduction gain" or simply "output level" might be more accurate, as genuine sensitivity is fundamentally constrained by the noise floor. Furthermore, an excessively high output level, interpreted as "sensitivity," can detrimentally affect the clipping level. Two primary measurement standards exist: The internationally preferred method expresses sensitivity in millivolts per pascal at 1 kHz, where a higher numerical value denotes increased sensitivity. The older American convention references a 1 V/Pa standard and is measured in decibels, yielding a negative value. In this system, a higher (less negative) decibel value, such as −60 dB, signifies greater sensitivity compared to −70 dB.
Measurement Microphones
Certain microphones are specifically designed for testing speakers, quantifying acoustic phenomena, including noise level assessment. These calibrated transducers typically include a calibration certificate detailing their absolute sensitivity across various frequencies. The performance classification of measurement microphones, often denoted as "Class 1" or "Type 2," pertains to sound level meter standards rather than intrinsic microphone specifications. A more exhaustive standard for characterizing measurement microphone performance has recently been implemented.
Measurement microphones primarily function as scalar pressure sensors, displaying an omnidirectional response constrained solely by the scattering characteristics of their physical form. Conversely, determining sound intensity or sound power necessitates pressure-gradient measurements, commonly achieved through the deployment of microphone arrays comprising at least two units, or by employing hot-wire anemometers.
Calibration
For accurate scientific measurements utilizing a microphone, its precise sensitivity (expressed in volts per pascal) must be ascertained. As this parameter can fluctuate throughout the device's operational lifespan, periodic calibration of measurement microphones is imperative. Such calibration services are provided by select microphone manufacturers and accredited independent testing laboratories. Ultimately, all microphone calibration procedures are traceable to primary standards maintained by national measurement institutes, including NPL in the UK, PTB in Germany, and NIST in the United States, which predominantly employ the reciprocity primary standard for calibration. Microphones calibrated via this methodology can subsequently serve as reference instruments for calibrating other microphones through comparative techniques.
The necessity for periodic testing of measurement microphones (typically annually or every few months) and following any potentially detrimental incident, such as physical impact (a risk mitigated by the foam-padded cases commonly supplied with these microphones) or exposure to excessive sound levels, varies according to the specific application.
Arrays
A microphone array consists of multiple microphones functioning synchronously. Its diverse applications include:
- Systems designed for isolating vocal input from ambient noise, particularly in devices such as telephones, speech recognition systems, and hearing aids.
- Surround sound and associated audio technologies.
- Acoustic source localization for object detection, exemplified by military applications in pinpointing artillery fire origins, and for aircraft location and tracking.
- Production of high-fidelity original recordings.
- Three-dimensional spatial beamforming for the localized acoustic detection of subcutaneous sounds.
Characteristically, an array comprises omnidirectional microphones strategically positioned around the periphery of an area, interconnected with a computer system that records and processes the data into a cohesive output.
Windscreens
Windscreens, also referred to as windshields, serve to mitigate the impact of wind on microphones. While pop-screens offer defense against unidirectional air blasts, foam 'hats' provide omnidirectional wind shielding for the microphone grille, and larger enclosures such as blimps, zeppelins, and baskets fully encase the microphone, thereby protecting its entire body. This comprehensive protection is crucial, as the pronounced low-frequency components of wind noise can induce significant vibrations in the microphone housing, thereby substantially increasing its noise output.
The materials employed for shielding, including wire gauze, fabric, or foam, are engineered to possess substantial acoustic impedance. Sound waves, characterized by relatively low particle-velocity air pressure fluctuations, can traverse these materials with negligible attenuation, whereas higher particle-velocity wind is obstructed to a considerably greater degree. While augmenting material thickness enhances wind attenuation, it concurrently risks degrading high-frequency audio content. Consequently, this imposes limitations on the practical dimensions of basic foam screens. Although foams and wire meshes can be partially or entirely self-supporting, pliable fabrics and gauzes necessitate mounting on frames or integration with more robust structural components.
Wind noise originates predominantly at the initial surface impacted by airflow; consequently, increasing the distance between the shield's periphery and the microphone capsule enhances noise attenuation. For shields approximating a spherical geometry, this attenuation scales approximately with the cube of the aforementioned distance. Furthermore, full basket windshields introduce a distinct pressure chamber effect, initially elucidated by Joerg Wuttke. This phenomenon enables the combined shield and microphone assembly to function as a high-pass acoustic filter, particularly beneficial for two-port (pressure gradient) microphones.
Given that surface turbulence constitutes the primary source of wind noise, mitigating macroscopic turbulence contributes significantly to noise reduction. Effective strategies have included employing aerodynamically smooth surfaces and designs that inhibit the formation of potent vortices. Historically, artificial fur has demonstrated considerable utility in this application, as its fibers generate micro-turbulence and passively dissipate acoustic energy. When maintained in an unmatted state, these fur fibers exhibit high acoustic transparency; however, their woven or knitted backing can introduce notable attenuation. A key limitation of artificial fur as a material is the challenge in achieving consistent manufacturing quality and preserving its optimal condition during field use. Consequently, there is a growing impetus to explore alternative materials.
Geophone – A transducer designed for detecting sound waves propagating through the earth.
- Geophone – transducer for sound within the earth
- Hydrophone – A transducer specifically engineered for sensing sound in aquatic environments.
- Ionophone – A microphone technology utilizing plasma for sound detection.
- Microphone blocker – A computer accessory designed to disable an internal microphone.
- Microphone connector
- Nominal impedance
Notes
References
Corbett, Ian. Mic It!: Microphones, Microphone Techniques, and Their Impact on the Final Mix. CRC Press, 2014.
- Corbett, Ian. Mic It!: Microphones, Microphone Techniques, and Their Impact on the Final Mix. CRC Press, 2014.
- White, Paul. Using Microphone Polar Patterns Effectively. Sound on Sound, SOS Publications Group, 2007.
- Eargle, John. The Microphone Book. Taylor & Francis, 2004.
- "The Invention of the Microphone". Scientific American. July 13, 1878. p. 16.
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