The conversion of sound energy into another form of energy, usually heat, when passing through an acoustical medium.

Ratio of sound absorbing effectiveness, at a specific frequency, of a unit of acoustical absorbent to a unit area of perfectly absorptive material.

The characteristic of a location which is said to be representative of the particular room, concert hall etc.

See "Remote Miking"

The noise level associated with a location in the absence of foreign excitation.

A room where the walls effectively absorb all incident sound, thereby creating a free-field condition for measurement purposes. A room in which there is little reverberant sound is said to be a "dead room".

The sound pressure level, for a specified frequency, at which persons with normal hearing begin to respond.

Electronic network designed to accentuate or attenuate signal levels at certain frequencies in order that the system response corresponds to the results of subjective tests of the ear's sensitivity. The A-weighting curve corresponds to the inverse equal loudness contour at a level of 40 phons.

Microphones placed directly on the surface of a large boundary - also called Pressure Zone Microphones or PZM microphones. Please see PZM microphones under related articles.

Placement of a microphone near a sound source, thus effectively eliminating all but the direct sound from the source

Coincidence stereo is the generic term for all stereo techniques where the two microphones are placed exactly in the same point. This technique uses directional microphones to create the stereo image. The stereo width is set by the microphones' off-axis attenuation and is both dependent on the stereo set-up and the quality of the individual microphone's polar pattern. Among the more well known coincidence stereo techniques are XY stereo, MS-stereo and Blumlein stereo.

Two or more microphones mounted on a common vertical axis.

Non-uniformity in off-axis frequency responses resulting an a distortion of the tonal quality of the source.

Measure of the separation in adjacent channels of a system expressed in -dB's. Equivalent to channel separation expressed in +dB's.

Removal of echoes and reverberation by the use of sound absorbing materials.

A linear numbering scale used to define a logarithmic amplitude scale, thereby compressing a wide range of amplitude values to a small set of numbers. A value X when expressed in dB is in relation to a reference value Xref: X (dB) = 20 Log10 X/Xref

Difference frequency distortion considers only the difference terms of intermodulation distortion. Such tones are not harmonically related and are therefore musically undesirable. It is a percentage of the signal amplitude.

The diffuse field response is significantly different from the on-axis response and shows the microphone's tonal qualities when placed in a highly reverberant environment with a large distance to the sound sources. This response is obtained by placing the microphone in an environment, where the sound pressure level is the same everywhere and the flow of energy is equally probable in all directions - a reverberant room. The diffuse field response normally shows a significant high frequency roll off compared to the on-axis response.

Sound that is completely random in phase; sound which appears to have no single source or direction.

The sound that arrives at the reception point directly (no reflections). Sound lacking in any reverberation.

Measure of the response of a microphone to sound incident from various angles, or of the radiation pattern of a loudspeaker. The sensitivity (referred to on-axis sensitivity) is plotted as a function of angle of incidence at various frequencies. Also called "polar pattern".

See "Close Miking"

Range between the quietest and loudest levels a device can produce or detect. For a microphone or measuring system it is normally specified as the range between the inherent noise level and a level leading to a specified amount of distortion.

One or several distinct repetitions of a sound. See "Reverberation".

Distribution of acoustic energy at a very much greater distance from a source than the linear dimensions of the source itself.

An environment in which there are no reflecting or obstructing boundaries and the sound field consists of uniformly progressing plane waves.

The frequency range is also called the microphone "bandwidth". Unlike the frequency response it is not represented by a graph, but will give the sound engineer a rough indication of the microphone's tonal span and weather it will match the full span of the sound source. The frequency range states the microphone's upper and lower limiting frequencies plus the tolerance field of the on-axis response in dB. If no tolerance field is indicated the frequency range is submitted as the -3dB points of the on-axis response.

The frequency response of a microphone is the characteristic graph obtained by recording the level in dB of the output signal of the microphone, while the microphone is exposed to a certain acoustic field of pure sinusoidal tones with equal intensity. The frequency response gives important informations about the tonal balance of the microphone under different acoustic conditions.

The sensitivity of a microphone to movement and shock. Expressed as an equivalent sound pressure level.

Stereo techniques using the human interaural geometry as a model for the microphone set-up.

Head related stereo is a generic term for a lot of different stereo techniques using both difference-in-time and difference-in-intensity stereo with microphone spacing and microphone off-axis attenuation related to the distance between the human ears and the shadow effect of the human head. Among the more standardized head related stereo techniques are ORTF stereo and DIN stereo.

Head related stereo should not be confused with binaural stereo where the human head is simulated through the use of a dummy head and torso.

Difference between standard operating level of 0VU (+4dBm) and a specified distortion level.

The unit of frequency measurement, representing cycles per second.

International Standard Recording Code. Information about the country of origin, the owner, the year of recording and the serial number of a recording encoded on compact disc.

The noise which is internally generated in a system in the absence of any excitation. For a microphone, usually expressed as an equivalent sound pressure level which would produce the same output voltage as the noise voltage. Also called "self-noise".

The noise which is internally generated in a system in the absence of any excitation. For a microphone, usually expressed as an equivalent sound pressure level which would produce the same output voltage as the noise voltage. Also called "self-noise".

Resistance to the transmission of sound by materials and structures.

An exciting feature of digital audio for producers, recording engineers etc. When directly recording multi-track music onto a digital medium, special bits are reserved for non-audio information called labels. The information contained in these bits can keep track of things like amount of microphones used for each take, their position, setting of the mixing console etc.

Subjective impression of the intensity of a sound.

The lower frequency at which the frequency response of a pressure microphone has fallen by 3dB. Determined by the venting system which equalises the static pressure difference between the inside and outside of the cartridge.

The process by which the threshold of audibility of one sound is raised by the presence of another (masking) sound.

Musical Instrument Digital Interface. A standard that defines and recommends hardware and procedure for control and communication between a controller and one or more MIDI-equipped musical instruments.

That part of a sound filed, usually within about two wavelengths from a noise source, where there is no simple relationship between sound level and distance.

Off-axis responses are normally submitted for directional microphones only. Linear off-axis qualities on directional microphones are extremely difficult to obtain. Therefore it is important to pass on information about the off-axis responses to the sound engineer, so he can compare the acoustic quality of different microphone types. The off-axis responses tells the sound engineer how the microphone will treat possible leakage from other sound sources with regards to both attenuation and frequency response. When measuring the off-axis responses the microphone is placed in a "free sound field" consisting of plane waves similar to the conditions when measuring the on-axis response. The microphone is turned off-axis so that the incidence of the sound waves on the microphone diaphragm (or the microphone's reference direction) are in a certain direction. Normally manufacturers are using angles like 30§, 60§, 90§ and 180§ showing the off-axis responses in the same diagram as the on-axis response.

The on-axis response is normally just referred to as the microphone's frequency response and will give information about the product's tonal behaviour when placed close to the sound source. It is normally obtained by placing the microphone in a "free sound field" consisting of plane waves frontally incident on the microphone diaphragm. Also called "axial free-field response" or "0 degrees incidence free-field response". It is vital to the method that the free field conditions are obtained under reliable circumstances: Either in an anechoic room or by using measurement equipment with time frame frequency analyser possibilities.

Placing microphones above the sound source in order to pick up more ambience with the instrument.

Overhead microphones are often used on drums and percussion plus on small classical ensembles or on sections of classical orchestras in order to pick up the instruments with a blend of natural ambiance. By placing the microphones above the musical instruments and at a slight distance, it is often possible to capture more of the instrument's natural timbre, because the sounds from all the different parts of the individual musical instrument will have time to reach the microphones with a more realistic dispersion, than in close miking situations. It is even possible to capture some of the first room reflections and thereby placing the instruments in their natural acoustic environment in the recording. Overhead microphone techniques covers both mono and stereo solutions and has no preference to the microphone's pick up pattern i.e. if the microphone is directional or omnidirectional.

The influence of the microphone's polar pattern:
The choice between directional or omnidirectional microphones in the overhead set-up is mainly a matter of taste. Naturally enough the use of omnidirectional microphones will add more ambiance to the recording than the use of directional microphones will. Omnidirectional microphones are furthermore not under influence of the proximity effect like the directional microphones are and will not change their characteristics if moved closer or further away from the sound source.

Stereo on overheads:
A stereo overhead set-up is often chosen in situations where more sound sources are present at the same time; like on a drum kit, a large percussion set-up or a classical ensemble. The stereo set-up is then used to place the different sound sources correctly with regards to their direction from the listeners point of view. It is also possible to have a better coverage of all the different sound sources using a two-microphone-technique. If the overhead microphones need to be supported by close miking techniques, the stereo image form the overhead microphones can be used as a help to place the close up microphones correctly in the total stereo image.

A unit of pressure corresponding to a force of 1 Newton acting uniformly upon an area of 1 square metre. Hence 1 Pa = 1N/square metre.

A technique for supplying the preamplifier supply voltage to condenser microphones whereby half of the d.c. flows through each signal conductor and returns to the voltage source via the cable shield. Commonly 12V or 48V (DIN 45 596).

The loudness level of a sound. It is numerically equal to the sound pressure level of a 1kHz free progressive wave which is judged by reliable listeners to be as loud as the unknown sound.

Broadband noise whose energy content is inversely proportional to frequency (-3dB per octave or -10dB per decade).

See "Directional characteristic".

The ratio between direct and diffuse sound in a recording can be altered in two different ways: Either by the microphone's polar pattern or by the distance from the microphone to the sound source.

In popular music recordings, where it is customary to record instruments on separate tracks, acoustic leakage problems may occur when an ensemble is recorded simultaneously. While it may seem natural to use a directional microphone for maximum separation, the amount of leakage can also be controlled by altering the source-to-receiver distance. Since an omnidirectional microphone does not exhibit any proximity effects, it may be placed closer to the source without any coloration problems.

The illustration shows the relationship between the microphone's polar pattern and its distance to the sound source where the direct to indirect sound radio is invariable.

An omnidirectional microphone placed at a distance of 1 meter receives the same proportions of direct and diffuse sound as a cardioid microphone placed at 1.7 meters.

The (normally high) d.c. voltage applied to the backplate-diaphragm capacitor of a condenser microphone via a high resistance, thus setting up a fixed charge condition. Changes in the backplate-diaphragm distance due to pressure variations result in a varying output voltage v(t).

Noise associated with the vocally-induced distortion in a microphone due to consonant sounding, "p", "b", "t" etc.

A technique of depositing a fixed charge-carrying layer on either the diaphragm or backplate of a condenser microphone, thus eliminating the need for an external polarisation voltage. Such microphones are termed "prepolarized condenser microphones" or "electret microphones".

A microphone in which only one side of the diaphragm is exposed to the impinging sound. The diaphragm responds the pressure variations uniformly and therefore pressure microphones are inherently omnidirectional. Also called "pressure operative microphone".

A microphone in which both sides of the diaphragm are exposed to the incident sound and the microphone is therefore responsive to the pressure differential (gradient) between the two sides of the membrane. Sound incident parallel to the plane of the diaphragm produces no pressure differential, and so pressure-gradient microphones have characteristically figure-of-eight directional characteristics. Also called "velocity microphone", since the output voltage is proportional to the air particle velocity.

An inherent characteristic of pressure gradient microphones, resulting in a boost in the low-frequency response when the microphone is brought closer to a source. The effect becomes significant when the source-to-microphone distance is approximately the same as the wavelength of the impinging sound.

PZM microphones or boundary layer microphones are taking advantage of special acoustic phenomenons occurring at the surface of a boundary.

"PZM" microphones is used as a generic term for microphones taking advantage of the special acoustic phenomenons occurring directly at the surface of a plane boundary i.e. a floor or a wall. PZM microphones are therefore also called Boundary Layer Microphones. Normally PZM microphones are omnidirectional (pressure) microphones specially designed for this purpose with the diaphragm placed flush with the surface of a plate or a disc working as pressure zone. Also normal omnidirectional microphones will work fine as PZM's if for example taped to the floor or directly on a wall.

The advantages:
Direct sound waves meeting a hard plane boundary will be reflected at the surface causing a 6dB acoustic pressure increase. A microphone diaphragm will therefore have a 6dB higher sensitivity when placed in the pressure zone, than an equal microphone placed in the free field. Diffuse sound will not be reflected at a plane boundary as diffuse sound has no direction (per definition). The PZM microphone will therefore give direct sound 3dB higher level than diffuse sound. If used outdoor PZM microphones have better conditions than conventional microphones with regards to wind noises, as the wind velocity in principle is 0 (zero) on the surface of the ground.

Placement of a microphone at a distance from the source, thereby picking up a larger proportion of the reflected sound. Also called "ambience miking".

Many repetitions of a sound successively closed in time.

The time, in seconds, required for sound pressure at a specific frequency to decay a specified amount after a sound source is stopped. RT60, the time required for a decay of 60dB, is commonly used.

The square root of the arithmetic average of a set of squared instantaneous values.

By letting the recording head of a tape recorder move at an angle relative to the direction of tape transportation it is possible to record relatively high frequencies (>4MHz) with reasonably low tape speeds. This fact makes rotating-head recorders well suited for recording video and digital audio. However, with advances in the manufacturing technology of both tapes and narrow recording heads it is now possible to obtain the same bandwidth/tape consumption performance with stationary-head recorders. By reduction of the amount of moving parts reliability of these recorders is also improved.

See "Inherent noise"

Any system mounting or suspension which mechanically isolates equipment from unwanted vibration.

Noise associated with vocal sounding of words with characteristically "s", "sch", or "ch" syllables.

The ratio of the maximum signal that a system can record or reproduce to the inherent noise of that system. For digital systems it is usually defined as the ratio between RMS value of the highest recordable sine wave and the RMS value of the quantizastion noise.

Energy that is transmitted by pressure waves in air or other materials and is the objective cause of the sensation of hearing. Commonly called noise if it is unwanted.

An electronic instrument for measuring the RMS level of sound, usually in accordance with an accepted national or international standard.

A dynamic variation in atmospheric pressure. The pressure at a point in space minus the static pressure at that point.

The expression of sound pressure as dB referred to a pressure, p0, of 20microPa. SPL is defined as: Lp = 20 Logp/p0 dB Where p is the RMS value (unless otherwise stated) of sound pressure in Pascal's. Suffices (e.g. 20dB(A)) indicate that the SPL is weighted.

Two omnidirectional microphones placed diagonally in a solid Ø20 cm sphere simulating the acoustic field around the human head.

Sphere stereo is one of many head related stereo techniques. Two omnidirectional microphones is separated by a solid sphere with a diameter of Ø20 cm. The microphones are placed diagonally with their diaphragms flush with the surface of the sphere. The geometric dimensions used in this technique emulates the basic interaural proportions of a human head i.e. the interaural shadow effect and interaural time delay. Mostly the head related stereo techniques uses directional microphones to accomplish these qualities, but as sphere stereo uses omnidirectional microphones, this technique can also be used as main stereo pair at larger distances without loss of low frequencies. Unfortunately the rather large solid sphere will act as an acoustic equalizer which will color the higher frequencies rather significantly. The sphere will not only cause a boost of higher frequencies, but will also introduce significant ripples on the frequency response.

A periodic wave having a fixed distribution in space which is the result of interference of progressive waves of the same frequency and kind. Characterised by the existence of anti-nodes and nodes that are fixed in space.

Two hyper-cardioid microphones spaced 4,5 cm and angled 135° creating the stereo image. The Stereo 180 technique uses two hyper-cardioid microphones spaced 4,5 cm apart and angled at 135° to each other.

A spaced microphone stereo recording technique which uses left versus right time and phase differences of the acoustic signals to give cues which project to the listener a vivid stereo image. Also known as the "A-B stereo recording technique".

Distortion in non-linear systems where harmonic components (integer multiples of a fundamental frequency) are produced. THD is normally expressed as a percentage of the fundamental.

The comparative phase response of a microphone pair. Also called "phase-matching".

An electronic filter which gives different weighting to signals at different frequencies, commonly to approximate the frequency response of the human ear under defined conditions. The A-weighting network is most commonly used (as in dB(A)).

Broadband noise having constant energy per unit of frequency.