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 Terms & Definitions


Absorb:  Sound waves are converted into energy such as heat that is then captured by insulation. 

Absorption: In acoustics, the changing of sound energy to heat. Sound absorption is the reduction of sound energy. The sound absorption  coefficient provides the relationship between absorption and reflection of a sound. A value of 0 represents total reflection and a value of 1 represents total absorption. Multiplying the sound absorption coefficient by 100 provides the percentage of sound energy being absorbed, e.g. a sound absorption coefficient of 0.75 stands for a sound absorption of 75% and a sound reflection of 25%. 

Absorption coefficient: The fraction of sound energy that is absorbed at any surface. It has a value between 0 and 1 and varies with the frequency and angle of incidence of the sound. 

Acoustic material: Any material considered in terms of its acoustical properties. Commonly and especially, a material designed to absorb sound. 

Acoustics:  The science of sound. It can also refer to the effect a given environment has on sound. 

Airborne sound: Sound that arrives at the point of interest, such as one side of a partition, by propagation through air. 

Alcons:  The measured percentage of Articulation Loss of Consonants by a listener. % Alcons of 0 indicates perfect clarity and intelligibility with no loss of consonant understanding, while 10% and beyond is growing toward bad intelligibility, and 15% typically is the maximum loss acceptable. 

Ambience:  The acoustic characteristics of a space with regard to reverberation. A room with a lot of reverb is said to be "live"; one without much reverb is said to be "dead". 

Ambient noise: The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest. 

Amplitude:  The instantaneous magnitude of an oscillating quantity such as sound pressure. The peak amplitude is the maximum value. 

Anechoic:  Without echo. 

Anechoic chamber: A room designed to suppress internal sound reflections. Used for acoustical measurements. 

Articulation: A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.

Artificial reverberation: Reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., added to a signal to make it sound more lifelike.

Attack: The beginning of a sound; the initial transient of a musical note.

Attenuate: To reduce the level of an electrical or acoustical signal. Reduction in sound level.

Audible frequency range: The range of sound frequencies normally heard by the human ear. The audible range spans from 20Hz to 20,000Hz

Auditory area: The sensory area lying between the threshold of hearing and the threshold of feeling or pain.

Aural: Having to do with the auditory mechanism.

Average room absorption coefficient: Total room absorption in sabins or metric sabins, divided by total room surface area in consistent units of square feet or square meters.

Average sound pressure level: Of several related sound pressure levels measured at different positions or different times, or both, in a specified frequency band, ten times the common logarithm of the arithmetic mean of the squared pressure ratios from which the individual level were derived.

A-weighting: A frequency-response adjustment of a sound-level meter that makes its reading conform, very roughly, to human response.

Axial mode: The room resonances associated with each pair of parallel walls.

Background noise: Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structure borne, and instrument noise.

Baffle: A moveable barrier used to achieve separation of signals from different sources. The surface or board upon which a loudspeaker is mounted.

Bandpass filter: A filter that attenuates signals both below and above the desired passband.

Bandwidth: The total frequency range of any system. Usually specified as something like: 20-20,000Hz plus or minus 3 dB.

Bass: The lower range of audible frequencies.

Block: Building a continuous barrier that traps or stops air movement from one side of a partition to the other, greatly reducing airborne sound transmission.

Boomy: Listening term, refers to an excessive bass response that has a peak(s) in it.

Break: A physical break in the assembly or construction which acts to decouple sound vibrations from traveling through the structure.

Bright: Listening term. Usually refers to too much upper frequency energy.

Broad band noise: Spectrum consisting of a large number of frequency components, none of which is individually dominant.

Cavity: A space between wall studs or attic joists where insulation is typically installed.

Channel balance: In a stereo system, the level balances between left and right channels. Properly balanced, the image should be centered between the left-right speakers. In a home-theater system, refers to achieving correct balance between all the channels of the system.

Coherence: Listening term. Refers to how well integrated the sound of the system is.

Coloration: Listening term. A visual analog. A "colored" sound characteristic adds something not in the original sound. The coloration may be euphonically pleasant, but it is not as accurate as the original signal.

Comb filter: A distortion produced by combining an electrical or acoustical signal with a delayed replica of itself. The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response. When plotted to a linear frequency scale, the response resembles a comb, hence the name.

Compression: In audio, compression means to reduce the dynamic range of a signal. Compression may be intentional or one of the effects of a system that is driven to overload. It is also the portion of a sound wave in which molecules are pushed together, forming a region with higher-than-normal atmospheric pressure.

Critical distance: The distance from a sound source at which direct sound and reverberant sound are at the same level.

Critical frequency: The frequency below which standing waves cause significant room modes.

Cross-talk: Unwanted breakthrough of one channel into another. Also refers to the distortion that occurs when some signal from a music source that you are not listening to leaks into the circuit of the source that you are listening to.

Cutoff frequency: Of an anechoic wedge or set of wedges, the lowest frequency above which the normal incidence sound absorption coefficient is at least .990.

Cycles per second: The frequency of an electrical signal or sound wave. Measured in Hertz (Hz).

Damp: To cause a loss or dissipation of the oscillatory or vibrational energy of an electrical or mechanical system.

DB: See decibel.

DB (A): A sound-level meter reading with an A-weighting network simulating the human-ear response at a loudness level of 40 phons.

DB (B): A sound-level meter reading with a B-weighting network simulating the human-ear response at a loudness level of 70 phons.

DB (C): A sound-level meter reading with no weighting network in the circuit, i.e., flat. The reference level is 20 uPa.

Decade: Ten times any quantity or frequency range. The range of the human ear is about 3 decades.

Decay rate: d, [T-1]; dB/s—for airborne sound, the rate of decrease of vibratory acceleration, velocity, or displacement level after the excitation has stopped.

Decibel: dB---the term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. (See level, sound transmission loss.) Thus, one decibel corresponds to a power ratio of 100.1.

Noise is measured in decibels, which operate on a logarithmic scale—every 6-decibel increase represents a doubling of sound intensity. A 20-decibel increase indicates a 10-fold increase, and the difference between 50 and 130 decibels is a 10,000-time rise in sound pressure. 

 Examples and Effects of Common Sounds in Decibels 

 50-70db Normal conversation, quiet office

 70db Intrusive, interferes with telephone use

 80db Annoying, interferes with conversation. Constant exposure may cause damage

 85db Eight-hour exposure can cause hearing damage

 70-85db Vacuum cleaner, hair dryer, dishwasher, city traffic

 85-99db Lawnmower, subway, motorcycle, food blender

 90-100db No more than fifteen minutes unprotected exposure recommended

100db+ Regular exposure for more than one minute risks permanent hearing loss. 100-120 Snowmobile,  chainsaw, jackhammer, dance floor, boom box, ordinary rock concerts 

125db Threshold of pain. 120-145 Extra loud rock concerts, jet takeoff, shotgun blast.

 Increasing the sound intensity by a factor of:

10 raises its level by 10 dB

100 raises its level by 20 dB

1,000 raises its level by 30 dB

10,000 raises its level by 40 dB and so on


Diaphragm (also diaphragmatic): Any surface that vibrates in response to sound or is vibrated to emit sound, such as in microphones and loudspeakers. Also applied to wall and floor surfaces vibrating in response to sound or in transmitting sound. 

Diffraction: A change in the direction of propagation of sound energy in the neighborhood of a boundary discontinuity, such as the edge of a reflective or absorptive surface.

Diffuse field: An environment in which the sound pressure level is the same at all locations and the flow of sound energy is equally probable in all directions.

Directivity index (DI): The difference between sound pressure level in any given direction in the acoustic far field and the average sound pressure level in that field.

Distortion: Anything that alters the musical signal. There are many forms of distortion, some of which are more audible than others.

Divergence: The spreading of sound waves which, in a free field, causes sound pressure levels in the far field of a source to decrease with increasing distance from the source.

DSP: Digital Signal Processing. DSP can be used to create equalization, compression, etc. of a digital signal.

Dynamic headroom: The ability of an audio device to respond to musical peaks. For example, an amplifier may only be capable of a sustained 100 watts, but may be able to achieve peaks of 200 watts for the fraction of a second required for an intense, quick sound. In this example the dynamic headroom would equal 3 dB.

Dynamic range: The range between the loudest and the softest sounds that are in a piece of music, or that can be reproduced by a piece of audio equipment without distortion (a ratio expressed in decibels). In speech, the range rarely exceeds 40 dB; in music, is greatest in orchestral works, where the range may be as much as 75 dB.

Echo: A delayed return of sound that is perceived by the ear as a discrete sound image.

Echograms: A record of the very early reverberatory decay of sound in a room.

EES: Early, early sound. Structure-borne sound may reach the microphone in a room before the air-borne sound because sound travels faster through denser materials.

Equalization: The process of adjusting the frequency response of a device or system to achieve a flat or other desired response.

Equalizer: A device for adjusting the frequency response of a device or system.

Euphonic: Pleasing. As a descriptive audio term, usually refers to a coloration or inaccuracy that none-the-less may be sonically pleasing.

Extension: How extended a range of frequencies the device can reproduce accurately. Bass extension refers to how low a frequency tone will the system reproduce, high-frequency extension refers to how high in frequency will the system play.

Far field: That part of the sound field in which sound pressure decreases inversely with distance from the source. This corresponds to a reduction of approximately 6 dB in level for each doubling distance.

Feedback, acoustic: Unwanted interaction between the output and input of an acoustical system, e.g., between the loudspeaker and the microphone of a system.

Field sound transmission class, FSTC: Sound transmission class calculated in accordance with Classification E 413 using values of field transmission loss.

Field transmission loss, FTL: Sound transmission loss measured in accordance with Annex A1 of Test Method E 336.

Fidelity: As applied to sound quality, the faithfulness to the original.

Filter, band pass: A filter that passes all frequencies between a low-frequency cutoff point or a high-frequency cutoff point.

Filter, high pass: A filter that passes all frequencies above a cutoff frequency. 

Filter, low pass: A filter that passes all frequencies below a certain cutoff frequency. 

Flanking transmission: Transmission of sound from the source to a receiving location by a path other than that under consideration. 

Fletcher-Munson Curve: Our sensitivity to sound depends on its frequency and volume. Human ears are most sensitive to sounds in the midrange. At lower volume levels humans are less sensitive to sounds away from the midrange, bass and treble sounds "seem" reduced in intensity at lower listening levels.  

Flutter: A repetitive echo set up by parallel reflecting surfaces. 

Free field: An environment in which a sound wave may propagate in all directions without obstructions or reflections. Anechoic rooms can produce such an environment under controlled conditions. 

Frequency: The measure of the rapidity of alterations of a periodic signal, expressed in cycles per second or Hz. 

Frequency response: The changes in the sensitivity of a circuit ,device, or room with frequency. 

Fundamental: The lowest frequency of a note in a complex wave form or chord. 

Fusion zone: All reflections arriving at the observer's ear within 20 to 40 msec of the direct sound are integrated, or fused together, with a resulting apparent increase in level and a pleasant change of character. This is the Haas effect. 

Gain: To increase in level. The function of a volume control. 

Hard room: A room in which the surfaces have very low values of sound absorption and are therefore highly reflective. 

Hearing sensitivity: The human ear is less sensitive at low frequencies than in the midrange. Turn your volume knob down and notice how the bass seems to "disappear". To hear low bass requires an adequate SPL level. To hear 25Hz requires a much higher SPL level than to hear 250Hz. 

Hertz: the unit of frequency, abbreviated Hz. The same as cycles per second. 

Helmholtz resonator: A reactive, tuned, sound absorber. A bottle is such a resonator. They can employ a perforated cover or slats over a cavity. 

Imaging: Listening term. A good stereo system can provide a stereo image that has width, depth and height. The best imaging systems will define a nearly holographic re-creation of the original sound. 

Impact insulation class, IIC: A single-number rating derived from measured values of normalized impact sound pressure levels I accordance with Annex A1 of Test Method E 492. It provides an estimate of the impact sound insulating performance of a floor-ceiling assembly. 

Impulse: A very short, transient, electric or acoustic signal. 

Impulse response: Sound pressure versus time measurement showing how a device or room responds to an impulse. 

In phase: Two periodic waves reaching peaks and going through zero at the same instant are said to be "in phase." 

Initial time-delay gap: The time gap between the arrival of the direct sound and the first sound reflected from the surfaces of the room. 

Insertion loss, IL: Of a silencer or other sound-reducing element, in a specified frequency band, the decrease in sound power level, measured at the location of the receiver, when a sound insulator or a sound attenuator is inserted in the transmission path between the source and the receiver. 

Intensity: Acoustic intensity is sound energy flux per unit area. The average rate of sound energy transmitted through a unit area normal to the direction of sound transmission. 

Interference: The combining of two or more signals results in an interaction called interference. This may be constructive or destructive. Another use of the term is to refer to undesired signals. 

Inverse-square law: Under far field/free field conditions, sound intensity varies inversely with the square of the distance from the source. In pure spherical divergence of sound from a point source in free space, the sound pressure level decreases 6 dB for each doubling of the distance. 

ITD: Initial time-delay gap. 

Isolate: A dampening mechanism made a part of the assembly or system, which reduces structure borne vibrations from passing through the structure. 

Joist: Horizontal wood framing member set from wall to wall to support the boards of a floor or ceiling.

KHz: 1,000Hz.

Law of the first wave front: The first wave front falling on the ear determines the perceived direction of the sound. 

LEDE: Live end dead end. 

Level, L: Ten times the common logarithm of the ratio of a quantity proportional to power or energy to a reference quantity of the same kind. (See sound power level, sound pressure level.) the quantity so obtained is expressed in decibels. 

Level reduction, LR: In a specified frequency band, the decrease in sound pressure level, measured at the location of the receiver, when a barrier or other sound-reducing element is placed between the source and the receiver. 

Live end dead end: An acoustical treatment plan for rooms in which one end is highly absorbent and the other end reflective and diffusive. 

Loudness: A subjective term for the sensation of the magnitude of sound. The subjective response to a sound level. 

Mass law: An approximation that describes the Sound Transmission Loss (TL) of a limp, flexible barrier in terms of mass density and frequency. For each doubling of the weight or frequency of a partition, mass law predicts a 6 dB increase in TL. 

Mean free path: For sound waves in an enclosure, it is the average distance traveled between successive reflections.

Metric sabin: [L2]---the unit of measure of sound absorption in the metre-kilogram-second system of units.

Mode: A room resonance. Axial modes are associated with pairs of parallel walls. Tangential modes involve four room surfaces and oblique modes all six surfaces. Their effect is greatest at low frequencies and for small rooms.

Muddy: Listening term. A sound that is poorly defined, sloppy or vague. For example, a "muddy" bass is often boomy with all the notes tending to run together.

Near field: Locations close to the sound source between the source and the far field. The near field is typically characterized by large sound pressure level variations with small changes in measurement position from the source.

Noise: Interference of an electrical or acoustical nature. Random noise is a desirable signal used in acoustical measurements. Pink noise is random noise whose spectrum falls at 3 dB per octave: it is useful for use with sound analyzers with constant percentage bandwidths. Unwanted, bothersome or distracting sound.

Noise criteria: Standard spectrum curves by which a given measured noise may be described by a single NC number.

Noise isolation class, NIC: A single-number rating calculated in accordance with Classification E 413 using measured values of noise reduction. It provides an estimate of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.

Noise reduction (NR): The difference in sound pressure level between any two points along the path of sound propagation. As an example, noise reduction is the term used to describe the difference in sound pressure levels between the inside and outside of an enclosure.

Noise reduction coefficient (NRC): The arithmetic average, to the nearest multiple of .05, of the sound absorption coefficients in the 1/3 octave bands centered at 250Hz, 500Hz, 1000Hz, and 2000Hz..

Normal incidence sound absorption: *; [dimensionless]---of a surface, at a specified frequency, the fraction of the perpendicularly incident sound power absorbed or otherwise not reflected.

Normal mode: A room resonance. See mode.

Normalized noise isolation class, NNIC: A single-number rating calculated in accordance with Classification E 413 using measured values of normalized noise reduction. (See normalized noise reduction.)

Normalized noise reduction, NNR: Between two rooms, in a specified frequency band, the value that the noise reduction in a given field test would have if the reverberation time in the receiving room were .5 s.

Null: A low or minimum point on a graph. A minimum pressure region in a room.

Octave: An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. Each octave you add on the bottom requires that your speakers move four times as much air!

Octave bands: Frequency ranges in which the upper limit of each band is twice the lower limit. Octave bands are identified by their geometric mean frequency, or center frequency.

One-third octave bands: Frequency ranges where each octave is divided into one-third octaves with the upper frequency limit being 2* (1.26) times the lower frequency. Identified by the geometric mean frequency of each band.

Overtone: A component of a complex tone having a frequency higher than the fundamental.

Particle velocity, u: [LT-1]; m/s---a fluctuating velocity superimposed by the presence of sound on the other velocities that the particles of the medium may have. In analogy with alternating voltage its magnitude can be expressed in several ways, such as instantaneous particle velocity or peak particle velocity. In air, the other velocities are those due to thermal agitation and wind currents.

Passive absorber: A sound absorber that dissipates sound energy as heat.

Peak sound pressure level: LPK[nd] ----ten times the common logarithm of the square of the ratio of the largest absolute value of the instantaneous sound pressure in a stated frequency band during a specified time interval to the reference sound pressure of 20 micro pascals.

PFC: Phase-frequency curve.

Phase: Phase is the measure of progression of a periodic wave. Phase identifies the position at any instant which a periodic wave occupies in its cycle. It can also be discribed as the time relationship between two signals.

Phase shift: The time or angular difference between two signals.

Phon: The unit of loudness level of a tone.

Pink noise: Noise with a continuous frequency spectrum and with equal power per constant percentage bandwidth. For example, equal power is any one-third octave band.

Pitch: A subjective term for the perceived frequency of a tone.

Plenum: An absorbent-lined cavity through which conditioned air is routed to reduce noise.

Polarity: The positive or negative direction of an electrical, acoustical, or magnetic force. Two identical signals in opposite polarity are 180 degrees apart at all frequencies. Polarity is not frequency dependent.

Pressure zone: As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.

Psychoacoustics: The study of the interaction of the auditory system and acoustics.

Pure tone: A tone with no harmonics. All energy is concentrated at a single frequency.

Random noise: A noise signal, commonly used in measurements, which has constantly shifting amplitude, phase, and a uniform spectral distribution of energy.

Rarefaction: The portion of a sound wave in which molecules are spread apart, forming a region with lower-than-normal atmospheric pressure. The opposite of compression.

RASTI: Rapid Speech Transmission Index expressed in a decimal range of 0.2 for "bad" to 1.00 for "Excellent"

Ray: At higher audio frequencies, sound may be considered to travel in straight lines, in a direction normal to the wave front.

RC (Room Criteria) curves: Undesirable rumble can result if NC curves are determined mainly by low frequency noise. Similarly, a hissing effect can result from NC level being controlled by higher frequency sounds. To achieve a better balance between low frequency and high frequency components, RC curves have been established for which the objective is to design spectra that meet an RC curve within + 2 dB at all frequencies. A spectrum that exceeds an RC curve by more than 5 dB at frequencies below 250Hz is likely to result in unacceptable rumble. Above 2000Hz, on the other hand, a spectrum more than 5 dB higher than the RC curve might have too much of a hissing quality.

Reactance: The opposition to the flow of electricity posed by capacitors and inductors.

Reactive absorber: A sound absorber, such as the Helmholtz resonator which involves the effects of mass and compliance as well as resistance.

Receiving room: In architectural acoustical measurements, the room in which the sound transmitted from the source room is measured.

Reflection: For large surfaces compared to the wavelength of impinging sound, sound is reflected much as light is reflected, with the angle of incidence equaling the angle of reflection.

Reflection-phase grating: A diffuser of sound energy using the principle of the diffraction grating.

Refraction: The bending of sound waves traveling through layered media with different sound velocities.

Resistance: The quality of electrical or acoustical circuits that results in dissipation of energy through heat.

Resonance: A natural periodicity, or the reinforcement associated with this periodicity.

Resonant frequency: Any system has a resonance at some particular frequency. At that frequency, even a slight amount of energy can cause the system to vibrate. A stretched piano string, when plucked, will vibrate for a while at a certain fundamental frequency. Plucked again, it will again vibrate at that same frequency. This is its natural or resonant frequency. While this is the basis of musical instruments, it is undesirable in music-reproducing instruments like audio equipment.

Response: See frequency response.

Reverberant sound field: The sound in an enclosed or partially enclosed space that has been reflected repeatedly or continuously from the boundaries.

Reverberation: The persistence of sound in an enclosed or partially enclosed space after the source of sound has stopped; by extension, in some contexts, the sound that so persists.

Reverberation room: A room so designed that the reverberant sound field closely approximates a diffuse sound field, both in the steady state when the sound source is on, and during the decay after the source of sound has stopped.

Reverberation time: The tailing off of a sound in an enclosure because of multiple reflections from the boundaries.

RFZ: Reflection-free zone.

Room mode: The normal modes of vibration of an enclosed space. See mode.

RT60: Reverberation time.

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