Technical Library

Acoustic Glossary

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136 terms

A

A-weighted Sound Level: A measure of sound pressure level designed to reflect the acuity of the human ear, which does not respond equally to all frequencies. The ear is less efficient at low and high frequencies than at medium or speech-range frequencies. Therefore, to describe a sound containing a wide range of frenquencies in a manner representative of the ear's response, it is necessary to reduce the effects of the low and high frequencies with respect to the medium frequencies. The resultant sound level is said to be A-weighted, and the units are dBA. The A-weighted sound level.
Absorption: A property of materials that allows a reduction in the amount of sound energy reflected. The introduction of an absorbent material into the surfaces of a room will reduce the sound pressure level in that room by not reflecting all of the sound energy striking the room's surfaces. The effect of absorption merely reduces the resultant sound level in the room produced by energy that has already entered the room.
Absorption Coefficient: A measure of the sound-absorbing ability of a surface. It is defined as the fraction of incident sound energy absorbed or otherwise not reflected by a surface. Unless otherwise specified, a diffuse sound field is assumed. The values of the sound-absorption coefficient usually range from 0.01 for marble slate to almost 1.0 for long absorbing wedges often used in anechoic rooms.
Acoustic Sealant: Permanently flexible non-hardening caulk used at perimeters, seams, and penetrations. A 1% open area can cost you most of an assembly's rating, so sealing is not optional detail work.
Acoustic Trauma: Damage to the hearing mechanism caused by a sudden burst of intense noise, or by a blast. The term usually implies a single traumatic event.
Acoustics: 1. The science of sound, including the generation, transmission, and effects of sound waves, both audible and inaudible. 2. The physical qualities of a room or other enclosure (such as size, shape, amount of noise) that determine the audibility and perception of speech and music within the room.
Air Gap / Cavity: The space between the two leaves of an assembly. Deeper cavities improve low-frequency isolation; filling them with mineral wool damps cavity resonance. Sealed and insulated beats deep and empty.
Airborne Sound: Sound that reaches the point of interest by propagation through the air.
Ambient Noise: The total of all noise in the environment, other than the noise from the source of interest. The term is used interchangeably with the term background noise.
Anechoic Room: A room in which the boundaries absorb nearly all the incident sound, thereby, effectively creating free field conditions.
ANSI: The American National Standards Institute.
Articulation Index (AI): A numerically calculated measure of the intelligibility of transmitted or processed speech. It takes into account the limitations of the transmission path and the background noise. The articulation index can range in magnitude between 0 and 1.0. If the AI is less than 0.1, speech intelligibility is generally low. If it is above 0.6, speech intelligibility is generally high.
Attenuation: The reduction of sound intensity by various means (e.g., air, humidity, porous materials, etc).
Audio Frequency: The frequency of oscillation of an audible sound wave. Any frequency between 20 and 20,000 hertz.
Audiogram: A graph showing individual hearing acuity.
Audiometer: An instrument for measuring individual hearing activity.

B

Background Noise: The total of all noise in a system or situation, independent of the presence of the desired signal. In acoustical measurements, strictly speaking, the term \"background noise\" means electrical noise in the measurement system. However, in popular usage the term \"background noise\" is often used to mean the noise in the environment, other than the noise from the source of interest.
Band: Any segment of the frequency spectrum.
Band Pass Filter: A wave filter that has a single transmission band extending from a lower cutoff frequency greater than zero to a finite upper cutoff frequency spectrum.
Broadband Noise: Noise with components over a wide range of frequencies.

C

Calibratior (Acoustical): A device that produces a known sound pressure on the microphone of a sound level measurement system, and is used to adjust the system to Standard specifications.
Ceiling Attenuation Class (CAC): Airborne sound blocked by a suspended ceiling tile between two rooms sharing a plenum. CAC 35+ is good for offices; 40+ for confidential spaces.
Cochlea: A spirally coiled organ located within the inner ear that contains the receptor organs essential to hearing.
Cutoff Frequencies: The frequencies that mark the ends of a band, or the points at which the characteristic of a filter change from pass to no-pass.
Cycle: The complete sequence of values of a periodic quantity that occurs during one period.
Cycles Per Second: A measure of frequency numerically equivalent to hertz.
Cylindrical Wave: A wave in which the surfaces of the constant phase are coaxial cylinders. A line of closely spaced sound sources radiating into an open space produces a free sound field of cylindrical waves.

D

Damping (Constrained Layer): Sandwiching a viscoelastic layer between two rigid panels so panel resonance is converted to heat instead of sound. It is especially effective at the low frequencies mass alone struggles with.
Decibel (dB): Logarithmic unit of sound level. +10 dB ≈ twice as loud perceptually; +6 dB doubles amplitude; +3 dB doubles acoustic power. Not linear — a wall that removes 30 dB knocks intensity down by ~1,000×.
Decoupling: Breaking the rigid path between two sides of an assembly so vibration can't walk straight through. Achieved with sound isolation clips and hat channel, staggered/double studs, or floating floors. Usually the single biggest STC gain available.
Diffraction: A modification which sound waves undergo in passing by the edges of solid bodies.
Directivity Index: A measure of the angular direction of the sound radiation from the noise source presented as a dB level either higher or lower than that which would be radiated by a spherical sound source.
Doppler Effect (Doppler Shift): The apparent upward shift in frequency of a sound as a noise source approaches the listener or the apparent downward shift when the noise source recedes. The classic example is the change in pitch of a railroad whistle as the locomotive approaches and passes by.
Dosimeter: A device worn by a worker for determining the worker's accumulated noise exposure with regard to level and time according to a pre-determined integration formula.

F

Far Field: Describes a sound source region in the free space where the sound pressure level obeys the inverse-square law (the sound pressure level decreases 6 dB with each doubling of distance from the source). Also, in this region the sound particle velocity is in phase with the sound pressure. Closer to the source where these two conditions do not hold constitutes the 'near field\" region.
Filter: A device for separating components of a signal on the basis of their frequency. It allows components in one or more frequency bands to pass relatively unattenuated, and it attenuates components in other frequency bands.
Flanking Noise: Sound that bypasses your treated assembly and travels via another path — shared joists, ductwork, outlets, door undercuts, or the ceiling plenum. Untreated flanking caps the real-world performance of an otherwise excellent wall.
Free Sound Field (Free Field): A sound field in which the effects of obstacles or boundaries on the sound propagated in that field is negligible.

G

Green Glue / Damping Compound: A viscoelastic compound spread between two layers of drywall. As the panels flex, the layer shears and converts vibration into trace heat. Two tubes per 4x8 sheet is the standard coverage.

H

Hair Cell: Sensory cells in the cochlea that transforms the mechanical energy of sound into nerve impulses.
Harmonic: A sinusoidal (pure-tone) component whose frequency is a whole-number multiple of the fundamental frequency of the wave. If a component has a frequency twice that of the fundamental it is called the second harmonic, etc...
Hat Channel: A 25-gauge steel furring channel (hat-shaped in profile) screwed into sound isolation clips. Drywall attaches to the channel — never to the framing — so the finished surface floats. Typically spaced 24 in. on center.
Hearing: The subjective human response to sound.
Hearing Level: A measured threshold of hearing at a specified frequency, expressed in decibels relative to specified standard of normal hearing. The deviation in decibels of an individual's threshold from the zero reference of the audiometer.
Hearing Loss: A term denoting an impairment of auditory acuity. The amount of hearing impairment, in decibels, measured as a set of hearing threshold levels at specified frequencies. Types of hearing loss are 1. Conductive: A loss originating in the conductive mechanism of the ear; 2. Sensor-neural: 3 A loss originating in the cochlea or the fibers of the auditory nerve; 3. Noise induced: A sensor-neural loss attributed to the effects of noise.
Hearing Threshold Level (HTL): Amount (in decibels) by which an individual's threshold of audibility differs from a standard audiometric threshold. HERTZ (Hz): Unit of measurement of frequency, numerically equal to cycles per second.
Hertz (Hz): Cycles per second. Perceived as pitch. Human hearing spans roughly 20 Hz (deep sub) to 20,000 Hz (upper air). Doubling frequency raises pitch by one octave.

I

Impact Insulation Class (IIC): Single-number rating of impact sound (footsteps, dropped items) through a floor/ceiling. Higher is quieter below. Code minimum is IIC 50; luxury/multifamily targets IIC 60+.
Impact Sound: The sound produced by the collision of two solid objects. Typical sources are footsteps, dropped objects, etc. on an interior surface (wall, floor, ceiling) of a building.
Impulsive Noise: A) Either a single sound pressure peak (with either a rise time less than 200 milliseconds or total duration less than 200 milliseconds) or multiple sound pressure peaks (with either rise time less than 200 milliseconds or total duration less than 200 milliseconds) spaced at least by 200 millisecond pauses, B) A sharp sound pressure peak occurring in a short interval of time.
Infrasonic: Sounds of a frequency lower than 20 hertz.
Insulation Batt (Mineral Wool): Cavity fill that absorbs sound bouncing between the leaves of a wall or floor. Dense mineral wool such as Rockwool Safe'n'Sound outperforms fiberglass for soundproofing. Friction-fit it; never compress it.
Intensity: The sound energy flow through a unit area in a unit of time.
Inverse Square Law: A description of the acoustic wave behavior in which the mean-square pressure varies inversely with the square of the distance from the source. The behavior occurs in free field situations, where the sound pressure level decreases 6 dB with each doubling of distance from the source.
ISO: The International Organization for Standardization.

L

Level: The logarithm of the ratio of a quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be specified.
Logarithm: The exponent that indicates the power to which a number must be raised to produce a given number. For example, for the base 10 logarithm, used in acoustics, 2 is the logarithm of 100.
Loudness: The subjective judgment of intensity of a sound by humans. Loudness depends upon the sound pressure and frequency of the stimulus. Over much of the frequency range it takes about a threefold increase in sound pressure (a tenfold increase in acoustical energy, or, 10 dB) to produce a doubling of loudness.
Loudness Level: Measured in phons it is numerically equal to the median sound pressure level (dB) of a free progressive 1000 Hz wave presented to listeners facing the source, which in a number of trials is judged by the listeners to be equally loud.

M

Masking: 1. The process by which the threshold of audibility for a sound is raised by the presence of another (masking) sound. 2. The amount by which the threshold of audibility of a sound is raised by the presence of another (masking) sound.
Masking Noise: A noise that is intense enough to render inaudible or unintelligible another sound that is also present.
Mass Law: Doubling the mass of a single-leaf partition buys roughly 5–6 dB of transmission loss. Real gains beyond that come from decoupling, damping, and added air gap rather than piling on more mass.
Mass Loaded Vinyl (Mlv): A dense, limp vinyl sheet (1 lb or 2 lb per sq ft) that adds mass without thickness. Best used as a layer inside an assembly with seams taped and edges sealed — any gap leaks sound straight through.
Medium: A substance carrying a sound wave.

N

Near Field: The sound field very near to the source, where the sound pressure does not obey the inverse-square law and the particle velocity is not in phase with the sound pressure.
NIOSH: The National Institute for Occupational Safety and Health.
Noise: 1. Unwanted sound. 2. Any sound not occurring in the natural environment, such as sounds emanating from aircraft, highways, industrial, commercial and residential sources. 3. An erratic, intermittent, or statistically random oscillation.
Noise Isolation Class: (NIC) A single number rating derived in a prescribed manner from the measured values of noise reduction between two areas or rooms. It provides an evaluation of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.
Noise Level: For airborne sound, unless specified to the contrary, it is the A-weighted sound level.
Noise Reduction (NR): The numerical difference, in decibels, of the average sound pressure levels in two areas or rooms after treatment. A measurement of \"noise reduction\" combines the effect of the sound transmission loss performance of structures separating the two areas or rooms, plus the effect of acoustic absorption present in the receiving room.
Noise Reduction Coefficient (NRC): Average absorption at 250/500/1k/2k Hz on a 0.00–1.00 scale. 0 = perfect reflector, 1 = perfect absorber. NRC shapes reverb and clarity inside a room — it does NOT block sound leaving it.
Non-impulsive Noise: All noise not include in the definition of impulsive noise.

O

Octave: The interval between two sounds having a frequency ration of two. There are 8 octaves on the keyboard of a standard piano.
Octave Band: A segment of the frequency spectrum separated by an octave.
Octave Band Level: The integrated sound pressure level of only those sine-wave components in a specified octave band.
Oscillation: The variation with the time, alternately increasing and decreasing, of (a) some feature of an audible sound, such as the sound pressure, or (b) some feature of a vibrating solid object, such as the displacement of its surface.
OSHA: The Occupational Safety and Health Administration.

P

Peak Sound Pressure: The maximum absolute value of the instantaneous sound pressure in a specific time interval. Note: in case of a periodic wave, if the time interval considered is a complete period, the peak sound pressure becomes identical with the maximum sound pressure.
Period: The duration of time it takes for a periodic wave form (like a sine wave) to repeat itself.
Permanent Threshold Shift (PTS): A permanent decrease of the acuity of the ear at a specified frequency as compared to a previously established reference level. The amount of permanent threshold shift is customarily expressed in decibels.
Phon: The unit of measurement for loudness level.
Pink Noise: Noise with constant energy per octave band width.
Pitch: The attributes of auditory sensation that orders sounds on a scale extending from low to high. Pitch depends primarily upon the frequency of the sound stimulus, but it also depends upon the sound pressure and waveform of the stimulus.
Plane Wave: A wave whose wave fronts are parallel and perpendicular to the direction in which the wave is traveling.
Presbycusis: The decline in hearing acuity that is attributed to the aging process.
Pure Tone: A sound for which the sound pressure is a simple sinusoidal function of the time, and characterized by its singleness of pitch.

R

Random Noise: An oscillation whose instantaneous magnitude is not specified for any given instant of time. It can be described statistically by probability distribution functions giving the traction of the total time that the magnitude of the noise lies within a specified range.
Reflection: The return of a sound wave from a surface.
Refraction: The bending of a sound wave from its original path, either because it is passing from one medium to another or by changes in the physical properties of the medium, e.g. a temperature or wind gradient in the air.
Resilient Channel: A thin springy metal channel screwed directly to framing to add limited isolation. Cheaper than clips but easy to short-circuit with a stray screw. Clip-and-channel systems outperform it and are far more forgiving to install.
Resonance: The relatively large amplitude of vibration produced when the frequency of some source of sound or vibration \"matches\" the natural frequency of vibration of some object, component, or system.
Resonator: A device that resounds or vibrates in sympathy with a source of sound or vibration.
Reverberant Field: The region in a room where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates.
Reverberation: The tail of reflected sound persisting after a source stops. Measured as RT60 — the time for sound to decay by 60 dB. NRC treatment shortens RT60; it does not stop sound leaving the room.
Reverberation Room: A room having a long reverberation time, especially designed to make the sound field inside it as diffuse (homogeneous) as possible.
Reverberation Time (RT): The reverberation time of a room is the time taken for the sound pressure level to decrease 60 dB from its steady-state value when the source of sound energy, is suddenly interrupted. It is a measure of the persistence of an impulsive sound in a room as well as of the amount of acoustical absorption present inside the room. Rooms with long reverberation times are called live rooms.
RMS Sound Pressure: The square root of the time-averaged square of the sound pressure.
Root-mean Square (RMS): 1. The root-mean-square value of a time-varying quantity is obtained by squaring the function at each instant, obtaining the average of the squared values over the interval of interest, and then taking the square root of this average. For a sine wave, if you multiply the RMS value by the square root of 2, or about 1.41, you get the peak value of the wave. The RMS value, also called the effective value of the sound pressure, is the best measure of ordinary continuous sound, but the peak value is necessary for assessment of impulsive noises. 2. A term describing the mathematical process of determining an 'average' value of a complex signal.
Rt60 (Reverb Time): How long it takes sound to fall by 60 dB after the source stops. Home studios usually target 0.3–0.5 s; a bare room with hard surfaces can run past 1 s and sound washed out.

S

Sabin: A measure of the sound absorption of a surface; it is the equivalent of one square foot of a perfectly absorptive surface.
Shielding: The attenuation of a sound, achieved by placing barriers between a sound source and the receiver.
Sociocusis: Loss of hearing caused by noise exposures that are part of the social environment, exclusive of occupational-noise exposure, physiological changes with age, and disease.
Sone: The unit of measurement for loudness. One sone is the loudness of a sound whose loudness level is 40 phons. Loudness is proportional to the sound's loudness rating, e.g. two sones are twice as loud as one sone.
Sound: 1. An oscillation in pressure, stress, particle displacement, particle velocity, etc. in an elastic or partially elastic medium, or the superposition of such propagated alterations. 2. An auditory sensation evoked by the oscillation described above. Not all sound waves can evoke an auditory sensation; e.g. ultrasound.
Sound Isolation Clip: A rubber-isolated bracket (RSIC, Resilmount, etc.) fastened to studs or joists that carries hat channel. The rubber element absorbs vibration, decoupling drywall from structure. Clip count is driven by area and spacing, not just wall length.
Sound Level: The weighted sound pressure level obtained by the use of a sound level meter and frequency weighting network, such as A, B, or C as specified in ANSI specifications for sound level meters (ANSI SI.4-1971, or the latest revision). If the frequency weighting employed is not indicated, the A-weighted is implied.
Sound Level Meter: An instrument comprised of a microphone, amplifier, output meter, and frequency weighting networks which is used for the measurement of noise and sound levels.
Sound Power: The total sound energy radiated by a source per unit time. The unit of measurement is the watt.
Sound Pressure: The instantaneous difference between the actual pressure produced by a sound wave and the average or barometric pressure at a given point in space.
Sound Pressure Level (SPL): The actual pressure of sound in air, measured in dB relative to 20 µPa (threshold of hearing). SPL is what a meter reads at your ear. STC/IIC/NRC describe how assemblies change SPL between spaces.
Sound Transmission Class (STC): Single-number rating of airborne sound loss through a wall/ceiling/door/window across 125 Hz–4 kHz. Higher blocks more. STC 33 = normal speech audible; STC 50 = loud speech barely audible; STC 60+ = privacy for music/TV.
Sound Transmission Loss (Stl): A measure of sound insulation provided by a structural configuration. Expressed in decibels, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient of the configuration.
Spectrum: The description of a sound wave's resolution into its components of frequency and amplitude.
Speech-interference Level (Sil): A calculated quantity providing a guide to the interference of a noise with the reception of speech. The speech-interference level is the arithmetic average of the octave band levels of the interfering noise in the most important part of the speech frequency range. The levels in the octave bands centered at 500, 1000 and 2000 Hz are commonly averaged to determine the speech-interference level.
Speed (Velocity) Of Sound In Air: 344 m/sec (128 ft/sec) at 70 degrees Fahrenheit in air at seal level.
Spherical Divergence: The condition of propagation of spherical waves relates to the regular decrease in intensity of a spherical sound wave at progressively greater distances from the source. Under this condition the sound pressure level decreases 6 decibels with each doubling of distance from source.
Spherical Wave: A sound wave in which the surfaces of constant phase are concentric spheres. A small (point) source radiating into an open space produces a free sound field of spherical waves.
Steady-state Sounds: Sounds whose average characteristics remain relatively constant in time. A practical example of a steady-state sound source is an air conditioning unit.

T

Temporary Threshold Shift (TTS): A temporary threshold impairment of hearing acuity as indicated by a change in the threshold of audibility.
Third-octave Band: A frequency band whose cutoff frequencies have a ratio of 2 to the one-third power, which is approximately 1.26. The cutoff frequencies of 891 Hz and 1112 Hz define the 1000 Hz third-octave band in common use.
Threshold Of Audibility (Threshold Of Detectability): The minimum sound pressure level at which a person can hear a specified frequency of sound over a specified number of trials.
Threshold Of Pain: The minimum sound pressure level of a sound outside the ear that will produce a transition from discomfort to definite pain.
Threshold Shift: A change in the threshold audibility at a specified frequency from a threshold previously established. The amount of threshold shift is customarily expressed in decibels.
Timbre: An attribute of auditory sensation allowing a subject to judge that two sound similarly presented and having the same loudness and pitch are dissimilar, e.g. trumpet vs. violin.
Tinnitus: Ringing in the ears or noise sensed in the head. Onset may be due to an acoustic trauma and persist in the absence of acoustical stimulation (in which case it may indicate a lesion of the auditory system).
Transducer: A device capable of being actuated by waves from one or more transmission systems or media and supplying related waves to one or more other transmission systems or media. Examples are microphones, accelerometers, and loudspeakers.
Transmission Loss (TL): The dB drop across an assembly at a specific frequency. STC is a single number summarising the TL curve; a wall's low-frequency TL is usually much worse than its STC suggests.
Triple-Leaf Effect: Three rigid panels with two air cavities resonate higher than two panels with one deeper cavity, hurting low-frequency performance. Avoid adding drywall inside a decoupled cavity — keep two leaves and one gap.

U

Ultasonic: Sounds or frequency higher than 20,000 hertz.

V

Vibration: An oscillatory motion of solid bodies described by displacement, velocity, or acceleration with respect to a given reference point.
Vibration Isolator: A resilient support for vibrating equipment designed to reduce the amount of vibration transmitted to the other structures.

W

Wave: A disturbance that travels through a medium by virtue of the elastic properties of that medium.
Wavelength (λ): Physical length of one wave cycle. λ = c / f. A 40 Hz wave in air is ~28 ft; 1 kHz is ~1.1 ft; 10 kHz is ~1.3 in. Wavelength governs how sound interacts with rooms and absorbers.
Weighting: Prescribed frequency filtering provided in a sound level meter.
White Noise: Noise with energy is uniform over wide range of frequencies, being analogous in spectrum characteristics to white light.
Windscreen: A porous device used to cover the microphone of a sound level measurement system that is designed to minimize the effects of winds and wind gusts on the sound levels being measured. Typically made of open cell polyurethane foam and spherically shaped.

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