Acoustic Glossary

To make your soundproofing experience more meaningful, the following glossary provides basic definitions of some of the industry’s most common terms.

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.

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.

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.

Attenuation: The reduction of sound intensity when sound waves pass through materials, like soundproofing foam or similar

Audio Frequency: The frequency of oscillation of an audible sound wave. Any frequency between 20 and 20,000 hertz.

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 frequencies 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.

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.

Calibrator (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.

Damping: The dissipation of energy with time or distance. The term is generally applied to attenuation of sound in a structure owing to the internal sound-dissipative properties of the structure or to the addition of sound-dissipative materials.

dBA: Unit of sound level. The weighted sound pressure level by the use of the A metering characteristics and weighted specified in ANSI Specifications for Sound Level Meters. dBA is used as a measure of human response to sound.

Decibel: A unit of sound pressure level, abbreviated dB.

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.

Echo: A wave that has been reflected or otherwise returned with sufficient magnitude and delay, so as to be detected as a wave distinct from that directly transmitted.

Equivalent A-Weighted Sound Level (Leq): The constant sound level that, in a given time period, would convey the same sound energy as the actual time-varying A-weighted sound level.

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.

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.

Frequency: The number of times per second that the sine wave of the sound repeats itself, or that the sine wave of vibrating object repeats itself. Now expressed in hertz (Hz), formerly in cycles per second (cps).

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.

Hertz (Hz): Unit of measurement of frequency, numerically equal to cycles per second.

Impact Insulation Class (IIC):An integer-number rating of how well a building floor attenuates impact sounds, such as footsteps. A larger number means more attenuation. The scale, like the decibel scale for sound, is logarithmic.

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.

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.

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.

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 sones 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 trials is judged by the listeners to be equally loud.

Masking Noise: A noise that is intense enough to render inaudible or unintelligible another sound that is also present.

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.

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): A measure of the acoustical absorption performance of a material, calculated by averaging its sound absorption coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest multiple of 0.05.

Octave Band: A segment of the frequency spectrum separated by an octave.

OSHA: The Occupational Safety and Health Administration.

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.

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.

Reflection: The return of a sound wave from a surface.

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 persistence of sound in an enclosed space, because of multiple reflections, after the sound source has stopped.

Reverberation Room: A room having a long reverberation time, especially designed to make the sound field inside it as diffuse (homogeneous) as possible.

ReverberationTime (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.

Sabin: A measure of the sound absorption of a surface; it is the equivalent of one square foot of a perfectly absorptive surface.

Sone: The unit of measurement for loudness. One sone is the loudness of a sound whose loudness level is 40 sones. Loudness is proportional to the sound’s loudness rating, e.g. two sones are twice as loud as one sone.

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.

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 Level: The weighted sound pressure level obtained using 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): 20 times the logarithm, to the base 10, of the ratio of the pressure of the sound measure to the reference pressure, which is 20 micronewtons per square meter. In equation form, sound pressure level in units of decibels is expressed as SPL (dB)=20 log p/p.

Sound Transmission Coefficient (STC): The average number of decibels of sound reflected away from a partition by a particular reflective barrier material.

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.

Speed (Velocity) of Sound in Air: 344 m/sec (128 ft/sec) at 70 degrees Fahrenheit in air at seal level.

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.

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.

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

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.

Wavelength: For a periodic wave (such as a sound in air), the distance between analogous points on any two successive waves. The wavelength of sound in air or in water is inversely proportional to the frequency of the sound. Thus, the lower the frequency, the longer the wavelength.

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.

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