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ACOUSTICAL TERMINOLOGY

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Acceleration of gravity
(see g)
Addition
(see Decibel addition)
Articulation index
A measure of the intelligibility of speech. The AI is determined by understanding correctly spoken words; the scale ranges from 0.0 (completely unintelligible) to 1.0 (fully intelligible). The articulation index can be used to estimate sentence intelligibility.
Air-borne sound
Sound that reaches the receiver (listener) by traveling mainly through the air.
Ambient sound
The all-encompassing sound associated with a given environment. This is usually a composite of sounds from many sources, near and far. No particular sound is dominant.
Amplitude
The magnitude of the pressure of a sound wave or a vibrating surface. Te amplitude is usually expressed in linear units.
American National Standards Institute (ANSI)
A standards-setting body that involves a large number of technical and professional societies. The Acoustical Society of America is responsible for the acoustical standards activity.
Acoustical Society of America (ASA)
A professional society covering all aspects of acoustics including noise, vibration, psychoacoustics, musical acoustics, ultrasonics, etc. The society is also responsible for preparation of acoustical standards.
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
A technical society for the heating, refrigerating and air-conditioning industry that published the ASHRAE Guide and Data Book and technical standards for the industry.
American Society for Testing and Materials (ASTM)
A professional society that prepares standards. One of the committees of the society is reponsible for the standards, recommendations, and procedures in the area of acoustics.
Attenuation (dB or dB/m[ft])
The reduction of sound with distance. It is often used to describe the change in sound level as one moves toward or away from a sound source, e.g., a 6 dB change (for a point source) with halving or doubling of a distance out-of-doors. A 3 dB change with halving or doubling of distance will occur with a line source, e.g., a highway. However, attenuation can also describe the reduction of sound per unit distance.
Audiologist
A health professional specializing in testing, evaluation, and protection of hearing.
Audiogram
A written record showing the results of a hearing test. The record contains the hearing level at each of the test frequencies. It might contain more than one years’ testing.
Audiometer
An instrument used to perform a hearing test. It permits testing at various frequencies at a wide range of hearing levels. Some audiometers are operated by the tester; some are operated by the one being tested; and some are controlled by an integrated circuit.
Average
The “energy” average of a series of discrete levels obtained by taking the sum of the anti-logs of one-tenth of the levels, dividing by the number of discrete levels and taking 10 times the logarithm of the result. This average is used for sound levels, sound pressure levels, and sound power levels.
A-weighting
A frequency network that reduces the importance of low frequencies in a manner similar to the human hearing mechanism. The network is standardized and is contained in sound level meters and other acoustical instruments.
A-weighted sound level
The sound level that is measured using the A-weighting network of a sound level meter or other acoustical instrument. It can also be computed from octave or 1/3-octave band sound pressure levels that have been adjusted for A-weighting.
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B

Background sound
Sound from all sources other than the particular sound that is of interest,i.e., the sound other than that being measured, the sound other than the speech being listened to, etc.
Bandwidth (hz)
The width, in hertz, of a filter used in acoustical measurements. The width can be an octave or larger, a one-third octave, a fixed percentage of the frequency, or a constant number of frequencies.
Base line audiogram
A valid audiogram obtained within 6 months of an employee’s first exposure to 90 DSA (or higher) time-weighted average noise level. This is the audiogram against which subsequent audiograms are compared to determine the Standard Threshold Shift. The base line audiogram may be replaced if a subsequent audiogram indicates better hearing levels.
Base line sound level (dB)
Measured sound level prior to the implementation of any noise mitigation project. The base line sound level is the ambient sound level plus the source sound emission.
Beat frequency
Two (or more) pure tones almost equal in frequency so that the sound level of the combined pure tones varies with time at a frequency that equals the difference in frequency between the two tones.
Breeching
The duct work used to convey the combustion products from the boiler to the chimney.
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C

Criterion (Various Units)
The physical or behavioral response which is to be achieved by mitigation in an acoustical design. The criterion may be expressed as a behavioral metric, such as annoyance or speech intelligibility. It may also be expressed as objective sound level.
Coincidence frequency (dip or notch)
The frequency at which a sound wave incident upon a surface has a projected wave length equal to the length of the bending wave excited in the surface material. The effect of this “trace matching” is a marked reduction in the sound transmission loss in the frequency region in which this occurs. The frequency is a function of the angle at which the sound impinges on the surface, the surface weight, and bending stiffness of the surface.
Composite Noise Rating
single value metric which includes the effect of the ambient sound level, the emitted sound level, and other factors related to intermittency, time of operation, and others.
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D

Damping
The internal property of a material that reduces the vibration in the material after the forced vibration has been removed. Materials with low damping ring for a long time (a bell), and materials with a high damping have a short vibration time.
Decibel addition
The total of a series of discrete levels obtained by taking the sum of the anti-logs of one-tenth of the levels and taking 10 times the logarithm of the result. This sum is used for sound levels, sound pressure levels, and sound power levels.
Decibel (dB)
The label used for levels in acoustics. It is 10 times the logarithm of the ratio of the acoustical properties that are being described. The properties must be proportional to power.
Directivity
The radiation of sound in a preferred direction
Displacement level (dB re 10′ m[in.])
The level of a vibration displacement on a logarithmic scale. The displacement and reference quantities can be either rms or peak values.
Displacement (vibration) (m[ft] or cm[in.])
The distance that a surface moves during vibration. Displacement may be in terms of the rms (effective) motion of the surface or the peak-to-peak motion.
Day-night sound level (Ldn, dB)
The equivalent A-weighted sound level calculated for a 24-hour period for the increased sensitivity of people to sound during nighttime hours (2200 hrs to 0700 hrs).
Doppler effect (shift)
The apparent change in frequency of a sound caused by the motion of the sound source toward or away from the receiver. The faster the sound source (or the observer) is moving, the greater the apparent change in frequency.
Double wall resonance (fo, Hz)
The resonance created when two surfaces on either side of an air cavity, which acts as a spring connecting the two surfaces, are excited by a sound. This resonance can be computed from the surface weights of the two surfaces and the depth of the air space. It is also a function of the angle at which the sound impinges on the surface.
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E

Ear muff
A hearing protector device that is placed over the ears to protect them. Earmuffs have a band that goes over or around the back of the head or under the chin.
Earplug
A hearing protector device that is placed in the ear canal to protect against noise. Earplugs can be premolded, custom-fitted, or made from an expandable foam.
Emissions (decibel)
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The sound emitted sound will depend on several factors such as the directivity of the source produced by a source which reaches impinges on a receiver. Sound Emissions add with the ambient sound level to produce a base line sound level.
Energy average
(see Average)
Equivalent sound level (Leq, dB)
The sound level that describes the energy contained in a fluctuating sound level over a certain period of time. Any time frame may be considered, e.g., one minute, one hour, or one day.
Excess attenuation (dB)
The various attenuations in a sound propagation path out-of-doors not due to increasing distance. Tese might be attenuation due to humidity, ground effects, wind, barriers, etc.
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F

Fast
(see Integration time)
Free field
A sound field in which there are no reflections. The best example is out-of-doors, away from all buildings.
Frequency (f, Hz)
The time rate of repetition of a period phenomenon, such as a sound wave. The unit of frequency is the Hertz (Hz), which corresponds to one cycle per second. The frequency is the reciprocal of the period.
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G

g
The acceleration of gravity. This is sometimes used as reference acceleration for vibration acceleration levels. By standard, this has been defined as 9.80667 m/sec2 386.087 in/sec2 = 32.1739 ft/sec2.
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H

Hanning Window
A type of time window that tapers time-history data to eliminate discontinuities at the beginning and end of the data record to be analyzed. Also called a cosine squared window to describe its shape in the time domain.
Hearing loss
The degradation of hearing acuity due to noise or other factors.
Hearing level (HL, dB)
The deviation of the hearing of a person from the normal threshold of hearing as measured in decibels. Positive values of hearing level indicate a hearing loss.
Hearing protector
A device that can be used to reduce the amount of sound that reaches the ear. Hearing protection is used to reduce hearing loss when the user is exposed to high noise levels.
Hearing threshold
(see Threshold of hearing)
Hertz (Hz)
The unit of frequency. It has been known as cycles per second.
HL
(see Hearing level)
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I

International Electro-technical Commission (IEC)
An international standards-making organization that establishes standards for acoustical instrumentation.
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J

K

L

Level (decibel, dB)
A unit used in acoustics in the measurement of physical quantities. It is 10 times the logarithm of the ratio of two variables, the denominator is usually the reference value of the sphyusical quantity, which are related to power in some manner. The most common terms are sound pressure level, sound level, and sound power level.
Loudness level (phons)
A logarithmic scale of loudness where a doubling or halving of loudness represents a 3 phon change in loudness level.
Loudness(sones)
A subjective attribute of sound. A doubling or halving of loudness represents a change of about 10 decibels. The loudness of a sound depends on its sound pressure level and its frequency content.
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M

Mass-air-mass resonance
(see Double wall resonance)
Mass-law sound transmission loss (dB)
The theoretical sound transmission loss of a homogeneous, limp panel. It depends only on the surface weight of the panel and does not take into account the resonances that result from stiffness, coincidence, or air cavities.
Maximum sound level (Lm, dB)
This is the maximum sound level in decibels that is measured during the measurement period. This should not be confused with Speak sound levels.
Mitigation
The process of reducing sound to meet a selected criterion, usually through the application of engineering or administrative controls.
Muffler
A device used to reduce the noise in piping or duct systems. It can also be a device at the end of a high-pressure pneumatic line. These are sometimes called silencers.
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N

Narrow-band analyzer
An analyzer whose bandwidth for analysis of sound is less than one-third octave in width. The bandwidth may be a percentage of the center frequency or a fixed number of frequencies in Hertz.
NCA curves
NC curves that have higher octave-band sound pressure levels in the lower frequencies. Tese curves have been superseded by the PNC curves.
NC curves
A set of curves used to rate sounds. THe curves give octave-band sound pressure levels for each of the ratings that are in 5 dB steps. The curves were developed using octave bands with frequency limits that are no longer used. THey have been superseded by the PNC curves.
Noise isolation class (NIC)
A rating curve for sound isolation analogous to the sound transmission class curve. The NIC curve is intended for use when the sound isolation is measured in the field rather than in the laboratory.
Noise
Unwanted sound. This term is loosely used for sound. A sound can be noise at times and not at others.
Noise level (dBA)
A term used interchangeably with sound level. The term implies the A-weighted sound level.
Normal incidence sound absorption (alpha n)
he sound absorption of a material when the sound wave is normally incident upon the material. Normal means that the direction of sound propagation is perpendicular to the material.
Noise reduction (NR, dB)
The arithmetic difference in decibels between the levels measured on either side of a noise control device. This could be a muffler in a duct system, a barrier, or a wall separating two spaces.
Noise reduction coefficient (NRC)
The arithmetic average of the sound absorption coefficients measured at 250, 500, 1000, and 2000 Hz in accordance with ASTM Designation E 423.
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O

Octave band (OB)
A band of frequencies whose upper frequency limit is two times the lower frequency limit. Tis is comparable to the octave on a musical instrument.
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P

Pascal (Pa, n/m2)
The unit 9of pressure. It is used as a reference pressure for sound pressure levels.
Peak level (L peak, dB)
The highest sound pressure measured during a time period. This is the instantaneous pressure and not the rms (effective) pressure that is used for sound level and sound pressure level. This is measured using a sound level meter capable of measuring impact or impulse sounds, which not every sound level meter is capable of doing.
Percentile
A distribution of numbers, the fraction (or percentage) of the distribution that exceeds the specified value.
Period (sec)
The length of time between repetitions of a periodic phenomenon. The period is the reciprocal of frequency. Periods in acoustics are usually in fractions of a second.
Peak-to-peak (m[in.])
A vibration term for the total displacement of a vibrating surface.
PNC curves
A series of curves for rating sounds. The curves in increments of 5 units give in octave-band sound pressure levels for each curve. Te curves have been published in the ASHRAE Guide and Data Book and are widely accepted. These curves supersede the NC curves.
Presbycusis (dB)
The gradual loss of hearing with age. Te loss seems to differ between men and women. It may be taken into account in some workers’ compensation claims. It may be taken into account in evaluation of a threshold shift in an audiometric program.
Pure tone (f, Hz)
A sound that contains only one frequency.
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Q

R

Reverberant sound field
A sound field in which sound waves are equally likely to arrive at the observation point from any direction. A very hard-surfaced room is an example of a reverberant room.
Reverberation time (sec)
The time required for the sound level to decay 60 dB from its original level after the sound source is stopped. Reverberation time is one of the criteria used for occupied spaces.
rms (root-mean-square)
The square root of the time-weighted average of the squared value of a time-varying phenomenon. It is the “effective” value of the physical quantity. The most commonly known example is ordinary electrical power, where the rms value of voltage is about 115 volts, whereas the actual voltage varies as a sine wave over one cycle from plus 160 volts to minus 160 volts. Sound pressure levels are measured using rms quantities.
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S

Single Event Noise Exposure Level (SEL or LAX)
The Sngle Event Noise Exposure Level is the A-weighted level that, if maintained for a constant period of 1 s, would convey the same sound energy as is actually received from a given noise event.
Sensorineural hearing loss
An irreversible hearing loss involving the hair cells and nerves of the inner ear.
Sentence intelligibility
A measure of the comprehension of sentences which ranges from 0% to 100%. It depends strongly on the content and context of the sentence material. The articulation index can be used to estimate the sentence intelligibility.
Speech interference level (SIL, dB)
A measure of the ability of a sound to interfere with speech. It is the arithmetic average of the 500, 1000, and 2000 Hz octave-band sound pressure levels.
Sound power (W, watts)
The power that is radiated by a sound source. It is an extremely small power for most normal acoustical sources.
Sound pressure (p, pascals)
The fluctuations in pressure above and below normal atmospheric pressure caused by a sound wave. For normal sounds, the variations in pressure are approximately one-millionth of an atmosphere.
Sound power level (Lw, dB re 10-12 watt)
The level in decibels of a sound power source. The reference power for the logarithmic scale for sound power level is 1 picowatt (10-12 watt).
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T

Threshold of hearing (dB)
The lowest sound pressure level that can be heard by a young person who has had no problems with their hearing. The threshold of hearing is a function of frequency; the sound pressure level is higher at low frequencies and about 0 dB around 1000 Hz.
Threshold shift
The change in hearing level between two consecutive audiograms. It is determined at each of the audiometric test frequencies; sometimes it is averaged over several frequencies.
Transmission loss
(see Sound transmission loss)
Transmissibility
A measure of the effectiveness of a vibration isolator or hanger. The smaller the transmissibility, the better the vibration isolation. It is the ratio of the vibration input to the top of the isolator to the output at the bottom of the isolator.
Triple wall resonance
The same as a double wall resonance (which see) except there are three surfaces and two air spaces, resulting in several resonances.
Time-weighted average (TWA, dB)
The average noise exposure of a person computed from the measured time-varying sound levels
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U

V

Velocity level (dB re 10-8 m/sec)
The level of a vibration velocity in decibels. The level is sometimes labeled vdB. Both the measured and the reference quantities must be either peak or rms values.
Vibration hanger
A device consisting of an elastomeric element, a metal spring, or a combination of both used to islate a vibrating machine or a ceiling from the overhead support structure.
Vibration isolator
A device consisting of an elastomeric element, a metal spring, or a combination of both used to islate a vibrating machine from the support structure.
Vibration
The periodic motion of a surface or a machine. t may be perceived as a sensation by the body, or it may set the air into motion and create a sound wave.
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W

Wavelength (lambda, m[ft])
For a sound consisting of a pure tone, the distance traveled by the sound wave over one cycle. The wavelength is the speed of sound divided by the frequency.

X

Y

Z