Noise Terms Glossary

Noise Terms Glossary – Memtech Acoustical

The ear is less efficient at low and high frequencies than at medium or speech-range frequencies. To describe sound in a manner representative of the human 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 called dBA. The A-weighted sound level is also called the noise level. Sound level meters have an A-weighting network for measuring A-weighted sound levels. Most measurements of occupational, industrial and environmental noise are taken using A-weighting.

ACOUSTICS
The science of sound. Its production, transmission and effects.

ACOUSTICAL
The properties of a material to absorb or reflect sound (adjective) acoustically, (adverb).

ACOUSTICAL ANALYSIS
A review of a space to determine the level of reverberation or reflected sound in the space (in seconds) as influenced by the building materials used to construct the space. Also, a study of the amount of acoustical absorption required to reduce reverberation and noise.

ACOUSTICAL CONSULTANT
A professional, usually with an engineering degree, whose primary roll is providing advice on acoustical requirements and noise control in a variety of situations.

ACOUSTICAL ENVIRONMENT
The acoustical characteristics of a space or room influenced by the amount of acoustical absorption, or lack of it, in the space.

ARCHITECTURAL ACOUSTICS
The control of noise in a building space to adequately support the communications function within the space and its effect on the occupants. The qualities of the building materials used determine its character with respect to distinct hearing.

ARTICULATION CLASS
A single number rating used for comparing acoustical ceilings and acoustical screens for speech privacy purposes. AC values increase with increasing privacy and range from approximately 100-250. This classification supersedes Speech Privacy Noise Isolation Class (NIC) rating method.

ARTICULATION INDEX (AI)
A measure of speech intelligibility influenced by acoustical environment rated from 0.01 to 1.00. The higher the number the higher the intelligibility of words and sentences understood from 0-100%.

ABSORPTION
The properties of a material composition to convert sound energy into heat thereby reducing the amount of energy that can be reflected.

AREA EFFECT
Acoustical materials spaced apart can have greater absorption than the same amount of material butted together. The increase in efficiency is due to absorption by soft exposed edges and also to diffraction of sound energy around panel perimeters.

ATTENUATION
The reduction of sound energy as a function of distance traveled.

A WEIGHTING
An electronic filtering system in a sound meter that allows meter to largely ignore lower frequency sounds in a similar fashion to the way our ears do.

AMBIENT NOISE/SOUND
Noise level in a space from all sources such as HVAC or extraneous sounds from outside the space. Masking sound or low-level background music can contribute to ambient level of sound or noise.

AUDIOGRAM
Graph of hearing threshold level as a function of frequency (ANSI S3.20-1995: audio gram).

AUDIOMETER
An instrument for measuring hearing acuity.

BAFFLE
A free hanging acoustical sound absorbing unit. Normally suspended vertically in a variety of patterns to introduce absorption into a space to reduce reverberation and noise levels.

BARRIER
A material that when placed around a source of noise inhibits the transmission of that noise beyond the barrier. Also, anything physical or an environment that interferes with communication or listening e.g., a poor acoustical environment con be a barrier to good listening and especially so for persons with a hearing impairment.

BASELINE AUDIOGRAM
The audio gram obtained from an audiometric examination administered before employment or within the first 30 days of employment that is preceded by a period of at least 12 hr of quiet. The baseline audio gram is the audio gram against which subsequent audio grams will be compared for the calculation of significant threshold shift.

BEL
A measurement of sound intensity named in honor of Alexander Graham Bell. First used to relate intensity to a level corresponding to hearing sensation.

BOOMINESS
Low frequency reflections. In small rooms, acoustical panels with air space behind can better help control low frequency reflectivity.

CLOUD
In acoustical industry terms, an acoustical panel suspended in a horizontal position from ceiling/roof structure. Similar to baffle but in a horizontal position.

COCKTAIL PARTY EFFECT
Sound in a noisy crowded room generated mostly by conversation. Levels rise and fall as people compete with one another to be heard. Perception of speech can be nearly impossible in high levels of noise.

COCHLEA
A snail shaped mechanism in the inner ear that contain hair cells of basilar membrane that vibrate to aid in frequency recognition.

CONTINUOUS NOISE
Noise with negligible small fluctuations of level within the period of observation (ANSI S3.20-1995: stationary noise; steady noise).

CREST FACTOR
Ten times the logarithm to the base ten of the square of the wide band peak amplitude of a signal to the time-mean-square amplitude over a stated time period. Unit dB (ANSI S3.20-1995: crest factor).

CYCLE
In acoustics, the cycle is the complete oscillation of pressure above arid below the atmospheric static pressure.

CYCLES PER SECOND
The number of oscillations that occur in the time frame of one second. (See Frequency). Low frequency sounds have fewer and longer oscillations.

DECIBEL (dB)
Unit of level when the base of the logarithm is the10th root of 10 and the quantities concerned are proportional to power (ANSI S1.1-1994:decibel).

DECIBEL, A-WEIGHTED (dBA)
Unit representing the sound level measured with the A-weighting network on a sound level meter. (Refer to Table 4-1 for the characteristics of the weighting networks).

DECIBEL, C-WEIGHTED (dBC)
Unit representing the sound level measured with the C-weighting network on a sound level meter. (Refer to Table 4-1 for the characteristics of the weighting networks).

DEAF
Loss of auditory sensation with or without use of assistive listening device. Loss of hearing more severe than is generally characterized as ‘hearing impaired’.

DERATE
To use a fraction of a hearing protectors noise reduction rating (NRR) to calculate the noise exposure of a worker wearing that hearing protector. (See NRR below).

DIFFUSION
The scattering or random reflection of a sound wave from a surface. The directions of reflected sound is changed so that listeners may have sensation of sound coming from oil directions at equal levels.

DOSE
The amount of actual exposure relative to the amount of allowable exposure, and for which 100% and above represents exposures that are hazardous. The noise dose is calculated according to the following formula:
D = {C1/T1 + C2/T2 +…+ Cn/Tn} H 100 where Cn = total time of exposure at a specified noise level Tn = exposure time at which noise for this level becomes hazardous

EAR
An incredible hearing mechanism consisting of outer, middle and inner ear segments that cause sound pressures to be picked up by the ear that are transmitted through auditory nerves where signals are interpreted by brain as sound.

ECHO
Reflected sound producing a distinct repetition of the original sound. Echo in mountains is distinct by reason of distance of travel after original signal has ceased.

ECHO FLUTTER
Short echoes in small reverberative spaces that produce a clicking, ringing or hissing sound after the original sound signal has ceased. Flutter echoes may be present in long narrow spaces with parallel walls.

EFFECTIVE NOISE LEVEL
The estimated A-weighted noise level at the ear when wearing hearing protectors. Effective noise level is computed by (1) subtracting derated NRRs from C-weighted noise exposure levels, or (2) subtracting derated NRRs minus 7 dB from A-weighted noise exposure levels. Unit, dB. (See Appendix).

EQUAL-ENERGY HYPOTHESIS
A hypothesis stating that equal amounts of sound energy will produce equal amounts of hearing impairment, regardless of how the sound energy is distributed in time.

EQUAL LOUDNESS CONTOURS
Curves represented in graph form as a function of sound level and frequency which listeners perceive as being equally loud. High frequency sounds above 2000 Hz are more annoying. Human hearing is less sensitive to low frequency sound. (See also Phon).

EQUIVALENT CONTINUOUS SOUND LEVEL
Ten times the logarithm to the base ten of the ratio of time-mean-square instantaneous A-weighted sound pressure, during a stated time interval T, to the square of the standard reference sound pressure. Unit, dB; respective abbreviations, TAV and TEQ; respective letter symbols, LAT and LAeqT (ANSI S1.1-1994: time-average sound level; time-interval equivalent continuous sound level; time-interval equivalent continuous A-weighted sound pressure level; equivalent continuous sound level).

EXCESS RISK
Percentage with material impairment of hearing in an occupational-noise-exposed population after subtracting the percentage who would normally incur such impairment from other causes in a population not exposed to occupational noise.

EXCHANGE RATE
An increment of decibels that requires the halving of exposure time, or a decrement of decibels that requires the doubling of exposure time. For example, a 3-dB exchange rate requires that noise exposure time be halved for each 3-dB increase in noise level; likewise, a 5-dB exchange rate requires that exposure time be halved for each 5-dB increase.

FENCE
The hearing threshold level above which a material impairment of hearing is considered to have occurred.

FREE FIELD
Sound waves from a source outdoors where there are no obstructions.

FREQUENCY
For a function periodic in time, the reciprocal of the period. Unit, hertz (HZ) (ANSI S1.1-1994: frequency).

FREQUENCY ANALYSIS
An analysis of sound to determine the character of the sound by determining the amount of sounds of various frequencies that make up the overall sound spectrum. i.e. Higher frequency sound or pitch vs. low frequency.

HEARING IMPAIRMENT
A degree of hearing loss, temporary or permanent, due to many causes. Hearing loss con be caused by illness, disease, or by exposure to excessively high noise levels. Affects 25 – 50 million people in USA of all ages. Hearing impairment as generally used means a hearing loss of mild, moderate, or severe degree as opposed to “deafness” which is generally described as little or no residual hearing with or without the aid of an assistive listening device. Hearing impaired persons are particularly adversely affected by long reverberation times.

HEARING RANGE
16 – 20000 Hz (Speech Intelligibility)
600 – 4800 Hz (Speech Privacy)
250 – 2500 Hz (Typical small table radio)

HEARING THRESHOLD LEVEL (HTL)
For a specified signal, amount in decibels by which the hearing threshold for a listener, for one or both ears, exceeds a specified reference equivalent threshold level. Unit, dB (ANSI S1.1-1994: hearing level; hearing threshold level).

HERTZ (Hz)
Frequency of sound expressed by cycles per second. (See Cycle).

IMMISSION LEVEL
A descriptor for noise exposure, in decibels, representing the total sound energy incident on the ear over a specified period of time (e.g., months, years).

IMPACT
Single collision of one mass in motion with a second mass that may be in motion or at rest (ANSI S1.1-1994: impact).

IMPULSE
Product of a force and the time during which the force is applied; more specifically, impulse is the time integral of force from an initial time to a final time, the force being time-dependent and equal to zero before the initial time and after the final time (ANSI S1.1-1994: impulse).

IMPULSIVE NOISE
Impulsive noise is characterized by a sharp rise and rapid decay in sound levels and is less than 1 sec in duration. For the purposes of this document, it refers to impact or impulse
noise.

INTENSITY
(See loudness).

INTERMITTENT NOISE
Noise levels that are interrupted by intervals of relatively low sound levels.

INVERSE SQUARE LAW
Sound levels full off with distance traveled. Sound level drops off 6 dB from source point for every doubling of distance.

LOUDNESS
A listener’s auditory impression of the strength of a sound. The average deviation above and below the static value due to sound wave is called sound pressure. The energy expended during the sound wave vibration is called intensity and is measured in intensity units. Loudness is the physical resonance to sound pressure and intensity.

MASKING
The process by which the threshold of hearing of one sound is raised due to life presence of another.

MOUNTING
Standards established by ASTM to represent typical installation for purpose of testing materials. i.e. A mounting test specimen mounted directly to test room surface. Or mounting furred out to produce air space behind.

NOISE
(1) Undesired sound. By extension, noise is any unwarranted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device.
(2) Erratic, intermittent, or statistically random oscillation (ANSI S1.1-1994: noise).

NOISE CRITERIA (NC)
Noise criteria curves used to evaluate existing listening conditions at ear level by measuring sound levels at loudest locations in a room. NC criteria can be referred to equivalent dBA levels. NC curves are critical to persons wills hearing loss.

NOISE ISOLATION CLASS (NIC)
A single number rating of the degree of speech privacy achieved through the use of an Acoustical Ceiling and sound absorbing screens in an open office. NIC has been replaced by the Articulation Class (AC) rating method.

NOISE REDUCTION (NR)
The amount of noise that is reduced through the introduction of sound absorbing materials. The level (in decibels) of sound reduced on a logarithmic basis.

NOISE REDUCTION COEFFICIENT (NRC)
The NRC of an acoustical material is the arithmetic average to the nearest multiple of 0.05 of its absorption coefficients at 4 one-third octave bands with center frequencies of 250, 500, 1000, 2000 Hertz.

NOISE REDUCTION RATING (NRR)
The NRR, which indicates a hearing protectors noise reduction capabilities, is a single-number rating that is required by law to be shown on the label of each hearing protector sold in the United States. Unit, dB.

OCTAVE
A pitch interval of 2:1. The tone whose frequency is twice that of the given tone.

OCTAVE BANDS
Sounds that contain energy over a wide range of frequencies are divided into sections called bands. A common standard division is in 10 octave bands identified by their censer frequencies 31.5, 63, 125, 250, 500, 1000, 2000, and 4000 Hz.

OTOLOGIST
A medical doctor specializing in the care and treatment of ear disorders.

OTOLARYNGOLIST
A medical doctor specializing in the diagnose and treatment of ear, nose and throat disorders.

PERMANENT THRESHOLD SHIFT (PTS)
Permanent increase in the threshold of audibility for an ear. Unit, dB (ANSI S3.20-1995: permanent threshold shift; permanent hearing loss; PTS).

PHON
Loudness contours. A subjective impression of equal loudness by listeners as a function of frequency and sound level (dB). An increase in low frequency sound will be perceived as being much louder than an equivalent high frequency increase.

PITCH
The perceived auditory sensation of sounds expressed in terms of high or low frequency stimulus of the sound.

PRESBYCUSIS
The loss of hearing due primarily to the aging process. High frequency loss is frequently a result of early hearing loss.

PULSE RANGE
Difference in decibels between the peak level of an impulsive signal and the root-mean-square level of a continuous noise.

REFLECTION
The amount of sound wave energy (sound) that is reflected off a surface. Hard non porous surfaces reflect more sound than soft porous surfaces. Some sound reflection can enhance quality of signal of speech and music.

RESONANCE
The emphasis of sound of a particular frequency.

REVERBERATION
Sound after it is ended at the source will continue to reflect of[ surfaces until the sound wave loses energy by absorption to eventually die out.

REVERBERATION TIME
The time taken for sound to decay 60 dB to 1 / 1,000,000 of its original sound level after the sound source has stopped. Sound after it has ended will continue to reflect off surfaces until the wave loses enough energy by absorption to eventually die out. Reverberation time is the basic acoustical property of a room, which depends only on its dimensions and the absorptive properties of its surfaces and contents. Reverberation has an important impact on speech intelligibility.

SABIN
A unit of sound absorption based on one square foot of material. Baffles are frequently described as providing X number of sabins of absorption based on the size of the panel tested, through the standard range of frequencies 125 – 4000 Hz. The number of sabins developed by other acoustical materials are determined by the amount of material used and its absorption coefficients.

SABINE FORMULA
A formula developed by Wallace Clement Sabine that allows designers to plan reverberation time in a room in advance of construction and occupancy. Defined and improved empirically, the Sabine Formula is T = 0.049(V/A) where T = Reverberation time or time required (for sound to decay 60 dB after source has stopped) in seconds. V = Volume of room in cubic feet. A = Total square footage of absorption in sabins.

SEPTUM
A thin layer of material between 2 layers of absorptive material. i.e. Foil, lead, steel, etc. that prevents sound wave from passing through absorptive material.

SIGNAL TO NOISE RATIO
The sound level at the listeners ear of a speaker above the background noise level. The inverse square low impacts on the S/N ratio. Signal to Noise Ratios are important in classrooms and should be in range of + 15 to +20 dB.

SIGNIFICANT THRESHOLD SHIFT
A shift in hearing threshold, outside the range of audiometric testing variability (5 dB), that warrants follow-up action to prevent further hearing loss. NIOSH defines significant threshold shift as an increase in the HTL of 15 dB or more at any frequency (500, 1000, 2000, 3000, 4000, or 6000 Hz) in either ear that is confirmed for the same ear and frequency by a second test within 30 days of the first test.

SOUND
(1) Oscillation in pressure, stress, particle displacement, particle velocity, etc. in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillations.
(2) Auditory sensation evoked by the oscillation described above (ANSI S1.1-1994: sound).

SOUND ABSORPTION
The property possessed by materials, objects and air to convert sound energy into heat. Sound waves reflected by a surface causes a loss of energy. The energy not reflected is called its absorption coefficient.

SOUND ABSORPTION COEFFICIENT
The fraction of energy striking a material or object that is not reflected. For instance if a material reflects 70% of the sound energy incident upon its surface, then its Sound Absorption Coefficient would be 0.30. SAC = absorption/area – sabins per sq. ft.

SOUND INTENSITY
Average rate of sound energy transmitted in a specified direction at a point through a unit area normal to this direction at the point considered. Unit, watt per square meter (W/m2); symbol, I (ANSI S1.1-1994: sound intensity; sound-energy flux density; sound-power density).

SOUND INTENSITY LEVEL
Ten times the logarithm to the base ten of the ratio of the intensity of a given sound in a stated direction to the reference sound intensity of 1 picoWatt per square meter (pW/m2). Unit, dB; symbol, L (ANSI S1.1-1994: sound intensity level).

SOUND LEVEL
A subjective measure of sound expressed in decibels as a comparison corresponding to familiar sounds experienced in a variety of situations.

SOUND PRESSURE
Root-mean-square instantaneous sound pressure at a point during a given time interval. Unit, Pascal (Pa) (ANSI S1.1-1994: sound pressure; effective sound pressure).

SOUND PRESSURE LEVEL
(1) Ten times the logarithm to the base ten of the ratio of the time-mean-square pressure of a sound, in a stated frequency band, to the square of the reference sound pressure in gases of 20 micropascals (µPa). Unit, dB; symbol, Lp.
(2) For sound in media other than gases, unless otherwise specified, reference sound pressure in 1 µPa (ANSI S1.1-1994: sound pressure level).

SOUND LEVEL METER
A device that converts sound pressure variations in air into corresponding electronic signals. The signals are filtered to exclude signals outside frequencies desired.

SOUND TRANSMISSION CLASS (STC)
The preferred single class rating system designed to give the sound insulation properties of a structure for the rank ordering of a series of structures

SPECTRUM
The description of a sound wave’s components of frequency and amplitude.

SPEECH
The act of speaking. A child learns to speak by imitating those people around him. It is important that a child can hear proper speech. ‘We speak what we hear.’

SPEECH INTELLIGIBILITY
The ability of a listener to hear and correctly interpret verbal messages. In a classroom with high ceilings and hard parallel surfaces such as glass and tile, speech intelligibility is a particular problem. Sound bounces off walls, ceilings and floor, distorting the teacher’s instructions and interfering with students’ ability to comprehend.

SPEECH PRIVACY
The degree to which speech is unintelligible between offices. Three ratings are used: Confidential, Normal (Non Obtrusive), Minimal.

TEMPORARY THRESHOLD SHIFT
Temporary increase in the threshold of audibility for an ear caused by exposure to high-intensity acoustic stimuli. Such a shift may be caused by other means such as use of aspirin or other drugs. Unit, dB. (ANSI S3.20-1995: temporary threshold shift; temporary hearing loss).

TIME WEIGHTED AVERAGE (TWA)
The averaging of different exposure levels during an exposure period. For noise, given an 85-dBA exposure limit and a 3-dB exchange rate, the TWA is calculated according to the following formula: TWA = 10.0 H Log(D/100) + 85 where D = dose.

TINNITUS
‘Ringing in the ears’ of which there is no observable cause.

THRESHOLD SHIFT
The deviation in decibels of a measured hearing level from one previously established.

VARYING NOISE
Noise, with or without audible tones, for which the level varies substantially during the period of observation (ANSI S3.20-1995: non-stationary noise; non-steady noise; time-varying noise).

VOLUME
The cubic space of a room bounded by walls, floors, and ceilings determined by Volume = length x Width x Height of space. Volume influences reverberation lime.

WAVELENGTH
Sound that passes through air produces a wavelike motion of compression and rarefaction. Wavelength is the distance between two identical positions in the cycle or wave. Similar to ripples or waves produced by dropping a stone in water. Length of sound wave varies with frequency. Low frequency equals longer wavelengths.