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GLOSSARY OF NOISE
TERMINOLOGY
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| Glossary of Noise Terms | Noise Facts | Abbreviations |
Glossary
of Noise Terms
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: audiogram).
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 audiogram 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 audiogram is the audiogram against which
subsequent audiograms 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 wideband 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 slopped) 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.
