» Glossary of Acoustic Terms
Acoustics First® Glossary of Acoustic Terms
The opposite of reflection. Sound absorption results from the conversion of sound energy into another form, usually heat or motion, when passing through an acoustical medium. When a sound wave encounters resistance, absorption occurs. Absorption is measured in sabins (after Wallace Clement Sabine). One sabin is the amount of absorption offered by one square foot of open air.
Ratio of sound absorbing effectiveness, at a specific frequency, of a unit area of acoustical absorbent to a unit area of perfectly absorptive material. The portion of energy absorbed when a sound wave strikes a material. The absorption coefficient of a material is dependent on the frequency of the sound wave. An absorption coefficient of 1.0 = total absorption, 0.0 = total reflection. Note:
See Area Effect
for explanation of absorption coefficients exceeding 1.00.
Acoustical tile -
A porous architectural material, usually constructed from fiberglass or pressed board, which is most absorptive at the high frequencies.
The sound characteristics of a room. The science of the production, control, transmission, reception and effects of sound and the phenomenon of hearing.
The residual “room sound” of a listening environment.
Ambient noise -
All pervasive noise associated with a given environment.
Analog representations of sound replicate its waveform, while transferring it through different media. All sound is analog. Audio may be analog or digital.
Literally “without echo”.
Due to exposed edges and diffraction of sound energy around perimeters, acoustical materials spaced apart can exhibit greater absorption than same amount of material with no gaps. The
surface of an anechoic wedge has a total surface area greater than the flat surface it replaces.
A measure of the intelligibility of speech.
ASTM E 84 -
A test method for determining the surface burning characteristics of building materials, sometimes referred to as the “Steiner tunnel test”. This test method is for single products and evaluates both flame spread and smoke development, assigning different classes based upon test results:
To reduce the level (volume, loudness, energy) of an acoustical (or electrical) signal.
A frequency that falls within the range of the human hearing, usually taken as 20 Hz to 20 kHz.
A-Weighted Sound Level
A measure of sound pressure level designed to reflect the response of the human ear, which does not respond equally to all frequencies, by reducing the effects of the low and high frequencies with respect to the mid-range frequencies. The resultant sound level is said to be A-weighted, and the units are dBA.
Axial mode -
The room resonances associated with each pair of parallel walls (including ceiling and floor).
Background noise -
The ambient noise level above which
signals must be presented or noise sources
A device used to inhibit the
propagation of sound waves. Baffles are usually
suspended vertically from ceiling to reduce
Heavy, dense and massive material used
to block sound.
Bass Trap -
A low frequency absorber. Low frequencies
are particularly difficult to absorb due to
their long wavelengths. Bass traps are designed
and constructed to absorb these longer waves and
control unwanted room resonances. Broadband
absorbers extending to lower frequencies are
often called Bass Traps, imprecisely. The term
“bass trap” is counterintuitive since these
devices eliminate low frequency room
cancellations, allowing bass to be heard.
To reduce airborne sound transmission.
Boomy / Boominess -
Listening term that usually refers to
an excessive amount of low frequency (bass)
physical gap in the assembly or construction,
which acts to decouple sound vibrations from
traveling through a structure.
Listening term that usually refers to excessive
upper frequency (treble) energy.
The destructive interference of two or
more sound waves. Waves of similar frequencies
and amplitude, but of opposite phase (180°F) produce mutual cancellation effects.
Class (CAC) -
Rates a ceiling's
efficiency as a barrier to airborne sound
transmission between adjacent closed offices.
Shown as a minimum value, previously expressed
as CSTC (Ceiling Sound Transmission Class). A
single-figure rating derived from the normalized
ceiling attenuation values.
Ceiling Cloud -
An acoustical panel suspended in a
horizontal position from ceiling or roof
Comb filter -
A distortion produced by combining an
acoustical (or electrical) signal with a delayed
replica of itself (offset in time). The result
is constructive and destructive interference
that results in peaks and nulls being introduced
into the frequency response. This response, when
plotted to a linear frequency scale, resembles a
comb (teeth) rather than a smooth curve.
The ability of a listener to
focus attention on a single talker among a
mixture of crowd conversations and background
noise while, at the same time, ignoring speech
from other locations. Understanding is possible
due to the ear / brain discrimination of
unwanted sound as well as lip reading and body
language. A single microphone recording of the
same conversation, absent these additional
factors, may be totally unintelligible and
unusable as evidence in court.
A term used to indicate audible
alterations to sound due to its environment.
Coloration can be a result of standing waves or
Interference - The addition of two
waveforms of similar phase. Constructive
interference is responsible for the production
of standing waves in which a signal and its
successive reflections are continually added to
one another. The opposite is destructive
Critical distance -
The distance from a sound source at
which the direct energy (energy radiated
directly from the source) is equal to the
reverberant energy (radiated from walls, floor
Cutoff frequency -
The lowest frequency above which the
normal incidence sound absorption coefficient is
at least .990 for an anechoic wedge, or set of
complete positive (forward) and negative
(backward) movement of a vibration corresponding
to a high and low pressure wave.
Cycles per second -
The frequency of an electrical signal
or sound wave, measured in Hertz (Hz).
mounting device, shaped like a D, used in
conjunction with hanger wire to mount baffles.
loss of energy in a vibration system, usually
acoustical condition in which reverberation is
absent, as in a room whose surfaces are covered
with highly absorptive materials.
The length of time taken for a signal to drop in
strength to a specific portion of its initial
value. Decay time is often frequency
dependent. The decay time of a room at a
specific frequency is the time necessary for a
sound of that frequency to decay 60 dB. (RT60)
Decibel (dB) -
The measuring unit of sound pressure, and hence
loudness. The decibel is a numerical ratio
between the sound pressure of a given sound and
the sound pressure of a reference sound (usually
.0002 microbar). Common decibel levels
encountered vary from the rustling of grass (15
dB) to conversation (50 dB), to live rock groups
(110 dB) to jet plane engines at close range
(130 dB). See
The bending of a sound wave around an obstacle,
or through an opening, such as slats. The
scattering of sound waves at an object smaller
than one wavelength, and the subsequent
interference of the scattered wavefronts.
A sound field in
which the sound pressure level is the same
everywhere and the flow of energy is equally
probable in all directions.
Diffuse sound -
Sound that is completely random in phase. Sound
which appears to have no single source/
A device for the
complex scattering of sound energy in all
directions. Traditional spatial diffusers, such
as the polycylindrical (barrel) shapes may also
double as low frequency traps. Temporal
diffusors, such as binary arrays and quadratics,
scatter sound in a manner similar to diffraction
of light, where the timing of reflections from
an uneven surface of varying depths causes
interference which spreads the sound.
scattering or random distribution of a sound
wave after striking a surface.
Sound waves arriving at the listening location
directly from the source. Differing from
reflected sound, which arrives at the listening
location after bouncing off the surrounding
- The apparent shift in frequency when the sound
source, or the observer, is in motion.
- A dense architectural wall construction
material applied to wood or metal studs, A/K/A
Sheet Rock or Gypsum Wall Board (GWB), useful
primarily as a sound barrier but is low
frequency absorber in some applications.
E400 Test -
Early reflection -
Reflected energy that occurs in close proximity
to the source but is slightly out of
synchronization (time / phase) with the source
Eased (edge) -
An acoustical panel edge detail also known as a
quarter-inch (¼") bevel.
distinctly discernible reflection, or repetition
of a source signal. Note: The term is often used
incorrectly to refer to reverberation which
consists of densely spaced, indistinguishable
A set of curves of
equivalent loudness, which model the ear’s
frequency response throughout the audible
spectrum. The curves, obtained from actual
testing, show how much more sound power is
required at one frequency than another to obtain
a sound of equal loudness. The results show that
the human ear is less sensitive to sound at the
extreme high and low frequencies.
The adjustment of timbre, or tone quality,
achieved by changing the amplitude of a signal
at different frequencies. (Abbreviated: EQ.)
Tone controls are simple forms of equalization.
Far field -
Distribution of acoustic energy at a very much
greater distance from a source than the linear
dimensions of the source itself.
Flame spread -
A measure of the time it takes for fire to
spread, when compared to red oak, whose Flame
Spread Index (FSI) is 100 in accordance with
ASTM E 84.
transmission of sound around a perimeter or
through holes within partitions (or barriers)
that reduces the otherwise obtainable sound
transmission loss of a partition. Examples of
flanking paths within buildings are ceiling
plena above partitions; ductwork, piping, and
electrical conduit penetrations through
partitions; back-to-back electrical boxes within
partitions, window mullions, etc. Flanking
occurs when a free standing partition size is
less than the wavelength of sound to be blocked.
used to describe an even frequency response in
which no frequency is accentuated.
The equal loudness
contours plotted by the researchers Fletcher and
Munson. Human ears are most sensitive to sound
between 1,000 Hz and 4,000 Hz. Above and below
those approximate frequencies, a tone must be
several dB greater in order to be perceived as
equally loud as a tone in the 1,000 Hz to 4,000
Hz range. See
repetitive echo set up by parallel reflecting
Free field -
An environment in which there are no reflective
surfaces within the frequency region of
The speed of vibration of a sound wave, measured
in cycles per second, or Hertz. Frequency
determines pitch; the faster the frequency, the
higher the pitch.
The placement of
vocal or musical information ahead of (closer
to) or behind (farther from) center position,
The basic pitch of a musical note.
freestanding device used to inhibit the
propagation of sound waves. Gobos are usually
employed to prevent microphone leakage between
two instruments being recorded simultaneously.
Typically a set of portable dividers covered
with acoustical treatments.
The principle now used to achieve diffraction of
acoustical waves, analogous to optical grating
by which light is broken into its component
colors as when passing through a prism.
reflection phase -
diffraction grating to produce diffusion of
Grazing Effect -
The way in which sound is absorbed by the
audience; stepping or raking the seating reduces
the absorption, and improves sight lines.
Haas effect -
Also called the precedence effect. Delayed
sounds are integrated if they fall on the ear
within 20 to 40 msec of the direct sound. The
level of the delayed components contributes to
the apparent level of the sound.
A reactive, tuned,
sound absorber. A bottle is such a resonator.
It can employ a perforated cover or slats over a
cavity. An acoustic guitar or violin body is a
The measuring unit of frequency or the speed of
vibration of a sound wave. Synonymous with
“cycles per second” (CPS).
Sound dislocated from its correct position, to
be more left and / or right of center.
Class (IIC) -
A system for
rating the ability of a structure to isolate
impact noise (i.e. footsteps, and other
vibrational disturbances). Normally used in
reference to floor and ceiling constructions,
the IIC method utilizes whole positive numbers
for rating purposes.
Impact noise -
The noise heard as a result of vibrations
transferred through the structure of a room.
Foot thumps are impact noise.
(also: Impaler) -
fastener that is a metal plate with tines
(prongs or spikes) sticking out of it. It is
used along with adhesive to mount acoustical
panels. The impaling clip is attached to the
wall with the tines facing outward and the panel
is pushed onto it (impaled).
short, transient, acoustical (or electrical)
Two periodic waves reaching peaks and going
through zero at the same instant are said to be
gap (signal delay) -
The time interval
between the arrival of a direct sound and its
first reflection from the surfaces of the room.
amount of sound energy radiated per unit area,
measured in watts per square centimeter.
Inverse square law-
Any condition in which the magnitude of a
physical quantity follows an inverse
relationship to the square of the distance. In
pure spherical divergence of sound from a point
source in free space, the sound pressure level
decreases 6 dB for each doubling of the
Resistance to the transmission of sound by
materials and structures. The separation of
airborne or mechanically transmitted energy.
A slice cut into the edge of an acoustical panel
so that it can accept a spline (extrusion) to
connect and mount an adjoining panel, providing
a level surface at the joint or seam.
Kilohertz / kHz -
1,000 Hz increments. See
Late reflection -
Reflected energy that occurs a greater distance
away from the source than an early reflection.
Sometimes referred to as “slap-back” or echo.
Law of the first
wavefront falling on the ear determines the
perceived direction of the sound.
unwanted sound picked up by (or “leaking” into)
a microphone from another instrument or
loudspeaker. Sound from one room that is heard
Live end, dead
end. An optimal acoustical treatment plan for
rooms in which one end is highly absorbent and
the other end reflective and diffusive.
reverberant acoustical condition, usually used
in reference to a room whose many reflective
surfaces encourage a lengthy reverberation time.
A wave in which
vibrations are in the direction of propagation
of sound as are sound waves in air.
Subjective impression of the intensity of a
The process by which one sound is used to
obscure the presence of another.
Mass law -
The law of physics that states that a material’s
ability to reduce the transmission of sound is
proportional to its weight. According to the
mass law, to increase a wall’s transmission loss
by 6 dB it is necessary to double the thickness
(weight) of the wall. See
Inverse Square Law.
Mass Loaded Vinyl -
A high density vinyl material that acts
as a barrier to sound transmission. Directly
applied to surfaces, suspended or used in wall,
floor and ceiling construction. See
- To rigidly connect two isolated
objects. (Also called a mechanical “short”.)
Example: Two isolated wall partitions are
mechanically coupled if rigid electrical conduit
is fastened to both walls. Air ducts and
plumbing are prime candidates for causing
mechanical shorts. When an acoustically
isolated room “leaks”, it is often a mechanical
short created during construction.
- The elimination of mechanical
Mechanical Coupling. Typically
accomplished by inserting a flexible loop
between rigid components. Flexible metallic
conduit (Greenfield) is used for decoupling
electrical conduit, accordion shaped canvas
collars decouple rigid air-ducts and flexible
tubing does the same for plumbing. For
structural decoupling, resilient materials are
used to separate rigid components.
Mineral Board -
A raw material, made of inorganic mineral
fibers, used for acoustical panels.
Mineral Wool -
A non-rigid acoustical substrate (A/K/A
“rock wool” or “slag wool”) made from molten
Mode) - A room resonance. Axial
modes are associated with pairs of parallel
walls. Tangential modes involve four room
surfaces and oblique modes all six surfaces.
Their effect is greatest at low frequencies and
in small rooms.
Common abbreviation for “monaural”, meaning from
a single source.
Standards established by ASTM to test
the acoustics of materials by representing a
typical installation. Specimens may be attached
directly to the test room surface as in Type “A”
Mount, spaced apart, or furred-out to produce an
air space behind. (Ceiling grid test is E400
Near field -
That part of a sound field usually within
about two wavelengths from a noise source, where
there is no simple relationship between sound
level and distance. The area in a room which is
in the immediate vicinity of the sound source.
Node (Dead Spot) -
A point or line where minimal air motion
Unwanted sound. Interference of an electrical
or acoustical nature. Random noise is a
desirable signal used in acoustical
measurements. Pink noise is random noise whose
spectrum falls at 3 dB per octave. It is useful
for use with sound analyzers with constant
Noise Criteria (NC) -
Standard spectrum curves by which a
given measured room’s ambient noise may be
described by a single NC number.
Noise isolation class,
NIC - A single-number rating
calculated in accordance with Classification E
413 using measured values of noise reduction. It
provides an estimate of the sound isolation
between two enclosed spaces that are
acoustically connected by one or more paths.
Coefficient (NRC) -
average of the Sound Absorption Coefficients of
a material at 250 Hz, 500 Hz, 1000 Hz and 2000
Hz. This is the range having the most impact
upon speech intelligibility.
Notch filter -
A filter of extremely narrow bandwidth
used to eliminate discrete frequencies. Notch
filters are usually tunable, and can be used to
eliminate specific room or instrument
or minimum point on a graph. A minimum pressure
region in a room.
Oblique mode -
musical spacing between a frequency and its
double. For example, the distance between “A”
(440Hz) and “high A” (880Hz) is an octave. The
audible range is about ten and one-half octaves.
Octave band -
A frequency spectrum which is one octave
wide (i.e. all frequencies from 125 Hz to 250
Hz). In recording and audio testing, the octave
itself is divided into thirds for increased
Out of phase -
Two related signals offset in time.
See In Phase.
Passive absorber -
A sound absorber that dissipates sound
energy as heat.
Path length difference
- The difference in time/distance of
source energy from reflected energy.
length of time (measured in seconds) it takes
for a wave to complete a cycle. t = 1/f
time relationship between two signals.
Phase interference -
The addition and/or subtraction of
two waves of similar or multiple frequencies,
causing peaks and dips in the overall response
Phase shift -
The time or angular difference between
empirical unit of loudness. Since the ear has
different sensitivities at various frequencies
(Fletcher-Munson), it does not hear equivalent
sound pressure levels as being equally loud.
Pink noise -
Broadband noise whose energy content is
inversely proportional to frequency.(-3dB per
octave) This gives the noise equal energy per
human perception of frequency. In general, the
higher the frequency, the higher the pitch.
Privacy Index (PI)-
A measure for rating the speech
privacy performance of an architectural space
(or lack of speech intelligibility), where the
PI is calculated from the Articulation Index
(AI). A privacy level of PI above 95% represents
confidential speech privacy whereas a PI of less
than 80% is poor privacy.
The positive (forward) or negative (backward)
direction of an acoustical, electrical, or
magnetic force. Two identical signals in
opposite polarity are 180°F apart at all
frequencies. Polarity, unlike phase, is not
Polar Plot -
The graphic representation of diffusion
or scattering, over all incident angles at a
Pressure zone -
As sound waves strike a solid surface,
the particle velocity is zero at the surface and
the pressure is high, thus creating a
high-pressure layer near the surface.
The study of the perception of sound.
Random noise -
Noise whose instantaneous amplitude is
not specified at any instant of time.
A decrease in density and pressure in a
medium, such as air, caused by the passage of a
sound wave. Opposite of compression.
Reactive absorber -
A sound absorber, such as the Helmholtz
resonator which involves the effects of mass and
compliance as well as resistance.
Reactive silencer -
A silencer in air-conditioning systems
that uses reflection effects for its action.
Reflected sound -
Sound arriving at the listening location
after bouncing off one or more of the
surrounding surfaces. The sum total of all
reflected waves determine the room’s
reverberation time and acoustical character.
The bouncing of a sound wave off of a
surface. Sound is reflected much as light is
reflected, with the angle of incidence equaling
the angle of reflection.
Grating - See
The bending of sound waves traveling
through layered media with different sound
Free from rigid contact, such as a
spring-mounted floating floor. Resiliency
reduces the transfer of noise and vibration from
one structure to another.
The sympathetic vibration of an object (or air
column) at a specific frequency when it is
excited into motion by a sound wave of similar
frequency in the immediate vicinity.
Resonant frequency dip
- The degradation of transmission
loss of a barrier at a specific frequency due to
inner resonance. The exact frequency at which
this phenomenon occurs is a function of the mass
and stiffness of the barrier. Laminated “safety”
glass has lower resonance, passing less sound
than plain glass.
exposed edge at the side of ceiling tile that is
at right angles to the general face of the
ceiling, visible (revealed) below the supporting
The persistence of sound in an enclosure
after a sound source has been stopped. This is a
result of the multiple reflections of sound
waves throughout the room arriving at the ear so
closely spaced that they are indistinguishable
from one another and are heard as a gradual
decay of sound. The area in which this occurs is
the Reverberation Field.
Reverberation Room -
A test chamber so designed that the
reverberant sound field within the room has an
intensity that is approximately the same in all
directions and at every point. It is commonly
used to measure sound absorption, ASTM C-423 and
transmission loss, ASTM E-90.
The time, in seconds, required for
sound pressure at a specific frequency to decay
60 dB after the source is stopped. 60 dB of
decay is equal to one millionth of their
original level. The reverberation time of a room
varies with frequency and is a function of the
room volume as well as the total number of
absorption units in the room. It can be
determined by the Sabine equation:
Note: If the dimensions are in meters, change
the constant from 0.049 to 0.161.
Other computations are available using the
Eyring or Fitzroy equations.
Reverberant field -
The area in a room in which the multitude
of decaying reflections has created a
reverberant and diffuse condition.
Reverberation time and
Room Modes -
A measure of sound absorption of a
surface. One sabin is equal to 1 square foot of
open window. Sabins are calculated by
multiplying the absorption coefficient of a
material multiplied by its area.
A sheer, loosely woven fabric used as
“backing” for acoustical panels.
A thin layer of material between two
layers of absorptive material, such as foil,
vinyl, lead, gypsum, steel, etc., that prevents
sound wave from passing through absorptive
ratio - The difference between
nominal or maximum operating level and the noise
floor expressed in dB.
Shifting center -
An apparent shift of the
position of an instrument or voice in the stereo
image due to a discrepancy in the phase
relationships of the signals from either side.
See Image Shift.
Slap back -
A discrete reflection from a nearby
(Rating or Index, SDI) - The
ratio of smoke emitted by a burning material to
the smoke emitted by the red oak standard (ASTM
Having or producing a full, deep, or
Energy that is transmitted by
pressure waves in air (as well as water or
solids) and is the objective cause of the
sensation of hearing. The phenomenon caused by
the vibration of the eardrum. The drum itself
is set into motion by pressure waves traveling
through the air, originating at the sound
Sound Board -
Generic term for composition
material available at building supply stores to
dampen impact noise in floors and provide
de-coupling in walls. Installing with nails or
screws reduces its effectiveness.
Sound isolation -
The degree of acoustical
separation between two locations, especially
Sound level -
The intensity of sound
measured with a sound level meter and one of its
Sound level meter
- A pressure-sensitive device
which measures loudness.
Sound power -
The total sound energy
radiated by a source per unit of time.
Sound pressure -
A dynamic variation in
atmospheric pressure. The pressure at a point in
space minus the static pressure at that point.
Level (SPL) - The fundamental
measure of sound pressure. The measurement of
what sound we hear expressed in decibels in
comparison to a reference level.
Sound Stage -
A room or studio that is
usually soundproof, used for the production of
movies. Or: The psycho acoustic phenomena where
a two-dimensional image (left-to-right and
front-to-back) is created in the mind suggesting
the physical relationship of the listener to the
individual performers. A well designed
listening space will create the impression of a
much larger sound stage than the physical
placement of the speakers, or the size of the
room would otherwise allow.
(airborne) - The conduction of
a sound wave through air. The speed of airborne
sound transmission varies with temperature and
humidity, and is 1130 feet/second in air at 70°F.
(structure borne) - The
conducting of a sound wave through a physical
structure (such as a wall, floor, ceiling or
door). Because of the increased speeds of sound
through common building materials (wood @ 11,700
feet/second, steel @ 18,000 feet/second) as well
as the physical connection of such materials in
the structural framework of a building,
structure borne sound transmission is much more
difficult to stop than airborne sound
transmission, and thus requires special measures
to be dealt with effectively.
- A single number rating for
describing sound transmission loss of a wall or
partition. A rating system designed to
facilitate comparison of the sound transmission
characteristics of various architectural
materials and constructions.
loss - Ratio of sound energy
emitted by an acoustical material or structure
to the energy incident upon the opposite side.
A mirror-like reflection. This is
another instance where sound reflection
properties are simplified by attributing to them
the properties of light rays. A direct
reflection from any surface incurring little or
no attenuation. See
intelligibility - A measure of
sound clarity that indicates the ease of
understanding speech. It is a complex function
of psychoacoustics, signal-to-noise ratio of the
sound source, and direct-to-reverberant energy
within the listening environment. See
Speech Privacy -
Privacy Index (PI). The degree to
which speech is unintelligible between offices.
Three ratings are used: Confidential, Normal
(Non Obtrusive) and Minimal.
Walls are splayed when they are
constructed at angles of varying degrees from
normal rectangular form.
An attachment method and related
hardware for acoustical wall panels that works
in conjunction with kerfed edges. Similar to a
tongue and groove application. See
Standing Wave -
A sound wave continuously
reinforced by its own reflections, influencing
the character of all sound within a room. Since
the standing waves are a direct result of the
size and geometry of the space itself, each room
has a unique set of standing waves. The
presence of these waves can easily be determined
by a combination of mathematical calculation and
An integral sub-multiple of
the fundamental frequency.
The underlying material to which a
covering is applied, or by which it is
supported. A substrate (sometimes referred to as
“core”) can also have important functional
characteristics such as acoustical performance,
impact resistance, and tackability.
design - A basic acoustical
design to create a desirable balanced listening
metal member of the “T” cross section used in
ceiling suspension systems.
Tegular (edge) -
Edge detail on ceiling panels allowing a
panel to hang flush or partially below a
Threshold of hearing-
The minimum sound pressure level of a
pure tone that can be perceived by a person with
good hearing. A sound pressure of 20x10-6
Pascal (0.0002 mBar) is defined as 0dB SPL.
Threshold of pain -
The minimum sound pressure level of a
pure tone which causes a sensation of pain in
the ear. (At approximately 140 dB SPL).
descriptive rather than technical term usually
applied to a well defined sound notable for its
clarity and distinction. “Tight” usually refers
to the absence of excessive reverberation and
out of phase reflections.
subjective tonal quality of a sound. The timbre
of any musical or non-musical sound is
determined largely by the harmonic structure of
the sound wave. Rich sounding musical tones
tend to have a great number of inner harmonics
which contribute to their lush timbre, while
thin sounding musical tones tend to be lacking
in the presence of harmonics.
Time Weighted Average
(TWA): The yardstick (standard of
measurement) used by the Occupational Safety and
Health Administration (OSHA) to measure noise
levels in the workplace. It is equal to a
constant sound level lasting eight hours that
would cause the same hearing damage as the
variable noises that a worker is actually
exposed to. (This hearing loss, of course,
occurs over long-term exposures.) Same as LOSHA.
The propagation of sound through a medium
or barrier. (See Sound Transmission.
The portion of sound
energy transmitted through a material.
Transmission loss (TL)
- The number of dB by which a barrier
reduces the transmission of sound. Transmission
loss varies significantly with frequency. For an
accurate representation of soundproofing
ability, Transmission Loss should be indicated
at several frequencies for any given barrier.
Up front -
to a sound notable for its prominence among
which oscillates about some specified reference
point. Vibration is commonly expressed in terms
of frequency such as cycles per second (cps),
Hertz (Hz), cycles per minute (cpm) or
revolutions per minute (rpm) and strokes per
minute (spm). This is the number of oscillations
which occurs in that time period. The amplitude
is the magnitude or distance of travel of the
support that tends to isolate a mechanical
system from steady state excitation.
cubic space capacity of a room bounded by walls,
floors, and ceilings determined by the formula:
Volume = Length x Width x Height. Volume
influences reverberation time. Also: Colloquial
meter indicating the RMS value of a signal.
Since the human perception of loudness
corresponds to the RMS value of the signal, VU
meters indicate volume (VU stands for Volume
Units). Zero VU is considered to be standard
listening term. In frequency, it is generally
considered to be the range from approximately
150Hz - 400Hz. A system with the "proper" warmth
will sound natural within this range.
unit of acoustical (or electrical) power.
The distance measured perpendicular to
the wave front in the direction of propagation
between two successive points in the wave, which
are separated by one period. The distance
between the beginning and end of a wave or
cycle. Wavelength is determined by the formula:
wavelength and frequency are inversely
proportional, low frequencies have much longer
wavelengths than high frequencies. For example,
a 1000 Hz signal would have a wavelength of
approximately 13.5 inches, whereas a 40 Hz tone
has a wavelength over 28 feet in length.
An electronic filter in a sound level
meter, which approximates, under defined
conditions, the frequency response of the human
ear. The A-weighting network is most commonly
White noise -
Broadband noise having a constant energy
per unit of frequency. Random noise having
uniform distribution of energy with frequency.
A two piece metal clip, one sliding
over the other, used to attach panels to a wall