Acoustic Terms
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The scientific study of sound, its production,
transmission, and effects.
The inspection of a site to determine the level of
reverberation or reflected sound (in seconds), which is caused by the building
materials used. It also determines the amount of acoustic absorption required to
reduce reverberation and noise.
The acoustic properties of an environment or room
affected by the amount of acoustic absorption in the space.
Sound traveling through the air to reach the point of
interest.
The overall noise level in a given environment,
typically a combination of sounds from many sources, both near and far. Traffic,
HVAC, and even low-level background music can contribute to the ambient noise or
sound level.
Control of noise within a building space to adequately
support the communication function within the space and its impact on occupants.
A measure of rating building elements such as acoustic
ceilings and acoustic screens for speech privacy purposes. AC values increase
with increasing privacy.
Any physical object or medium that interferes with
communication or listening.
In acoustics, an acoustic panel is a panel suspended
horizontally from a ceiling or room structure. It is similar to a partition, but
in a horizontal position.
It is dissipated and converted into small amounts of
heat as the damping layer stretches and compresses or shears. A viscoelastic
damping compound is placed between two rigid or solid confining layers, such as
drywall or plywood. This conversion of vibration to heat reduces the resonance
of the rigid layers, much like placing a hand on a drum head to stop the
resonance.
It is a subjective measure of the perceived intensity
and duration of a sound and can range from soft to loud. Loudness depends on
sound pressure, intensity, and frequency. Every 10 dB is roughly equivalent to a
doubling or halving of the loudness. Also known as intensity..
A single-number rating of building elements such as
acoustic ceilings and acoustic screens for speech privacy purposes derived from
measured noise reduction values between two enclosed spaces connected by one or
more paths that cannot be isolated, such as open office environments.
The average amount of sound pressure reduction measured
logarithmically in decibels (dB) between two rooms containing a sound source and
separated by a wall, and the measurement taken in the other room. Noise
reduction is also used to determine the reduction in sound pressure level caused
by an absorbent material before and after it is installed in a room containing a
sound source.
An acoustic treatment plan, typically for performance
purposes, where one end of the performance space is highly absorbent and the
other more reflective or diffuse. For example, in a good theater, the stage will
be EXIT so that the performer can hear the purest sound produced. The audience
will be at the LIVE END of the theater and will be able to enjoy the warm sound
produced by the reverberation and diffusion of the performance as their sound
works with the room's acoustic treatments.
Sound pressures having frequencies higher than the upper
limit of human hearing, which is approximately 20,000 Hz (20 KHz) to 25,000 Hz
(25 KHz).
The scattering, or random reflection, of a sound wave
from a surface. The direction of the reflected sound is altered so that
listeners perceive the sound as coming equally from all directions . The amount
of diffusion can be measured by taking echo measurements at several different
locations in the room and then averaging the differences in decay times.
The distance between two identical crests in a sound
wave or cycle. Wavelength can easily be pictured as the ripples in water from a
thrown stone. Sound wavelengths vary with frequency. The higher the frequency,
the shorter the wavelength. These frequencies disperse more quickly and are
easier to control, whereas low-end frequencies produce longer wavelengths that
take longer to disperse and are more difficult to control. Low-end frequencies
with longer wavelengths are emitted from sound sources that cause most noise
complaints, such as traffic noise and many stereo systems, such as those found
in today's home theaters.
Reflected sound that produces a distinct repetition of
the original sound reaching the listener when the delay is >0.01 seconds after
the direct sound. For an echo to be audible, the reflecting object must be
approximately 18 m away.
The analysis of a sound to determine the character
(volume of sounds at various frequencies) that make up the overall sound
spectrum, i. e., the higher frequency sound or pitch versus the lower frequency
sound or pitch. A sound rarely consists of a single pure frequency.
The average young person's hearing range is between
16-20 Hz and 20,000 Hz. Human speech ranges from 600 Hz to 4,800 Hz. The average
middle-aged person's highest frequency is between 12,000 and 14,000 Hz..
It describes the ability to focus one's listening
attention on a single speaker amidst a mix of background noises. This effect
demonstrates one of the amazing capabilities of our auditory system, which
allows us to communicate in a noisy place like a cocktail party.
Each doubling of mass produces a 6dB improvement in
transmission loss. For example, if you were to measure a single sheet of 20mm
plasterboard (not attached to any wall) compared to two layers of 20mm
plasterboard, you would see a 6dB gain in sound loss (TL).
Every element has a natural vibration frequency,
determined by a combination of factors such as mass and stiffness. Changing one
factor will change the resonant frequency. It is much easier to make an object
vibrate at its resonant point. In acoustics, a frequency in air can be amplified
by reinforcing a partition vibrating at resonance. For example, if a wall
resonates at 100 Hz, frequencies around 100 Hz will pass through that wall much
more easily.
Mechanical periodic or random oscillations around a
specific reference or equilibrium point. Vibration is expressed in terms of the
number of revolutions or frequency occurring in a given period of time, such as
Hertz (Hz), cycles per second (cps), revolutions per minute (cpm), revolutions
per minute (rpm), and strokes (spm). A few examples are the imbalance of a
rotating object, such as drums, tuning forks, motors, or washing machines.
The continuation of sound wave energy in an enclosed
space after the original sound source has stopped. After this original sound
source stops, the sound wave energy will continue to be reflected and absorbed
until it loses enough energy to fade away. More reverberation may be good for
music, but it can be poor for speech intelligibility.
Echo flutter can be heard as a ringing effect created when a sound bounces back
and forth between two hard, flat parallel surfaces within a room. Echo flutter
can be controlled with diffusers and sound-absorbing products.
In acoustics, a cycle is the complete oscillation of the
pressure of a sound wave above and below atmospheric static pressure.
The arithmetic average of four sound absorption
coefficients of an acoustic material at frequencies of 250 Hz, 500 Hz, 1000 Hz,
and 2000 Hz, to the nearest multiple of 0.05. A Noise Reduction Coefficient of 0
= perfect reflection, and a Noise Reduction Coefficient of 1 = perfect
absorption. The Noise Reduction Coefficient is based on human speech
frequencies, so it provides a simple measure of how well a surface material
absorbs human sound.
Any degree of complete or partial impairment in the
ability to perceive sound. Sound waves vary in frequency and amplitude. Hearing
loss can result from age, disease, or exposure to excessively high noise levels.
Environments with longer reverberation times make it more difficult for those
with hearing loss to understand speech.
The inability to understand speech between rooms or
areas, such as offices and conference rooms, typically found in an open office
plan. The three ratings used are: Confidential (Very Private), Normal
(Unobtrusive), and Minimal (Poor or Low Privacy).
The ability or process of one sound to render another
sound inaudible to the ear. It is most commonly used to aid speech privacy in
open office environments and is more commonly and less accurately known as
"white noise" or "pink noise."
A random signal of each frequency, with each higher
octave falling by 3 dB. Lower octaves have more power, and higher octaves have
less power. Pink noise is used to test loudspeakers and "tune" a room for
optimal sound reproduction or masking systems.
Reducing the sound pressure intensity of sound
transmitted from one point to another.
Building materials, structural elements, and design
features that make structures more impermeable to sound transmission from room
to room or from the outside to the inside of buildings.
When expressed in decibels (dB), the SPL of a sound is
the ratio of the sound pressure to the reference pressure times 20 times the
logarithm to base 0. The reference pressure should be clearly stated and defined
by standards.
This describes the amount of sound energy absorbed by a
given material. For example, if a material being tested reflects 60% of the
sound energy striking that material, the Sound Absorption Coefficient would be
0.40.
In acoustics, a sound barrier is any material or
structure placed around a sound source to prevent noise from being transmitted
beyond the barrier. A poor acoustic environment, such as a room with a long
reverberation time, can be a "barrier" to speech intelligibility or good
hearing.
The subjective measurement of sound or Sound Pressure
Level (SPL), expressed in decibels (db), as a comparison to familiar sounds
experienced in various situations.
An instrument containing a microphone, amplifier, output
meter, and various frequency weighting settings used to measure sound levels or
noise at different frequencies.
Spaced acoustic materials can have greater absorption
than the same amount of material placed together. The increase in efficiency is
due to the greater surface area on the acoustic panel, diffraction around the
panels, and absorption from the air gap.
A measure of speech intelligibility affected by an
acoustic environment, rated from 0.01% to 1.00%. The higher the number, the
higher the intelligibility of the spoken word in a sentence, from 0 to 100%.
Sound level measured by a sound level meter using
A-weighting, which changes the sound level meter's sensitivity with frequency so
that it becomes less sensitive at frequencies to which the ear is less
sensitive; it is often used to determine permissible sound levels in buildings.
The resulting sound level is said to be A-weighted and is in units of decibels
(dBA). The A-weighted sound level is also called the noise level.
The sum of all unwanted residual noise produced from all
direct and reflected sound sources in an area that may represent an interference
with or interfere with good listening and speech intelligibility.
Sound waves whose magnitude does not change with respect
to the direction of measurement in a sound medium free of all limiting surfaces
or obstacles.
The number of cycles per second of a particular tone.
Acoustic frequency is normally measured in units called Hertz (Hz). One Hz is 1
cycle per second, two Hz is 2 cycles per second, and so on.
The process by which vibrations are converted into heat
over time and distance.
In acoustics, noise is defined as any sound within the
acoustic field, both desirable and undesirable. Music and birdsong are examples
of desirable sounds, while noise is more commonly used to refer to undesirable
sounds such as traffic, aircraft, industrial, and other disruptive noises. Noise
does not have to be excessively loud to be annoying or disruptive.
The noise criterion is determined by noise criterion
(NC) curves, which are a series of octave-band spectral curves used to provide a
single number rating of the noise level of an enclosed space. The room's
octave-band spectrum is compared to this set of curves to determine the room's
noise criterion (NC) level, which indicates the extent to which unwanted noise,
such as mechanical or HVAC noise, interferes with speech intelligibility.
A measure of sound pressure over the frequency spectrum,
represented by curves on a graph, where a listener perceives different
frequencies at equal volumes. Human hearing is less sensitive at lower
frequencies, while higher frequencies above 2000 Hz can be more annoying at
higher volumes.
An acoustic absorber or sound baffle used primarily in
sound recording studios and home theaters to absorb sound at frequencies below
about 100 hertz (Hz). Bass traps, like all acoustically absorbent materials,
work by converting sound energy in the room into small amounts of heat through
friction.
Sound produced by the collision of two solid objects and
carried within a structure. Examples include footsteps, falling objects, and
slamming doors.
A unit of frequency equal to one cycle per second.
Short for Outdoor-Indoor Transmission Class, which
covers the range 80 Hz to 4000 Hz. While similar to STC, this method is used to
rate the amount of Transmission Loss (TL) a partition or wall can transmit from
outside to inside, such as determining how much traffic noise an exterior wall
can block. Unlike STC, transmission class is expressed in dB.
The weight of a given material in a given volume.
Generally, the heavier or denser a material, the more sound it blocks and the
higher its Transmission Loss (TL). It is more difficult for a sound wave to
travel through a heavy (high-mass) wall than a lightweight (low-mass) wall.
It is the ability of the human ear to accurately
understand spoken words in a given environment, such as an open office plan, and
is rated on a scale of 0.01% to 1.00%, as defined by the Articulation Index. The
higher the number, the higher the intelligibility of the spoken word in a
sentence, from 0 to 100%.
A flexible support that isolates an object from its
vibration source. An example would be using a Vibration Isolator to prevent the
vibration of a rooftop HVAC unit from being transferred into the building.
The time in seconds it takes for a sound to decay by 60
dB, or one-millionth of its original sound level, after the source has stopped
in an enclosed space. Often referred to as RT60.
A logarithmic unit used to express the difference or
magnitude of sound intensity level or strength. It is equal to ten times the
common logarithm of the ratio of two levels. [DECI + BEL] A whisper is about 20
dB, typical conversation is between 60–70 dB, and the pain threshold of the
human ear is around 120 dB. The decibel is not directly related to the
sensitivity of the human ear, and because double dB operates as a curve, it does
not equal a doubling of perceived loudness. 10 dB is a typical doubling or
halving of perceived loudness. Note that because these are logarithmic values,
they cannot be added.
In an empty space, the sound pressure level will
decrease by 6 dB on a scale according to the Z-SCALE each time the distance from
the sound source is doubled.
In acoustics, a thin sheet of material such as foil,
steel, or lead that helps prevent sound waves from passing through an absorbent
material
A representation of the wave components of a sound, each
with its own frequency and amplitude. A sound spectrum is a representation of
that sound in terms of the amount of vibration at each frequency.
A method for ranking walls, doors, windows, noise
barriers, partitions, and other acoustic products by a single number, measured
across 16 frequencies ranging from 125 Hz to 4000 Hz, assuming the noise source
is generally uniform across the frequency spectrum. STC involves measuring the
transmission loss (TL) of a sound source across various frequency bands, from
one room to another, separated by the test median. STC is not a very good rating
system for most real-world situations that produce sounds at frequencies below
125 Hz, such as traffic noise, machinery noise, and many music systems, such as
those found in today's home theaters. Again, STC ignores lower frequencies in
its calculation, so a "high STC" wall may perform very well at problematic low
frequencies.
The comparison of the level of the desired signal, such
as speech and music, to the level of background or unwanted noise. The higher
the ratio, the lower the background or unwanted noise will interfere with the
desired signal.
A continuous uniform (sound) wave with a constant
amplitude and frequency.
Vibrational mechanical energy that propagates as waves
in substances such as air and water and is described by the properties of sound
waves such as frequency, amplitude, wavelength, intensity, period, speed, and
direction. The speed of sound depends on the temperature and medium through
which it travels. The average speed of sound in air at sea level and 20ºC (68ºF)
is 343 m/s (roughly 767 mph).
The property of materials such as air, walls, or
acoustic panels that convert sound wave energy into heat. When a sound wave
strikes a surface, what is not reflected is absorbed.
The changing difference between the instantaneous
pressure of a sound wave at a point in space and the static atmospheric pressure
at that point