Acoustic
Test Chambers  Anechoic
Chamber
The following are Acousticrelated terms and the accompanying definitions.
Absorption
The conversion of sound energy into another form of energy, usually heat, when
passing through an acoustical medium.
Absorption coefficient Ratio of sound absorbing effectiveness, at a specific frequency, of a unit area of
acoustical absorbent
material to a unit area of perfectly absorptive material
Acoustics
The science of the production, control, transmission, reception and effects of sound and of the phenomenon of hearing.
Ambient noise Allpervasive noise associated with a given environment.
Amplitude distribution A method of representing timevarying noise by indicating the percentage of time
that the noise level is
present in a series of amplitude intervals.
Anechoic room A room whose boundaries effectively absorb all incident sound over the frequency
range of interest, thereby creating
essentially free field conditions.
Audibility threshold The sound pressure level, for a specified frequency, at which persons with normal
hearing being to respond.
Background noise The ambient noise level above which signals must be presented or noise sources
measured.
Cumulative distribution A method of representing timevarying noise by indicating the percentage of time
that the noise level is
present above (or below) a series of amplitude levels.
Damping (1) The action of frictional or dissipative forces on a dynamic system causing the
system to lose energy and reduce the
amplitude of movement.
Damping (2) Removal of echoes and reverberation of the use of soundabsorbing materials.
Decibel scale A linear numbering scale used to define a logarithmic amplitude scale, thereby compressing a wide range of amplitude values to a small set of numbers.
Diffraction The scattering of radiation at an object smaller than one wavelength and the
subsequent interference of the scattered wavefronts.
Diffuse field 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 is phase; sound which appears to have no
single source.
Directivity factor The ratio of the meansquare pressure (or intensity) on the axis of a transducer at
a certain distance to the
meansquare pressure (or intensity) which a spherical source radiating the same power would produce at that point.
Far field Distribution of acoustic energy at a very much greater distance from a source
than the linear dimensions of the source
itself; the region of acoustic radiation used to the source and in which the sound waves can be considered planar.
Free field An environment in which there are no reflective surfaces within the frequency
region of interest.
Hertz The unit of frequency measurement, representing cycles per second.
Impedance, specific acoustic The complex ratio of dynamic pressure to particle velocity at a point in an
acoustic medium, measured
in rayls (1 rayl = 1 N sec/m).
Infrasound Sound at frequencies below the audible range, i.e. below about 16 Hz
Isolation Resistance to the transmission of sound by materials and structures.
Masking The process by which threshold of audibility on one sound is raised by the
presence of another (masking) sound.
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.
Newton The force required to accelerate a 1 kg mass at 1 m/s. Approximately equal to
the gravitational force on a 100g mass.
Noise emission level The dB(A) level measured at a specified distance and direction from a noise
source, in an open environment,
above a specified type of surface. Generally follows the recommendation of a national or industry standard.
Noise reduction coefficient, NRC The arithmetic average of the sound absorption coefficients of a material at 250,
500, 1000, and
2000 Hz.
Noy A linear unit of noisiness or annoyance.
Particle velocity The velocity of air molecules about their rest position due to a sound wave.
Pascal, Pa A unit of pressure corresponding to a force of 1 Newton acting uniformly upon anarea of 1 square meter. Hence 1 Pa = 1 N/m.
Phon The loudness level of a sound. It is numerically equal to the sound pressure level
of a 1 kHz free progressive wave which is
judged by reliable listeners to be as loud as the unknown sound.
Pink noise Broadband noise whose energy content is inversely proportional to frequency
(3dB per octave or 10dB per decade.)
Power spectrum level The level of the power in a band one hertz wide referred to a give reference power.
Random noise Noise whose instantaneous amplitude is not specified at any instant of time.
Instantaneous amplitude can only be
defined statistically by an amplitude distribution function.
Reverberation The persistence of sound in an enclosure after a sound source has been
stopped. Reverberation time is the time, in
seconds required for sound pressure at a specific frequency to decay 60 dB after a sound source is stopped.
Root mean square (rms) The square root of the arithmetic average of a set of squared instantaneous
values.
Sabine A measure of sound absorption of a surface. One metric Sabine is equivalent to 1
sq. meter of perfectly absorptive surface.
Semianechoic field A free field above a reflective plane
Sone A linear unit of loudness. The ratio of loudness of a sound to that of a 1 kHz tone
40dB above the threshold of hearing.
Sound Energy that is transmitted by pressure waves in air or other materials and is the
objective cause of the sensation of hearing.
Commonly called noise if it is unwanted.
Sound Intensity The rate of sound energy transmission per unit area in a specified direction.
Sound level The level of sound measured with a sound level meter and one of its weighting
networks. When Aweighting is used, the
sound level is given in dB(A).
Sound level meter An electronic instrument for measuring the rms level of sound in accordance with
an accepted national or
international standard.
Sound power The total sound energy radiated by a source per unit time.
Sound power level The fundamental measure of sound power. Defined as
where P is the rms value of sound power in watts, and Po is 1
pW.
Sound pressure A dynamic variation in atmospheric pressure. The pressure at a point in space
minus the static pressure at that
point.
Sound pressure level The fundamental measure of sound pressure. Defined as:
where p is the rms value (unless otherwise stated) of
sound pressure in Pascal's, and po is 20 Pa for measurements in air.
Sound transmission class, STC A singlenumber rating for describing sound transmission loss of a wall or
partition.
Sound transmission loss Ratio of the sound energy emitted by an acoustical material or structure to the
energy incident upon the
opposite side.
Standing wave A periodic wave having a fixed distribution in space which is the result of
interference of progressive waves of the
same frequency and kind. Characterized by the existence of maximum and minima amplitudes that are fixed
in space.
Ultrasound Sound at frequencies above the audible range, i.e. above about 20 kHz.
Wavelength The distance measured perpendicular to the wavefront in the direction of
propagation between two successive points in the
wave, which are separated by one period. Equals the ratio of the speed of sound in the medium to the
fundamental
frequency.
Weighting network An electronic filter in a sound level meter which approximates under defined
conditions the frequency response of
the human ear. The Aweighting network is most commonly used.
White noise Broadband noise having constant energy per unit of frequency.
