Acoustic Definitions and Terms

Engineering Applications

The following are Acousticrelated terms and the accompanying definitions.

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

The science of the production, control, transmission, reception and effects of sound and of the phenomenon of hearing.

Ambient noise
All-pervasive noise associated with a given environment.

Amplitude distribution
A method of representing time-varying 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 time-varying 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 sound-absorbing 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.

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 mean-square pressure (or intensity) on the axis of a transducer at a certain distance to the mean-square 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.

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).

Sound at frequencies below the audible range, i.e. below about 16 Hz

Resistance to the transmission of sound by materials and structures.

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.

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.

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.

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.

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.

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

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.

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 A-weighting 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 single-number 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.

Sound at frequencies above the audible range, i.e. above about 20 kHz.

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 A-weighting network is most commonly used.

White noise
Broadband noise having constant energy per unit of frequency.