Acoustical Glossary of Terms

Common acoustical terms and definitions.

A-weighting: a frequency response adjustment of a sound-level meter that makes its reading conform, very roughly, to human response.

Background noise: the total noise that interferes with the measurement of a particular sound of interest. Background noise may include contributions from air-borne sound, structure-borne vibrations, and electrical noise in instruments.

Broadband noise: a sound whose sound pressure level distribution over the frequency spectrum has no dominant peaks, varying smoothly with frequency.

Decibel: a base-10 logarithmic unit used to express the magnitude of a change in the sound level.

Discrete tone: a sound source in which its frequency distribution is prominent at one or more individual frequencies.

Fluctuating noise: noise that does not remain at any constant level over any given time period.

Frequency: the measure of the rapidity of alterations of a periodic signal per unit time, expressed in cycles per second (Hz).

Insertion loss: the difference in sound levels before and after installation of a barrier, where the noise source, terrain, ground and atmospheric conditions have been judged as equivalent.

Intermittent noise: noise occurring at regular or irregular intervals.

Impulsive noise: a noise of short duration (typically less than one second), particularly of high intensity, such as that produced by an explosion, discharge of a firearm, the passage of a supersonic aircraft, etc.

L10 (dBA):  is the noise level just exceeded 10% of the measurement period, A-weighted and calculated by Statistical Analysis.

Ldn (dBA):  Day-night equivalent level : A-weighted, Leq. Sound Level, measured over the 24 hour period, with a 10 dB penalty added to the levels between 23.00 and 07.00 hours. 

Noise: annoying or unwanted sound.

Noise spectrum: a representation of the magnitude (e.g., amplitude and phase) of the components of a complex sound as a function of frequency.

Octave band: a frequency range extending from one frequency to exactly double that same frequency. An octave band is specified by its center frequency.

Sound: any air pressure variation that the human ear can detect.

Sound intensity: in a specified direction at a point, the average rate of sound energy transmitted in the specified direction through a unit area normal to this direction at the point considered.

Sound pressure: change in pressure of the air (often very small) above and below average atmospheric pressure, expressed in Pascals (Pa).

Sound power: the total amount of acoustical energy radiated by a sound source, expressed in Watts (W).

Speed of sound: 344 m/s in dry air, at a temperature of 21°C (70°F).

Threshold of hearing: the minimum sound pressure level that an average ear of normal hearing can hear in a noiseless environment (i.e. the sound pressure level below which a normal human being cannot hear).

Threshold of pain: the sound pressure level at which an average human being experiences physical pain in the ear (140dB).

Transmission loss (TL): of a partition, for a specified frequency band, the difference between the average sound pressure levels in the reverberant source room and receiving room (expressed in decibels) plus 10 times the logarithm to the base 10 of the ratio of the area of the common partition to the total Sabine absorption in the receiving room.

Wavelength: the distance between one peak or crest of a wave of sound and the next corresponding peak or crest