Acoustics Glossary

A-Z list of terminology commonly used within acoustics.



Tangential mode: a room mode, which involves reflections between two pairs of parallel surfaces (e.g. walls).

Temporary threshold shift: the component of threshold shift, which shows progressive reduction with the passage of time, when the apparent source has been removed.

Threshold of hearing: of a given listener the lowest sound pressure level of a particular sound that can be heard under specified measurement conditions, assuming the sound reaching the ears from other sources is negligible.

Threshold of pain: for a given listener the minimum sound pressure level of a specified sound, which will produce the sensation of pain in the ear.

Threshold shift: the deviation, in decibels, of a measured hearing level from one previously established.

Timbre: the quality of a sound, which is related to its harmonic structure.

Time averaged sound level: see under continuous equivalent sound level, LAeq,T.

Time constant: of a process or quantity, which decays exponentially with time; the time required for the value to reduce by a factor of 1/e, where e is the exponential number 2.7183…

Time weighting: one of the standard averaging times (F, S, I) used for the measurement of RMS sound pressure level in sound level meters, specified in BS EN ISO 61671-1.

Tinnitus: a subjective sense of noise in the head or ringing in the ears for which there is no observable cause.

Tonality: the degree to which a noise contains audible pure tones: broadband noise is generally less annoying than noise with identifiable tones.

Tone: a sound, which produces the sensation of pitch; see also under pure tone.

Traffic noise index: and index used for the assessment of environmental noise in the 1960s, based on a combination of the LA10 value and the LA90 value; it is now rarely used.

Transducer: a device for converting signals from one form to another; frequently, the requirement is to convert changes in some physical variable, such as temperature or sound pressure, into analogous changes in electrical voltage or charge.

Transfer function: of a vibrating system; the ratio of the output or response of the system to the input excitation, usually expressed as a complex function of frequency.

Transfer standard: a calibrated noise source designated to fit over a microphone.

Transient: a noise or vibration signal, which is not continuous but which decreases to zero then remains zero.

Transmissibility: of a vibrating system; the non-dimensional ratio of vibration amplitude at two points in the system; frequently, the two points are on either side of springs used as antivibration mounts, and the transmissibility is used as an indicator of the effectiveness of the isolation.

Transmission coefficient: see under sound transmission coefficient.

Transmission loss: see under sound reduction index.

Transverse sensitivity: of an accelerometer; the sensitivity to vibration in a direction perpendicular to the axis of the accelerometer.

Transverse wave: a wave in which the direction of vibration of the particles of the medium is perpendicular to the direction of wave travel; an example is a shear wave in a solid medium.

Triboelectric effect: the production of electric charge as a result of vibration in an accelerometer cable, leading to electrical noise in the accelerometer signal, unless the cable is secured to prevent movement.