Glossary of Useful Terms & Technical Definitions

A mechanical disturbance propagated in an elastic medium of such character as to be capable of exciting the sensation of hearing.
May be defined as unwanted sound.
Sound Pressure
The instantaneous difference between the actual pressure and the average or barometric pressure at a given location. The unit of measurement is the micro pascal.
Sound Pressure Level
Denoted SPL. Or Lp. Is the 20 times logarithm to the base 10 of the ratio of the effective pressure of a sound to the reference pressure of 20 micro pascals. The unit of measurement is decibel; denoted dB.
A Weighting
A frequency weighting characteristic, and is intended to approximate the relative sensitivity of the human ear to different frequencies of sound. Decibel values that have been corrected using the A weighting characteristics are described as dBA.
Denoted dB; A dimensionless measure of sound level, sound pressure level or sound power level.
Noise Reduction
Any reduction to sound pressure levels achieved by ‘natural means.’ i.e. distance, absorption in air etc. or by the use of noise reduction equipment. It is sometimes used to describe the specific performance criteria of equipment used to achieve a reduction of noise.
Insertion Loss
Denoted IL. Most commonly used to describe the specific performance criteria of silencers. It is defined as the change in inline radiated sound pressure level resulting from the use of the silencer.
A general term that refers to any reduction in sound pressure level. It is sometimes used as a synonym for noise reduction and insertion loss.
Transmission Loss
Denoted TL. Used to describe the difference in sound power between the source side and the receiver side of a panel. It Is most often used to specify the performance criteria of a panel.
Flanking Loss
A loss in acoustical performance when sound is transmitted as vibration along a mechanical path (ie. silencer shells) and is reradiated as sound. Vibration breaks must be provided along the mechanical path to preserve the full performance of the treatment.
Denoted Hz, Hertz or CPS. Of a periodic quantity is the number of times the quantity repeats itself in a unit interval of time. i.e. the frequency of a sound is the number of oscillations that occur in a set period of time. The unit of measurement is oscillations per second or Hertz which is the same as cycles per second. The normal range of human hearing is 20-20,000 Hz.
Sound Power
Is the total energy of a sound source that is available to radiate into the surrounding air. It is independent of location. i.e. distance and the acoustic values of the surrounding space. The unit of measurements is watts.
Sound Power Level
(Denoted PWL or Lw) of a sound source is 10 times the logarithm to the base 10 of the ratio of the sound power radiated by the source to the reference sound power. The reference sound power is standardized at 10-12 watts. An older reference sound power of 10-13 watts is sometimes used. To convert sound power levels re. 10-13 watts to sound power levels re. 10-12 watts, subtract 10 from the sound power levels re. 10-13 watts. The unit of description is decibel (denoted dB).
Free Field
A term used to indicate that the space surrounding the noise source does not cause any change to the noise propagation other than what could normally be expected in open space. i.e. changes due to distance and air absorption only. Thus, the resultant sound field has only a direct component.
Sound Absorption
Refers to the process by which a material takes in sound waves and reduces the energy in the sound, transforming it into a minute quantity of heat, leaving less energy in the reflected sound wave, and therefore a reduced sound level.