A-Weighted Sound Level is weighted sound pressure level obtained by the use of metering characteristics and the A – Weighting specified in American National standard sound level for measurement of noise and other sound levels (ANSI – S1.4-1983, 1985)

Absorption Co-efficient

Absorption Co-efficient is the ratio of sound energy absorbed by a surface of a medium (or material) exposed to a sound field to the sound energy incident on the surface.

Ambient Noise

Ambient Noise is the all-encompassing noise that is present within a given environment.


Attenuation is the energy loss of silencer or other device .Ten times the logarithm of the ratio of incident power to transmitted power.

Audible frequency noise

Audible frequency noise for Purpose of engineering evaluation, only those frequencies between about 30 and 10,000 Hz are considered to be audible.

Background Noise

Background Noise is the noise that is present within a given environment excluding noise from a specific source being evaluated or measured.

Broadband Noise

Broadband Noise is noise that is essentially flat over the entire frequency range. See narrow Band noise.

Decibel (dB)

Decibel (dB) is a standard by which Noise measured or evaluated. The decibel is dimensionless unit used to express logarithmically the ratio of one of the sound to another either in terms of sound power and Pressure.


dBA – One of the three basic sound level meter weighting networks (A,B and C scales) that provide different response characteristics to noise as a function of frequency .The A scale is the most heavily weighted against low frequency response of the human ear “A “scale decibel readings are often referred to as “dBA” levels.


Directivity is the difference in sound measured under free field conditions with a uniform source and that measured at the same distance, but at varying angles from the actual source.

Dynamic Insertion Loss (DIL)

Dynamic Insertion Loss (DIL) – is the reduction of noise provided by a give silencer under actual operating conditions. The difference in sound level without and with the silencer.


Frequency is the number of cyclical variations per second (cps) or Hz.

Sound Transmission Loss (TL)

Sound Transmission Loss (TL) is a logarithmic ration of the sound power on the one side of partition (or silencer shell) to the sound power radiated by the other side.

Wavelength (δ)

Wavelength (δ) is the length of one complete cycle of a sound wave. δ= (Speed of sound / frequency) = C/F

Frequency spectrum

Frequency spectrum is a quantity of sound pressures (dB) expressed as a function of frequency.

Hertz (Hz)

Hertz (Hz) is the unit if frequency in the cycles per second.

Insertion Loss (IL)

Insertion Loss (IL) is the reduction of noise due to the insertion of a silencer or other device in the systems. Under actual operating conditions, insertion loss is referred to as dynamic insertion loss (DIL)


Noise – simply defined is unwanted sound

Noise abatement

Noise abatement is the same as noise reduction

Noise control

Noise control is the prevention or lessening of the noise before it is generated

Noise criteria (NC)

Noise criteria (NC) is the maximum noise levels that are allowed at a specific location or distance from a noise source in a given environment.

Noise reduction (NR)

Noise reduction (NR) is the reduction of noise after it is produced.

Octave bands

Octave bands are frequency bands where the upper limit of each band is twice the lower limit. Octave bands are identified by their center frequency (Geometric mean – frequency)

One – third octave bands

One – third octave bands are the same as octave bands except each octave is divided into one-third octaves.

Sound is an oscillation

Sound is an oscillation in pressure in an elastic medium, which is capable of producing the sensation of hearing caused by a pressure oscillation.

Sound absorption Coefficient

Sound absorption Coefficient is the dimensionless ration of sound energy absorbed by a given surface to that incident upon the surface.

Sound Power Level (Lw)

Sound Power Level (Lw) in decibels is 10 times the logarithm to the base 10 of the ratio of a given power to a reference Power. (The reference power is usually 10^12 watt)

Sound pressure Level (Lp)

Sound pressure Level (Lp) in decibels is 20 times the logarithm to the base 10 if the ratio of a sound pressure to a reference pressure. (The reference pressure ie usually 0.0002 microbar or 20 micro Pascal.)