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Acoustic enclosures


Absorption Coefficient: (S )

A measure of the quantity of sound lost on impinging on a surface. It can be defined as:
                                           (Reflected sound energy)
                                  1 -     -----------------------------
                                           (Incident sound energy)
It is a property of the material on which the sound impinges and is dependant on thickness of material and frequency of the sound.

Absorptive Silencer

A silencer making use of the absorptive properties of materials incorporated in it to reduce the sound passing through it.

Ambient Noise

The existing background noise in an area can be sounds from many sources, near and far.

Anechoic Room

A specially constructed room in which as much sound as possible is absorbed at its boundaries. It is typically achieved by using sound absorbing wedges.


Noise reduction


It is a noise-reducing device - often colloquially known as a 'silencer'.


Background Noise

The existing noise associated with a given environment, can be sounds from many sources, near and far. (See also Ambient Noise.)


It is the escape of sound from any source enclosing structure, such as ductwork and metal casings.


Cross talk

It is the transfer of airborne noise from one area to another via secondary air paths, such as ventilation ductwork or ceiling voids.

Cut off Frequency

It is the frequency at which performance of an acoustic item or material starts to fall below normal or below criterion. Applied to anechoic wedge treatment it refers to the frequency below which the absorption coefficient is worse than 0.99.


Decibel (dB)

One tenth of a Bel, a Bel being a unit of amplification corrESLonding to a tenfold increase.

In terms of Sound Level Measurements it is related to datum levels as follows:

             For Sound Pressure Level (SPL) datum  = 2 X 10-5 Pascal.
                  For Sound Power Level (SWL) datum     = 1 X 10-12 Watts

             S.P.L. (dB) = 20 log            P
                                                1 X 10-5
             where p is the amplitude of the Sound Pressure Waves concerned measured in Pascal.

             S.W.L. (dB) = 10 log           w
                                                1 X 10-12
             where w is the sound power radiated by the source concerned measured in watts.

Directivity Factor

When sound radiates from any source sound levels can be higher in certain directions than others.This is called 'Directivity'. Directivity Factor is the ratio of the increased level to the average value.

Directivity Index

Is directivity factor expressed in decibels (dB).
It is usually designated by DIO where O is the angle between the axis of the source and the direction of the measuring point.

Discrete Frequency

It is a single frequency signal, or a single frequency noise sufficiently dominant over other frequencies to be distinctly audible.

Dynamic Insertion Loss (DIL)

It is a measure of the acoustic performance of an attenuator when handling the rated flow. Not necessarily the same as Static Insertion Loss because it may include regeneration and / or other velocity effects and will account for the effects of the actual fluid and fluid conditions for which the silencer is designed.


End Reflection

End reflection occurs when sound energy radiates from a hole. The sudden expansion to atmosphere causes some low frequency noise to be reflected back towards the source. Expressed in decibels (dB), the effect is dependent on hole size and frequency. Maximum at lowest frequency from smallest hole.


Flanking transmission

It is the transfer of sound between any two areas by any indirect path, usually structural. It can also apply to noise transmitted along the casing of a silencer.

Free Field

It is a sound field, which is free from all reflective surfaces. A simulated free field can be produced inside a anechoic room.

It is a sound field, which is based on a flat reflective surface with no other reflective surfaces present is known as a Hemispherical Free Field.

Frequency (Hz) - Sound

It is the number of sound waves to pass a point in one second.

Frequency - vibration

It is the number of complete vibrations in one second.


Helmholz Resonance

A resonance created by the mass of a "plug" of fluid acting on the resilience of "spring" of a volume of fluid. e.g. a "plug" of air in a bottleneck resonates on the volume when one blows across the neck. This principle can be used in silencers etc.

Hertz (Hz)

It is the unit of frequency equivalent to one cycle per second.


Insertion Loss

The reduction of noise level by the introduction of noise control device established by the substitution method of test, or by "before and after" testing. The term can be applied to all forms of treatment including silencers and enclosures. (See also Dynamic Insertion Loss and Static Insertion Loss).

Insulation (Sound)

It is the property of a material or partition to oppose sound transfer through its thickness.

Inverse Square Law

The reduction of noise with distance in terms of decibels, it means a decrease of 6dB for each doubling of distance from a point source when no reflective surfaces are apparent. This is only applied in free field conditions where the source is small in comparison with the distance.


Laminar Flow

It is colloquially used to describe the preferred state of airflow. Strictly means undisturbed flow at very low flow-rates where the air moves in parallel paths.

Level Difference

The difference in Sound Pressure Levels between two positions, e.g. inside and outside an enclosure. (This is not the same as Insertion Loss, Transmission Loss or Sound Reduction Index although in some circumstances they may be similar.)


Masking (Sound)

Extra sound introduced into an area to reduce the variability of fluctuating noise levels or the intelligibility of speech.

Mass Law

Heavy materials stop more noise passing through them than light materials. For any airtight material there will be an increase in its "noise stopping" ability of approximately 6dB for every doubling of mass per unit area.
It applies only over a certain frequency range and with this range Sound Reduction Index (SRI) is given approximately by
                                  SRI = 20 log 10 Mf - 43
where M is the surface density of the material in kg/m2 and f is frequency in Hz.


Natural Frequency

The frequency of a system or material at which it freely vibrates when a force is applied and removed (e.g. kicked)

Near Field

It is the area close to a large noise source where the inverse square law does not apply.


Unwanted Sound.

Noise Criterion Curves (NC)

An American set of curves based on the sensitivity of the human ear. They give a single figure for broadband noise. It is used for indoor design criteria. See Part 3 (ii)

Noise Rating Curves (NR)

A set of curves based on the sensitivity of the human ear. They are used to give a single figure rating for a broad band of frequencies. It is used in Europe for interior and exterior design criteria levels. They have a greater decibel range than NC curves. See Part 3 (i)

Noise Reduction

It is used to define the performance of a noise barrier. Established by measuring the difference in sound pressure levels adjacent to each surface (See also Sound Reduction Index).


Octave Bands

It is a convenient division of the frequency scale. Identified by their center frequency, typically 63 125 500 1000 2000 4000 8000 Hz.


Pink Noise

It is noise of a statistically random nature, having an equal energy per octave bandwidth throughout the audible range.

Pressure Drop

The difference between the pressure upstream and down stream of a silencer at given flow conditions. If the silencer is to be installed other than in a duct system of constant cross-section care must be taken with regard to measuring positions and methods to allow for difference in velocity head.

Pure Tone

It is a single frequency signal.


Random Noise

A confused noise comprised from large number of sound waves, all with unrelated frequencies and magnitudes.

Reactive Attenuator or Resonant Attenuator / Silencer

An attenuator in which the noise reduction is brought about typically by changes in cross section, chambers and baffle sections.


The noise generated by airflow turbulence. The noise level usually increases with flow speed.


This is the build up of excessive vibration in a resilient system. It occurs when the machine speed (disturbing frequency) coincides with the mounted machine natural frequency, or support system. Similar effects can occur in acoustic systems (i.e. sound energy in a gaseous fluid).

Resonant Frequency (Hz)

It is the frequency at which resonance occurs in the resilient system.


Reflected sound in a room, that decays after the sound source has stopped.

Reverberation Room or Chamber

A calibrated room specially constructed with sound reflective walls, e.g., plastered concrete. The result is a room with a "long smooth echo", in which a sound takes a long time to die away. The sound pressure levels in this room are very even.

Reverberation Time

The time taken in seconds for the average sound energy level in a room to decrease to one millionth of its originally steady level after the source has stopped, i.e., time taken for a 60 dB decrease to take place. It is usually related to frequency bands as it varies with frequency.

Room Constant

It is the sound absorbing capacity of a room, usually expressed in m2.



It is a unit of absorption comprising the sum of the products of absorption coefficients and areas of the materials of a room. It must be qualified by the units of area used e.g. Square Meter Sabines.

Sabine's Formula

Predicts the reverberation time of a room or enclosure from know room volume and absorption characteristics. Becomes inaccurate when absorption is high.


It is Colloquialism for attenuator, spoken by optimists.

Sound Insulation

It is the property of a material or partition to oppose sound transfer through its thickness.

Sound Level Meter (Noise Meter)

It is an instrument for measuring sound pressure levels. It can be fitted with electrically weighting networks for direct read-off in dBA, dBB, dBC, dBD and octave or third octave bands. (See Section 3).

Sound Power

It is a measure of sound energy in watts. It is a fixed property of a machine, irrESLective of environment.

Sound Power Level (SWL or PWL)

It is the amount of sound output from a machine, etc., cannot be measured directly. It is expressed in decibels of SWL.

Sound Pressure Level (SPL)

It is a measurable sound level that depends upon environment. It is a measure of the sound pressure at a point in N/m2. It is expressed in decibels of SPL at a specified distance and position. It can also be considered as a measure of intensity of terms of Sound Energy per unit area at the point considered, but is not a vector (i.e. directional) as Sound Intensity strictly is.

SPL Direct Field

It is the sound radiating directly from the source(s) to the receiver without reflection. The direct components of a sound level field are calculated from a given SWL by using inverse square law and directivity etc.

SPL Reverberant Field

It is the sound reaching the receiver after one or more reflections. The reverberant component of a sound level calculation from a given SWL by using room constant values from reverberation time and volume.

Sound Reduction Index (SRI)

A set of values measured by a specific test method to establish the actual amount of sound that will be stopped by the material, partition or panel, when located between two reverberation rooms. Average SRI can be calculated by averaging the set of values in the sixteen third octave bands from 100 Hz to 3150 Hz. It is a property of the material(s) or construction, not directly measurable in the field.

Sound Spectrum

It is the separation of sound into its frequency components across the audible range of the human ear.

Standing Waves

These occur due to room geometry. Sound levels at some locations in the room at certain frequencies will be intensified by additive interference of successive waves, and in other locations reduced by cancellation.

Static Insertion Loss (S.I.L)

The Insertion Loss of an attenuator under static (no flow) conditions (c.f. Insertion Loss, Dynamic Insertion Loss).


Transmission Loss

American preferred description for sound reduction index. A set of values measured by a specific test method to establish the actual amount of noise that will be stopped by the material, partition or panel when placed between two reverberation rooms.

Threshold of Audibility or Hearing

The minimum sound levels at each frequency that a person can just hear.

Threshold of pain

The sound level at which a person experiences physical pain. (Typically 120 dB)

Threshold Shift

A partial loss of hearing caused by excessive noise, either temporary or permanent, in a person's threshold of audibility.

Third Octave Bands

It is a small division of the frequency scale, three to each octave. It enables more accurate noise analysis.

Turbulent Flow

A confused state of airflow that may cause noise to be generated inside, for example, a ductwork system.


Velocity Head or Velocity Pressure (Pv)

It is a measure of the inertia of a flowing fluid used in assessing pressure losses in duct systems and / or silencers for air at atmospheric conditions.

             Pv   =   v2
                         ----        where Pv is in millimeters of water and v is the velocity in meters / seconds.
             Pv   =   v2
                        ------       where Pv is in inches of water and v is the velocity in feet / minute.


White Noise

It is noise of a statistically random nature having an equal energy level per Hertz throughout the audible range.


The distance between two like points on a wave shape, e.g. distance from crest to crest.

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