Acoustics Group

Sound reduction behind an environmental noise barrier

A barrier can be considered to be any solid obstacle which impedes the line of sight between source and receiver thus creating a sound shadow.  In this program the attenuation of sound energy by an environmental barrier is calculated using the Kurze-Anderson formula:

When LB = attenuation (dB) due to the barrier obstruction. N is the Fresnel number ( N > 0.2 and N< 12.5). The calculation of N is shown below, where λ is the wavelength and f is the frequency. For N> 12.5, experimental data show that there is an upper limit of 24 dB.

A noise barrier is only effective if it is large compared to the wavelength of the sound -- in other cases diffraction effects render it virtually transparent, therefore it must extend laterally sufficiently far to prevent diffraction occurring around the ends; the barrier length should be such that the distance from the source to the ends is at least twice the normal distance of the source to the barrier; or a barrier coders an angle of 160˚subtended from the receiver; or the barrier length should be more than 4-5 times of the height. The source and receiver positions can be strategically arranged, as shown below.

[source: Kurze, U. J. and Anderson, G. S., 1971, Sound attenuation by barriers. Applied Acoustics, 4, 35-53]

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Calculation of sound distribution and reverberation in street canyons using the image source method

This program calculates sound level distribution and reverberation in rectangular urban streets canyons with geometrically reflecting boundaries. The program is based on the image source method. Using the program the effects of basic street parameters can be analysed.For both sound level and reverberation calculation, the program allows the following inputs:
(1) Street length (x), width (y) and height (z).
(2) Sound source position.
(3) The height of a horizontal receiver plane, where there are 10 (width) x 10 (length) calculation points.
(4) Sound absorption coefficient of facades and ground. Air absorption (Np/m) can also be input. Typical air absorption values can be selected.
(5) For sound pressure level calculation, source pressure level at 1m from the source.

Clickhere to start calculation.
For further information and algorithm, please see here.

Calculation of absorption coefficient of micro-perforated absorbers

This program calculates absorption coefficient of micro-perforated panels and membranes mounted over an airtight cavity. Using the program a required absorber can be designed, and the effects of various parameters can be demonstrated. For comparison, two configurations, one with a single layer and one with double layers, can be considered. Sound incident angle can also be specified.

Basic sound absorbers include porous absorbers, single resonators, perforated panel absorbers, and panel and membrane absorbers. The perforated panel absorbers are more commonly used in practise.
Micro-perforated absorber is a newly developed sound absorber, it has a number of attractive features:
  • Unlike conventional perforated absorbers, micro-perforated absorber can be made from transparent materials like plastic glass.
  • Unlike commonly used fibrous absorptive materials, micro-perforated absorber is fibrous-free and thus, there is no health concern.
  • Micro-perforated membranes are lightweight and inexpensive.

The absorption performance of micro-perforated absorber is very good. Typically the absorption coefficient exceeds 0.4 over 3-5 octaves.

This calculation depends on the following parameter inputs:
1. Panel thickness
2. Aperture diameter
3. Aperture spacing
4. Air-space depth
5. Material (PVC or Metal)
6. Angle of sound incidence
7. Acoustic resistance of the membrane (normally 1 can be used)
8. Surface density of the membrane

Clickhere to start calculation.

For further information and algorithm, please see here.

Reverberation time calculation in a rectangular space

Reverberation time is the time taken for a sound to decay 60dB after the source is stopped , it is the most important index of reverberation. During calculating, input parameters needed are room dimensions and boundary absorption coefficients. A database of absorption coefficients is given, including the most common materials in room acoustics. The Eyring formulae is used as a modification of the Sabine formula:

V - volume of the room(m3)
S - total surface area of the room (m2)
ā - average absorption coefficient of all the boundaries
m - air absorption coefficient

It is important to note that Eyring formulae embody the assumption of a diffuse field space. Although there is no real sound field that strictly meets this condition, the formulae is accurate enough for many enclosures.

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Digital audio animation for the soundscape design of urban streets and squares

This is a computer tool which allows users to input idealised urban streets and squares in a 2D environment. After selecting and positioning a number of urban sound sources, the soundscape file with multiple sources can be played back, with reverberation effects.
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