THE SPEED OF SOUND, the frequency of sound, and the wavelength of sound are fundamentally linked. By knowing two factors, it is possible to determine the third. The speed of sound is generally taken as a constant (c):
Table A.1 shows 1/3 octave bands with corresponding wavelengths.
Table A.1 Wavelength of sound waves at 1/3 octave bands given in meters and feet
THE LEVEL OF NOISE created by a source can either be expressed as a sound power level (SWL) or a sound pressure level (SPL). The term “SWL” is used for sound power levels because they are expressed in units of watts. The term “SPL” is used for sound pressure levels because they are expressed in units of Pascals. They both express the same thing: how much energy is used to create a sound wave.
This equation assumes a point source in a hemispherical condition e.g., mechanical equipment fixed on a roof.
A.2.1 Adding sound levels
Because sound is measured on a logarithmic scale, adding together sound levels is not a linear process (i.e. 3 dB + 3 dB does not = 6 dB). To add together a number of sound sources in a room or outside a building, such as mechanical equipment, they must be logarithmically summed:
Once the difference between two noise levels is greater than 10 dB, there is no appreciable increase in the overall noise level. Table A.2 outlines how many dB should be added to a higher noise level dependent on the difference between the two noise levels.
Difference in dB between two sound levels | Level in dB to be added to the louder sound source |
0–1 | 3 |
2–3 | 2 |
4–9 | 1 |
10 or greater | 0 |
Table A.2 can be used as a simplistic method of adding noise levels, as shown in the example below:
Noise source 1 – 81 dB
Noise source 2 – 84 dB
Noise source 3 – 54 dB
Noise source 4 – 83 dB
Start by discounting any level which is –10 dB below any of the other sources (i.e., noise source 3). Next add the two loudest levels together (noise sources 2 and 4) using Table A.2. Then add the sum of these two levels to the remaining source (noise source 1).
A.2.2 Averaging sound levels
If you have several measurements of a single noise source taken over a number of separate surveys and find that the noise level varies, you may wish to average these noise levels. Because sound is measured on a decibel scale we cannot arithmetically average these levels and they must be logarithmically averaged.
A.2.3 Sound propagation
For a point source (noise from a small single object), sound pressure will reduce at –6 dB for every doubling of distance. For a line source (such as a road or railway line), sound pressure will reduce at –3 dB for every doubling of distance. This is used to determine noise levels at a proposed building façade when the noise source has been measured at a position closer than the proposed façade.
A.2.4 Attenuation from barriers
The performance of an acoustic barrier can be determined by knowing the differences in the path length that a sound wave would have to travel to get over the barrier in relation to how far it would have to travel if the barrier were not there.
A.1 Attenuation of sound by a barrier
