Draft National Wind Farm Development Guidelines - July 2010

When prominent low-frequency components are present, noise measures based on
A-weighting are inappropriate.
As with infrasound, there are several points worth noting:
• Low frequency noise is naturally occurring in the environment
• Low frequency noise is not normally a problem with wind farms, except under
conditions of unusually turbulent inflow air.
Where a low frequency noise assessment is carried out, it is likely to require the
measurement of sound levels indoors. This is in contrast to other measurements associated
with wind farm noise emission, including background noise level measurements to establish
noise level limits, which are carried out outdoors.
The evaluation method presented in DIN45680:1997 consists of the two following stages:
• Conduct a preliminary investigation consisting in subtracting the A-weighted noise
level from the C-weighted noise level. If the resultant is higher than 20dB, then the
measured sound is considered to contain low frequency noise.
• Conduct a third octave band assessment where each third octave band level is to
be compared against the hearing threshold levels presented in Table B-5.
In addition, the assessment should be able to demonstrate that measured levels of the
reported Low frequency noise are sufficiently above the background noise level.
Human thresholds to low frequency noise are much less sensitive than to sound in other
frequency ranges, such as the range of speech frequency. For example, the 50% hearing
threshold for humans at 20Hz, according to DIN 45680:1997, is 71dB. Where a tone at 20Hz is
less than 71dB, 50% of the population of people with normal hearing will not be able to
perceive it. In contrast, the hearing threshold for humans at 1000Hz is 0dB.
However, once low frequency noise is perceived, changes in the sound level are more
readily observed. Leventhall 14 states that an increase in low frequency noise of 5dB can
subjectively be perceived as a doubling in loudness. This contrasts with sound at mid and
high frequencies where a doubling in loudness is normally associated with a 10dB increase
in sound level.
Table B-5
Hearing threshold levels
Centre frequency of third octave band (Hz)
Hearing threshold level (dB)
8 103
10 95
12.5 87
16 79
20 71
25 63
31.5 55.5
40 48
50 40.5
63 33.5
80 28
100 23.5
14 Leventhall, G. (2003) A review of published research on low frequency noise and its effects.
Department for Environment, Food and Rural Affairs, London, United Kingdom.
Page 68 Draft National Wind Farm Development Guidelines – 2 July 2010