CPD PROGRAMME
Octave band centre frequency Band LP
A-weighting
A-weighted LP, dbA *10 log10 (10 L
63/10 +10 L 125/10 +…10L /10 8000
63 45
-26 19
) Figure 7: Example A-weighting of octave band sound pressure levels A young person may be able to hear
sounds from 20 Hz to around 20 kHz. Below this, frequencies are known as infrasound; higher than this normal hearing range is termed ultrasound. In the hearing range, perceived loudness is critical in the assessment of the acoustic environment, as this will infl uence the design and operation of systems so that they do not adversely affect people. The value of sound pressure level may
sound pressure level of about 70 dB will have the same perceived loudness as 50 dB at 1,000 Hz. The curves indicate that generally low
(or very high) frequency sounds need to have a greater sound pressure level to be perceived as loud as mid-range frequencies. These curves were developed from original experiments in the 1930s and have recently been altered in ISO2261
, notably with the
shift of the most sensitive frequency from 4 kHz to 3.5 kHz and amended to be less sensitive at frequencies below 1 kHz.
130 120 110 100 90 80 70 60 50 40 30 20 10 0
-10 10 100 1,000 frequency (Hz)
Figure 6: Phon – equal loudness contours (Data source: ISO 226:2003)
10k (estimated)
100 phon 80 60 40 20
(threshold)
be weighted in terms of the ear’s ability to hear them. There are several different methods of weighting that may be applied, but for the relatively low intensities within buildings, weighting network A, or dBA, is commonly used. This weighting practically simulates the 40 phon contour by attenuating the lower frequencies and clipping the highest frequencies. Where there is predominantly low frequency noise, dBA may well underestimate the apparent loudness. However, this weighting is widely used to determine noise levels for building services applications. For example, a series of octave band
sound pressure levels have been measured in a room (as in Figure 7), and the dBA weightings are applied to determine the total A-weighted sound level. Noise weightings B and C relate to higher phon levels and are not often used in building services applications – they take greater account of lower frequencies. When using a sound meter, the reading
is normally directly in dBA, and there is no need to convert; indeed, it is the more expensive instrumentation that allows an octave band analysis (or even a ‘third octave band’ where greater discrimination is required). If octave band frequency analysis is available, then the data may be transferred onto a set of noise criteria (NC) curves or noise rating (NR) curves. These are useful, as they not only give a value that can be compared to design standards but will also help identify frequency bands that may cause potential problems. The NC curves are of American origin
and were originally designed for use in offi ce applications to assess the noise. The
www.cibsejournal.com
NR curves are European, designed for general ‘environmental’ application, and are more tolerant of the lower frequency noise but less tolerant of high frequencies. Figure 8 provides an NR value for the spectrum used to determine 51 dBA above – this provides an NR47.
NR curves 120 100 80 60 40 20 0
63 125 250 500 1K 2K 4K 8K octave band centre frequency (Hz)
Figure 8: Noise rating curves dBA may be approximately translated
into NC or NR by taking 5 away from the dBA level. When the sound level is measured, it
will be (as set on the measuring device) ‘fast’ to show quickly varying noise, ‘slow’ to dampen out variations and give a more consistent reading, or ‘continuous’ to ‘average’ out the sound over an extended time period. Continuous ‘equivalent’ dBA is typically abbreviated to LAeq
used to record environmental noise. A future CPD will consider the application of these acoustic concepts to buildings and systems. © Tim Dwyer 2012
Further reading: CIBSE Guide B5: Noise and Vibration Control for HVCA, 2002.
References 1. ISO 226:2003 Acoustics – Normal equal- loudness-level contours.
March 2012 CIBSE Journal 67
NR80 NR75 NR70 NR65 NR60 NR55 NR50 NR45 NR40 NR35 NR30 NR25 NR20
125 44
-16 28
250 45 -9
36
500 46 -3
43
1000 45 0
45
2000 44 +1 45
4000 42 +1 43
8000 40 -1
39
Total* 53
51
and is widely
Shutterstock/1xpert
sound pressure level (dB)
sound pressure level (dB)
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