126
Moderate Intensity Noise, i.e. 55-80 dBA, although not a potential hazard
to hearing, may adversely affect concentration and will interfere with speech communication if greater than 65 dBA.
Low Intensity Noise, i.e. less than 55 dBA, may still result in complaints
from the general public of annoyance such as sleep disturbance.
13.5 Addition of Sound Levels
When two sounds are being emitted at the same time their total combined intensity is not the numerical sum of the decibel levels of each sound. For
accurate calculations they must be added as logarithms – usually using a
calculator. Alternatively a reasonable approximation of additions of decibel levels can be made using the table below:
Difference in dB A Add to the Higher
0 or 1 3
2 or 3 2
4 to 9 1
10 or more
Thus if two machines are both emitting noise levels of 90dBA the sum total noise level is 93 dBA. N.B. A doubling of the sound level results in an
increase of 3dBA.
13.6 Frequency Analysis
Unless a sound is a pure tone, which is unusual, most noises are made up of sounds of many frequencies and intensities, and when assessing it for health
or annoyance reasons it is useful to understand what the levels are over a range of frequencies, that is, to produce a sound spectrum. For convenience it
127 is usual to divide the range of frequencies into octave bands by using an
instrument, which measures the intensities over an octave, and quoting it as the intensity at a particular mid-octave frequency. The mid-octave frequencies
chosen for this are as follows: 31.5Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz and
sometimes 16 kHz Thus a noise spectrum will quote the intensities at each of the above mid-
octave frequencies. The human ear is most sensitive to frequencies between 20 Hz - 20 kHz of which the frequency of speech lies between 500 Hz - 4 kHz,
the vowel sounds being at the lower frequencies and the consonants at the higher.
13.7 Decibel Weightings
As noise is a combination of sounds at various frequencies and intensities, the noise intensity can be either expressed as a spectrum, or as a combination of
all frequencies summed together in one value. As the human ear is more sensitive to certain frequencies than others, it is possible to make allowances
for that in the electronic circuitry of a sound level meter. That is, certain frequencies are suppressed whilst others are enhanced in order to
approximate to the response of the human ear. This technique is known as weighting, and there are A, B, C and D weightings available for various
purposes. The one that has been adopted for a workplace spectrum is given in dBA. If the A-weighting is applied to a measurement in dB, the
corresponding level in dBA is a good indication of loudness as perceived by the human ear.
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Source: Public Domain Wikimedia Commons
Figure 13.2 - Graph showing frequency weightings and relative response 13.8 Equivalent Continuous Sound Level Leq
Expressing noise exposure from the standpoint of its of its potential to cause noise-induced hearing loss is simplified by using dBA instead of dB to
remove the frequency dependent factor. However, as well as varying in frequency, industrial noise varies in its intensity throughout the day and from
day-to-day and is often intermittent in nature. Some sort of average is therefore needed and the equivalent continuous sound level Leq has been
established for this purpose.
Equivalent Continous Sound Level Leq
Time
N o
is e
L e
v e
l d
B A
Source: Adrian Hirst
129 Leq can be defined as the steady sound pressure level, which over a period of
time has the same energy content and consequently the same hearing damage potential, as the actual fluctuating noise.
13.9 Noise Dose
