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© FANTECH
2016
NOISE CONTROL PRODUCTS
GENERAL ACOUSTIC INFORMATION
NOISE RATINGS
dB(A) LEVELS
The ear responds not only to the absolute sound pressure level of a sound, but also to
its frequency content. It actually gives a weighting to the level of sound according to its
frequency content, and ascribes a certain loudness. This means that if we want to know
how a person will judge the sound, we must somehow translate our objective measured
units of sound pressure level and frequency content into subjective units of loudness.
A sound level meter accepts all of the frequency components of a sound, and adds all
their absolute levels together to give an overall sound pressure level, dB (Linear).
The illustration below shows typical overall sound pressure levels produced by some
everyday sources.
However the ear is not as sensitive to lower frequency sound pressure levels as it is to
higher frequency sound pressure levels. In the 1930’s, experiments were carried out on
11 people by Harvey Fletcher at the Bell Telephone Laboratories in New York to
determine how loud tones of different frequencies sounded subjectively. Therefore the
"A" weighting (or the "A" in dB(A)) was devised so that the sound meter would filter each
frequency of sound by a certain amount before adding them together to give a
loudness that more closely follows the sensitivity of the human ear.
The ‘A’ frequency weighting corrections are shown below.
The ‘A’ frequency weighting suggests that if a tone of 40 dB is played at 1000 Hz, a 40
dB tone played at 63 Hz would sound 26 dB quieter, or be 14 dB(A). Due to its
simplicity and convenience, the ‘A’ frequency weighting has become popular and is
now used for many different noise sources at different levels. In fact, most legislation
regarding noise is written using dB(A)s, in addition nearly all manufacturers of fans and
other noise generating machines quote their noise levels in dB(A)s at 1, 1.5, or 3 metres
assuming spherical distribution. It is therefore important that we understand the ‘A’
frequency weighting and how dB(A)s are calculated.
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DECIBELS
dB(A)
jet aircraft taking off (25 metres)
threshold of pain
Deafening
Very Noisy
Noisy
Quiet
Very Quiet
rock concert (front row)
sheet metal shop (hand grinding)
jack hammer (1 metre)
lawn mower, heavy trucks (6 metres)
loud radio (in average domestic room)
electric drill (1½ metres), busy street
busy general office, restaurant
normal speech, general office
quiet office
quiet bedroom, whisper
still day in the country away from traffic,
tap dripping
threshold of hearing
Octave Band Centre Frequency, Hz 63 125 250 500 1000 2000 4000 8000
‘A’ frequency weighting corrections -26 -16 -9 -3 0 +1 +1 -1
CALCULATING dB(A) LEVELS
Published dB(A), or ‘A’ frequency weighted, sound pressure levels are theoretical
values. These are, in fact, calculated from the sound power level data and are quoted
at a specified distance i.e. 1, 1.5, or 3 metres. For example, using the Fantech model
AP0804AP10/23 (duty 7000 L/s @ 80 Pa, inlet side), by applying an ‘A’ frequency
weighting correction to the fan sound power levels for each frequency and then
logarithmically adding the values from left to right the resultant overall sound power
level for this unit will be 98 dB(A). A further calculation is required to convert this value
from the ‘A’ weighted sound power level to an ‘A’ weighted sound pressure level at a
prescribed distance from the noise source i.e. 77 dB(A) @ 3m.
See next page for a detailed example of this calculation.
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