The loudness of a sound as heard by the human ear depends not
only upon the overall intensity of the sound, but on its frequency
distribution as well. The ear acts as an analyzer, determining the
loudness in a rather complicated way from the sound intensities at
the various frequencies. This loudness can be measured by making a
proper summation of the sound intensity in each of eight octave
bands, and the resulting units are called "sones." By definition,
one "sone" is the loudness of a sound with a frequency of 1000
cycles per second and a sound pressure of 0.02 microbar.
Practically, one sone is approximately equal to the loudness of a
quiet refrigerator in a quiet kitchen.
The sone scale is linear to the human ear. A sound of 20 sones
is twice as loud as one of 10 sones. Because of its linearity, and
because it is related to loudness as perceived by the ear, the sone
is used as a single number method of rating the sound output of
non-ducted fans and power ventilators. Within the present state of
the art, when comparing sone loudness ratings, differences of ±6%
or less are not considered significant.
The AMCA method of rating in sones gives the loudness at a
distance of 5 feet from the unit in free space with no nearby
reflecting surfaces. Most practical applications, however, will
involve the determination of total loudness within a room.
The charts and formulae given in this bulletin are for the
purpose of estimating the loudness of fans as installed, and take
into consideration room size and acoustical qualities as well as
the number and loudness ratings of the fans. It will be necessary
to know the acoustical characteristics of other noise sources
within the room to be able to estimate the total loudness of all
sources. For the addition of sounds, it is assumed that the noise
spectrums are similar. The room effect chart is for the reverberant
field in the room, and applies everywhere except in the space near
the fan. Within this space, the direct sound field can be estimated
and added to the reverberant field to obtain total loudness.
The sound pressure level in a room as measure by a Sound Level
Meter on the "A" network, (dBA) can be estimated by use of the
Loudness -dBA correlation chart. This chart gives the relationship
between loudness in sones and dBA levels for normal fan spectrums
within ±2 dBA, which is about the same order as experimental errors
in sound measurements. It must be emphasized that this correlation
applies only after the calculation for room effect and direct sound
field have been completed and does not apply to the fan alone.
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