The decibel (dB)
is a measure of sound intensity; that is, the magnitude of the
fluctuations in air pressure caused by sound waves. The decibel
scale is logarithmic, not arithmetic. This means that a doubling
of sound intensity is not represented as a doubling of the decibel
level. In fact, an increase of just 3 dB means twice as much
sound, and an increase of 10 dB means ten times as much sound.
A sound pressure level of 0 dB represents
the threshold of hearing in the most sensitive frequency range of
a young, healthy ear, while the thresholds of tickling or painful
sensations in the ear occur at about 120 to 130 dB.
Decibels are usually measured with a filter
that emphasizes sounds in certain frequencies. The "A" filter
(dBA) is the one most frequently used. The "C" filter (dBC) puts
more weight on low-frequency sounds such as the bass in amplified
music.
The perception
of loudness by the human ear is not directly proportional to the
decibel level. For example, a sound 10 dB greater than another is
not perceived as being ten times as loud but only about three
times as loud.
The intensity of noise diminishes with
distance. Outdoors, and in absence of any close reflecting
surface, the effective decibel level diminishes at a rate of 6 dB
for each factor of two increase in distance. For example, a sound
measuring 100 dB at 10 metres would be 94 dB at 20 metres, 88 dB
at 40 metres, and so on.
How much noise is too much
noise?
One of the first questions people ask when told that noise can
be harmful is: how much noise is actually necessary before human
beings are adversely affected?
The answer isn't simple, because the effects of noise involve
multiple aspects of people's health and welfare.
From a philosophical point of view, since noise may be defined
as any unwanted acoustical stimulus which interferes with human
activity or rest, any amount of noise affects people (otherwise it
wouldn't be noise). Often, however, noise is better tolerated if
it is judged to be unavoidable. The noise of rain, for instance,
is much more acceptable than that coming from the isolated but
steadily repetitive drops from a leaking tap. Generally speaking,
periodical noises are more annoying than random ones.
But when objectively measurable effects are considered,
research has shown that there exists a strong correlation between
the physical intensity of the stimulus and the extent of the
effect. That is particularly true as regards hearing loss.
Extensive research has been done for decades on different groups
of industry workers, and several criteria have been developed in
order to assess specific situations.
One of those criteria, namely that contained in the ISO 1999
Standard, starts by defining "hearing loss" as a permanent
increase of the auditory threshold (the minimum audible sound
level) affecting the intelligibility of speech. This amounts to
some 25 dB for middle-frequency tones. Then a double entry table
is used to assess the percentage risk of "acquiring" hearing loss
if exposed in the workplace to a certain average sound level for 8
hours per day, 6 days a week for a given number of years:
| dBA |
Years of exposure |
| 5 |
10 |
15 |
20 |
25 |
30 |
35 |
40 |
45 |
| 80 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
| 85 |
1 |
3 |
5 |
6 |
7 |
8 |
9 |
10 |
7 |
| 90 |
4 |
10 |
14 |
16 |
16 |
18 |
20 |
21 |
15 |
| 95 |
7 |
17 |
24 |
28 |
29 |
31 |
32 |
29 |
33 |
| 100 |
12 |
29 |
37 |
42 |
43 |
44 |
44 |
41 |
35 |
| 105 |
18 |
42 |
53 |
58 |
60 |
62 |
61 |
54 |
41 |
| 110 |
26 |
55 |
71 |
78 |
78 |
77 |
72 |
62 |
45 |
| 115 |
36 |
71 |
83 |
87 |
84 |
81 |
75 |
64 |
47 |
Whenever working hours are halved, the criterion is applied
subtracting 3 dBA from the actual sound level.
Let's take, for example, the case of a disc jockey working at a
night-club 3 nights a week exposed to an average sound level of
106 dBA during a 4-hour shift. We have half the days and half the
time per day, so 3 dBA must be subtracted twice, yielding 100 dBA.
Suppose, further, that the disc jockey starts at the age of 15 and
stays in this trade till 30 -- that is, a 15-year exposure. The
table reveals a 37 percent risk of suffering speech
intelligibility handicap. That is, almost 4 out of 10 such persons
will experience difficulties in understanding normal speech -- at
the age of 30.
Labor regulations usually allow exposure to levels of 85 dBA or
even 90 dBA, but also compel employers to carry out hearing
conservation programs, which typically include periodic hearing
tests to screen out highly susceptible workers from hazardous
noise environments.
A closer look at the table reveals that at 80 dBA the risk is
0% for any extension of working life. Does this mean that 80 dB is
the safe ceiling? Not at all. This is because in the definition of
"risk" there is another element which for simplicity was not
mentioned earlier. As this table is intended to predict the risk
of workplace exposure, the percentage of people suffering from
hypoacusis just by aging (i.e. presbycusis) has already been
subtracted.
There are several studies that reveal that what was formerly
considered to be presbycusis is actually "sociocusis" -- that is,
hearing loss due to exposure to social or community noise.
In the seventies the U.S. Environmental Protection Agency (EPA)
was committed to providing "information on levels of environmental
noise requisite to protect public health and welfare with an
adequate margin of safety." The task was not easy, because though
a large amount of information on workplace exposure to noise was
readily available, very little information regarding non-workplace
exposure had been published. The approach was thus to extrapolate
the available data under a number of reasonable asumptions. The
results were published in a famous paper known as "The Levels
Documents."
The EPA criterion regarding hearing conservation states that in
order to protect virtually all the population against hearing
loss, the average sound level should not be greater than 70 dBA
during a 24-hour day. Equivalently, it should not be greater than
75 dBA for an 8-hour working day, provided that for the rest of
the day the level is kept considerably below that figure.
This criterion differs substantially from the ISO's in that it
considers that hearing loss has occurred when the auditory
threshold rises only 5 dB instead of 25 dB. It is thus a far
stricter criterion.
It should be noted that the EPA limit is an average,
meaning that much larger levels can usually be tolerated for brief
periods of time. Indeed, each time the duration of the exposure is
halved, the safe level increases by 3 dBA, so it would be safe to
be exposed to 78 dBA for 4 hours, 81 dBA for 2 hours, 84 dBA for 1
hour, and so on. However, exposure to levels above 100 dBA even
for very short periods is not recommended, since some susceptible
persons may suffer immediate and irreversible hearing impairment.
As seen above, occupational limits tend to be rather permissive
as compared to environmental ones, the reason being that they take
into account other factors appart from safety and welfare, such as
the technical or economical feasibility of decreasing noise levels
at the workplace.
Finally, the EPA document addresses not only the hearing
impairment issue but also issues of annoyance and interference
with activities. In this case, it states that the average outdoor
noise level should be no greater than 55 dBA in the daytime and 45
dBA at night. Indoors, the recommended limits are 45 dBA and 35
dBA respectively. period of time.
Actual environmental noise laws and regulations tend to be
somewhat more permissive, usually allowing for corrections under
given circumstances. For instance, when the noise source affects
the surroundings of an industrial area, a correction of as much as
25 dB is often applied to the basic EPA criterion. Noise from
motor vehicles is generally treated in a different way: since each
vehicle passes by a given location only for a short period of
time, much higher limits are imposed for individual vehicles, and
little attention is paid to the cumulative noise level arising
from continuous traffic flow.
