Quiet-List 1997

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How much noise is too much noise?



Dear Friends,

The following is an article that I've written to be posted to the quiet
internet site by Peter Donnelly. I'll welcome any comments which allow me to
improve it prior to actually posting it. Thanks.

Regards,

Federico Miyara


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How much noise is too much noise?

by Federico Miyara


One of the first questions people ask when told that noise constitutes a
deleterious stimulus is how much noise is actually necessary to adversely
affect human beings.

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 to 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 1,999 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 percentual risk of "acquiring" hearing
loss if exposed in a labor basis (i.e., 8 hours per day, 6 days a week) to a
certain average sound level in dBA during a given number of years:  

                  ------------------------------------
                 |        Years of exposition         |
                 |------------------------------------|    
                 |  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 |
     | dBA | 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 labor hours are halved, the criterion is applied subtracting 3 dBA
to 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 4 hours.
We have half the days and half the time per day, so 3 dBA must be subtracted 
twice, getting 100 dBA. Suppose, further, that the disc jockey starts at the
age of 15 and stays in this trade thru 30, that is, a 15-year exposition.

Well, the table reveals a 37%-risk of suffering speech intelligibility
handicap, that is, almost 4 out of 10 such guys will--at thirty!--experiment
difficulties to understand normal speech. Impressive, isn't it?    

Labor regulations usually allow exposition to levels of 85 dBA or even 
90 dBA, but also compel employers to carry out hearing conservation
programs, which typically include periodical hearing tests to screen out
highly liable workers from hazardous noise environments.

A closer look at the table reveals that at 80 dBA the risk is 0% for any 
extension of labor 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
intends to predict the risk of a labor exposition, the percentage of people
suffering from hypoacusis just by aging (i.e, presbycusis) has been
previously subtracted.  
  
Now, there are several studies that reveal that presbycusis is not just
presbycusis but "sociocusis", that is, hearing loss due to exposition to
social or community noise.

In the 70's, the U.S. Environmental Protection Agency (EPA) was committed to
provide "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 labor exposition 
to noise was readily available, very little information regarding not-labor
exposition 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 24 hours a day.
Equivalently, it should not be greater than 75 dBA during an 8-hour labor
day, provided that the rest of the day the level is kept considerably below
that figure.

This criterion differs substantially from ISO's in that it considers that
hearing loss has ocurred when the auditory threshold rises only 5 dB instead
of 25 dB. It is, thus, a far more strict criterion. 
   
It should be noted that the EPA limit is an *average*, meaning that much 
larger levels can usually be tolerated during brief periods of time. Indeed,
each time the duration of the exposition is halved, the safe level increases 
in 3 dBA, so it would be safe 78 dBA during 4 hours, 81 dBA during 2 hours, 
84 dBA during 1 hour, and so on. However, exposition to levels above 100 dBA 
even for very short periods are not recommended, since some liable persons 
may suffer immediate 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 technical or economical
feasibility to decrease noise levels at the workplace.

Finally, the EPA document not only addresses the hearing impairment issue,
but also the activity interference and annoyance one. In this case, it
states that the average noise level should be, outdoors, 55 dBA in the daytime, 
and 45 dBA in the nighttime, and indoors, 45 dBA and 35 dBA respectively.

Environmental noise bylaws tend to be, however, somewhat more permissive,
usually allowing for some corrections due, for instance, to the fact that 
the noise source has been there for many years, or, in the case of the motor
vehicles, that each vehicle pass by a given location only during a short
period of time.          




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