Quiet-List 1997

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Re: F.A.Q. for "quiet-list"

Dear friends,

Here are a few corrections to the quiet-list FAQ

>   The decibel (dB) is a measure of sound intensity; that is, the  
>magnitude of the fluctuations in air pressure caused by sound waves.  

I believe that some reference to the important difference between "dB" and
"dBA" (decibel measured after A-filtering the signal) is necessary, since 
most standards and regulations are based upon the "dBA" and not the "dB".

>Human perception of loudness is also logarithmic.

This is not true. 

Human perception of loudness is a quite complex phenomenon. First of all, it
depends strongly on frequency. At the same "physical" intensity, sounds in
the central frequency range (500 Hz to 5.000 Hz) are perceived as louder
than those outside this range. Second, the perception of loudness differs
substantially for sounds below 40 dB and above that sound pressure level
(SPL). The loudness is better measured in a unit called *son*. In the
central frequency range described above, roughly 40 dB equals 1 son. Above
40 dB, an increase of 30 dB implies a factor of ten increase in loudness. As
30 dB is the same as multiplying by 10 three times, that means that a factor
of 1,000 increase in intensity implies a factor of 10 increase in loudness.
Thus, the relationship is of a cubic root rather than a logarithm. So, 

     "Human perception of loudness varies as the cubic root of the 
      physical intensity of the sound (assuming sounds in the central 
      frequency range and SPL > 40 dB)."

Outside the central range and below 40 dB the whole thing is far more
complex than that so it seems to be useless to include any reference on it
in a general FAQ.

>   Distance diminishes the effective decibel level reaching the ear.  

I would rather say 

     "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." 
>At 120 decibels, the ear registers pain, but actual hearing damage 
>begins at a much lower level, about 85 decibels.

It depends on so many factors that I would leave this point to Q.5 

>   Because permanent hearing loss is usually a long-term process, it is
>impossible to know at exactly what point noise becomes loud enough to cause
>damage to the ears. However, many experts consider it unsafe to be exposed 
>indefinitely to a sound level of 85 dB or higher. 

This is a highly tolerant limit. I prefer the EPA (USA) limit of 70 dBA
(average) over a period of 40 years. First of all, I guess the word
"indefinitely" was not the intended one. 85 dBA indefinitely means two
things: first, you can never sleep, which in its turn implies severe health
problems after just a few days; second, it is equivalent to 90 dBA 8 hours a
day, which, according to the ISO 1999 standard, *guarantees*, for example,
that after 10 years a 10% of the exposed population will have hearing
impairments affecting speech intelligibility.

The EPA limit (70 dBA, 40 years) is the result of extrapolation of several
criteria, and insures that virtually the whole population is protected
against hearing loss. Since this is an *average* limit, for short exposures,
the safe level is obviously increased. Over 85 dBA, some rather liable
persons might begin to suffer an irreversible hearing loss when exposed
during as little as a single 8 hour shift (let alone the legally accepted 90

For an 8 hour labor day, the EPA criterion extends to 75 dBA, and for 1
minute of accidental or chance exposure, it sums up to 101 dBA (for example
walking 10 m apart from an operating pneumatic drill).   


Soon I'll be launching my Geocities home page including some own articles on
noise, and useful technical data. I'll make an effort to make that
information available on a bilingual basis (spanish-english). If you like,
I offer my page as another noise pollution resource.

Best regards,

Federico Miyara

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