**** Whistle Pitch Audibility ***

Date: Wed, 21 Feb 1996 22:49:46 -0500
From: Grenzstein@aol.com
To: sar-l@islandnet.com
Subject: Re: Whistles and voices
Message-ID: <960221224944_228218044@emout05.mail.aol.com>

>How about a whistle that uses low(er) pitches?
>I feel that railway whistles (??, meaning old-style metal whistles
>containing a pea) carry better than very high pitched whistles.
>Does anyone know of any papers on this subject?


Having been a music major in college, I studied sound and acoustics. Low
frequency sound definitely carries farther than high frequency. High
frequencies are usually more readily absorbed by surfaces too. The
absorption coefficients vary greatly between materials. Unfortunately, my
text books don't give an absorption coefficient for fauna. Air itself,
while carrying sound, also absorbs it. The rate of sound absorption of air
varies withtemperature and humidity.

A modern, typical example of low frequency sound carrying farther than high
frequency, is that car with the zillion watt sound system approaching from
far away. First you here that booming bass drum beat, then maybe the bass
guitar or low end synthesizer. As the car approaches you begin to hear the
mid range instruments and lastly the highs. You may hear that bass drum for
a long time before you hear any other sound from that car and for a long
time after the car disappears.

The average human ear can detect freqs from about 15 cps to 15,000 cps. As
we age, that band narrows considerably at the high end. The problem that
arises is even a low pitched whistle will not produce fundamental
frequencies near the very low end of the audible frequency range. Also,
frequency alone is not the only consideration. The sound pressure level
behind the frequency is also important. A low frequency whistle with a low
sound pressure level may not carry as far as a higher pitched whistle with
a higher sound pressure level.

It is necessary to find the acceptable medium. Producing the lower
frequencies with a whistle will require, of course a larger whistle which
will require more "lung power" to produce the sound pressure necessary to
carry the frequency the greater distance. And an instrument capable of
producing these lower frequencies would have to be so large as to make them
impractical for using in SAR. A perfect example is the Swiss Alpine horn
which is about 10 or 12 feet long, give or take a foot. These horns were
specifically designed for communicating in the Swiss Alps over long
distances.

The best method for testing these whistles scientifically would be with
sound equipment, even in the field. Human testing would be subjective at
best since all humans do not hear alike. All of our ears have different
capabilities due to age, possible ear damage, etc. Five tests in the same
conditions with five persons would most likely yield five sets of results.

To summarize, IMHO, if you are going to use whisltes, the lowest pitch
whistle that produces an acceptable sound pressure level, with the most
distinctive sound, should be used. One extra thought; the older the victim,
the lower the pitch of the whistle should be for most effectiveness,
especially in elderly victims. If you have a good baritone who really knows
how to use his voice in your unit, you may be inclined to use him. The
trained human voice is capable of producing freqs well below whistles with
adequate sound pressure levels.

Pete Bremy
West Milford, NJ

              ---------------------------------------------