Heath Consultants Aqua-Scop - Manual (Page 17)

III. PRINCIPLES OF SOUND:
A. Engineering References
This section will cover the basic physics of sound. Section B will
attempt to relate a few of these principles to the field of underground
leak detection and pinpointing.
Wave motion, of which sound is but one example, is classically
regarded as either transverse or longitudinal. In transverse wave
propagation the wave travels perpendicular to the motion of the
source (a plucked string is an example). Longitudinal waves
progress along the axis or body of the media (listening for a train
on its tracks is an example).
The audible limits of sound frequency for people are approximately
from 20 to 20,000 hertz. The human ear responds to sound pressures
from 0.0002 to 2000 dynes/cm (seven orders of magnitude). Sound
intensity is generally expressed on a logarithmic scale (Decibels,
or dB) with the reference intensity taken to be 10-16 watt/cm, so
that:
INTENSITY LEVEL IN DECIBELS = 10 log
In addition the sound pressure (with the reference pressure taken as
0.0002 dyne/cm) is given by:
SOUND PRESSURE LEVEL IN DECIBELS = 20 log
An upper limit exists for this sound pressure depending on the media
involved. For air the upper limit is approximately 191 dB. At this level
rarefaction of the wave causes cavitations and the wave breaks down since
the media can no longer support it. The pain threshold for audible sound is
at an intensity level of 120 Decibels.
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CHAPTER TWO