Heath Consultants Aqua-Scop - Manual (Page 27)

Influence of the Leak Noise (Material of the Lines):
You should know the material of the line being tested as the sound
transmission of leak sounds vary greatly from cast iron, to steel, to asbestos
cement, and PVC. A simple example to clarify this: If a steel line, suspended
from a cable is struck with a hammer, it will sound like a gong. A plastic or
asbestos cement line would give a thumping noise on impact but it wouldn’t
resonate the sound at all.
Pressure in the Lines: The pressure in the line is also a very important factor.
High pressure normally gives a loud noise, a high frequency, and good sound
characteristics for localization. It generally can be said that a pressure of
twenty pounds per square inch (PSI) is sufficient enough to locate leaks.
Under twenty PSI it may be necessary to use auxiliary means.
Special Circumstances Encountered: The conditions of the cover over
the main will especially influence the detection of the leak noise. A very
hard ground has a better efficiency for sound transmission and is of higher
resonance than muddy ground or sod. Sand and compact sand and stony
grounds are very efficient sound transmitters.
On the other hand, the transmittance of sound under frozen ground may be
so excellent that it is very difficult to locate the maximum (Loudest Point)
since a large section of the frozen earth will be vibrating.
Surface: The surface in the environment can greatly influence the detection.
It is necessary when pinpointing to consider the surface on which the Ground
Microphone is placed. A change in the kind of surface (from sod to asphalt,
for example) from one medium to the other may cause a very contradictory
test value. This can sometimes cause so many differences of noise that a
real comparison of noises for the leak detection may be difficult.
Time of Work: The choice of day or night testing is very dependent on the
local situation. In rural or residential areas work during the day is possible.
In areas with heavy traffic noise, loud machines, and/or factories, work
during evening or night hours may be necessary.
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