Heath Consultants Aqua-Scop - Manual (Page 20)

Absorption of sound by materials is a science in itself. Terms often
stated by acoustical wall manufacturers are seal and absorption loss.
Seal refers to sweep strips used on doors to attenuate the transmission
of higher frequencies. Absorption loss or door rating in decibels tends to
increase as the number of laminates (metal, wood, fabric, etc.) increases.
The higher frequencies are attenuated more dramatically than low
frequencies. Sheet lead is often used to reduce plenum crossover.
Finally, the effect of paved surfaces on sound pick-up should be
discussed. For lack of a better name, let us call it the Diaphragm Effect.
Because of the large area and low modulus, the resonant frequency will
be low. This surface, therefore, tends to amplify noises of low frequency
occurring in the soil beneath the paved area and helps immeasurably
in water leak location.
B. Relating Sound Principles to Leakage Detection
The detection and location of underground leaks is implemented by a
variety of instruments and numerous methods. For the purpose of this
section, we will discuss the use of the AQUA-SCOPE and some of
the general conclusions we might draw from the previous section in
that regard.
The Direct Contact Microphone is generally placed against the surface of
a pipe in a position nearly perpendicular to the pipe direction. The sensor
responds to vibration along the axis of the Direct Contact Microphone.
Therefore, the instrument is most sensitive to transverse sound waves
on the pipe. Likewise, sound induced by leaks will be oriented more
likely in a transverse mode. Now no sound can be transmitted through
the fluid or gas in the pipe in a transverse mode based on the previous
section. This means that the pipe wall must carry any transverse sound
transmission alone.
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