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3. Sound in Air:
Compared to many other materials, air is a very poor conductor
of sound waves, or what we call “sound energy”. Place your ear on a railroad
track and you will hear a train several miles away. Stand up and you will see
the train before you hear it. This is the reason deep snow can be a problem to
a cable fault.
FACTORS AFFECTING SOUND IN FAULT-LOCATING EFFORT
LOCATION OF THE FAULT ON THE CABLE
1. Fault on top of the cable.
An easy fault to locate as all of the thumpers energy
is coming straight up. You can normally hear this type without a listening device.
Most of the time you can feel the pressure wave with your foot or hand.
2. Fault on the bottom of the cable.
This fault is sometimes hard to hear because
the thumpers pressure wave goes down and reflects back up, much of the
energy is lost in the soil at the point of reflecting.
3. Fault on the side of the cable and the thump is arcing into another cable
phase.
The cable on the side will absorb most of the energy and very little of the
sound wave is available to the operator. The operator will not be able to hear the
thump without a good acoustical instrument of some type.
SOIL TYPES
Some types of soil can muffle the sound created by the “thump” more than other
types. Dry, porous soil like sand is a good example. Even though grains of sand by
themselves are dense and would be good conductors of sound if they were packed
together, sand is filled with tiny air pockets that are poor conductors of sound. After a
rain storm, the tiny pockets are filled with moisture and the overall sound conducting
ability will improve.
TEMPERATURE
Frozen soil is generally quite easy to locate in because the ground is quite hard
with ice frozen around the grains of soil and ice is a very good conductor of sound,
however this is not always the case. Repeated freezing and thawing can create air
pockets due to soil’s movement during the freezing and thawing process.
ASPHALT & CONCRETE SURFACES
How well a thump sound can be heard above an asphalt or concrete surface depends
on how firm this material is in contact with the soil, or how much air space is between
the soil and bottom side of the material the microphone is set on. Dead air spaces can
be created under these surfaces by soil settling or by soil surface movement caused
by freezing and thawing.
