3
SOUND
GENERAL CHARACTERISTICS
Sound is mechanical energy in the form of pressure waves. You cannot see sound
waves, but you can form a mental picture of how sound works by comparing it to
ripples moving out in water when a rock is dropped into a quiet water surface. As the
ripples move away, they lose energy and get smaller. If they strike a solid object, they
reflect off that object and start traveling back in the direction they came from. Sound
waves act very much like these ripples in the lake. They radiate out in all directions
from the source of the sound. They grow weaker and lose more energy the farther
they travel, and if they bounce off objects, they will reflect back in the direction they
came from.
Thumpers used to locate cable faults create sound waves which radiate out from the
fault in all directions. When the high voltage thumper pulse reaches the fault and arcs
from the conductor to neutral, it creates an “explosion” which is a rapid ionization of
air. If this occurred in open air, it would sound like a large caliber rifle discharging.
Since the explosion caused by the thumper pulse occurs underground, the sound
waves are muffled and what the operator will hear most often is a soft “THUMP”. Thus
the name for the high voltage impulse generator = “Thumper”
Many times, the sound of the thump is loud enough in the area of the fault that the
operator can hear it without using equipment to amplify the sound. Sometimes, the
voltage arc at the fault releases enough energy to actually move the soil at ground
level. In these cases, the thump can be felt with the operator’s foot or hand.
SOUND TRAVELING IN DIFFERENT MATERIALS
Sound travels at different speeds and with greater ease or difficulty in different
materials. This can have an effect on the operator’s efforts to locate a cable fault.
1. Speed of Sound and Sound Resistance:
Sound waves travel at different
speeds in different materials. In open air, sound travels about 1,100 feet per
second (750 mph). In steel, sound travels at about 16,000 feet per second
(11,000 mph). In general, sound travels faster in hard or dense materials. Sound
waves also travel “easier” and with less loss in dense or hard materials. For
example, sound waves will travel farther in steel than in air before losing enough
energy that they can no longer be heard. For the same reason, sound travels
better in water than it does in air.
2.
Sound Reflections:
Sound waves are reflected when they hit any object.
Like ripples on a lake surface reflecting off a rock or floating object, the ripples
reflected will be much smaller than the original wave that hit the rock. Sound
waves traveling through soil will reflect off building foundations, underground
pipes, sidewalks, or even off the cement base of a pad mount transformer.
Summary of Contents for S.D.A.D.II
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