Understanding
Acoustic Leak Detection
What are the Sounds
of Water Leaks?
Water leaks
in underground, pressurized pipes may make many different sounds:
- “Hiss” or “Whoosh” from
pipe vibration and orifice pressure reduction
- “Splashing” or “Babbling Brook” sounds
from water flowing around the pipe
- Rapid “beating/thumping” sounds
from water spray striking the wall of the soil cavity
- Small “clinking” sounds
of stones and pebbles bouncing off the pipe
The “Hiss” or “Whoosh” sound, which often
sounds like constant static noise, is the only one which is always
present for leaks in pipes with 30 psi or higher water pressure.
The other sounds may or may not be present, and usually they are
not as loud. So, we decide “Is there a leak?” by listening
for the “Hiss” or “Whoosh.”
Small
Leak on Cast Iron Water Main
What Factors Affect
These Sounds?
There are several
factors that affect the loudness and the frequency range of the
sounds made by water leaks transmitted on the pipes and transmitted
to the surface of the ground:
- Water pressure
in the pipe
- Pipe material
and pipe diameter
- Soil type
and soil compaction
- Depth of
soil over the pipe
- Surface
cover: grass, loose soil, asphalt, concrete slab, etc.
The loudness
or intensity of the leak sound is directly proportional to the water
pressure inside the pipe (up to a limit):
Sound
Intensity (loudness) vs. Water Pressure
Metal pipes,
such as iron mains, copper services, and steel pipes, transmit water
leak sounds that are louder and higher frequency than do PVC pipes
or asbestos-cement pipes. Thus, knowledge of the pipe material is
important.
Large diameter
pipes, whether they are PVC, concrete, steel, or iron, transmit
much less sound from water leaks than small diameter pipes. And,
large diameter pipes transmit lower frequency sounds than small
diameter pipes.
Sandy soil
and very loose soils, particularly over a freshly buried pipe
line,
do not transmit the sounds of water leaks very well, nor do water
saturated soils such as bogs and swamps. Hard, compacted soil
transmits
the sounds of water leaks best. Soil absorbs the sounds of water
leaks very quickly. Leaks in water lines that
are only 3 or 4 feet deep are much easier to hear at the grounds
surface than leaks in deeper lines. At 7 or 8 feet deep, only
very large leaks with good water pressure will produce enough noise
to be heard at the surface.
Finally, the
ground cover, whether it is an asphalt street, loose dirt, concrete
slab, or grass lawn, also makes an important difference. Hard street
surfaces and concrete slabs resonate with the sounds of the
water
leak, and the leak may be heard for 5 to 10 feet or more on either
side of the water pipe. Grass lawns and loose dirt surfaces
do not
offer such a resonating plate-like surface, and their surface variations
make firm contact more difficult.
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How Do Leak Sounds
Travel on Pipes?
Metal pipes,
particularly iron mains between 6 inches and 12 inches, copper
services,
and steel pipes transmit the sounds of water leaks for hundreds
of feet in every direction. Asbestos-cement pipe and PVC pipe
do
not transmit the sounds nearly as far.
Distances transmitted
for the “Hiss” or “Whoosh” sounds
of water leaks are a function of the pipe diameter as well as the
pipe material:
Pipe Material and Diameter |
|
Distance
Sounds Travel
for 2 GPM Leak at 60 PSI |
6 inch
Cast Iron Pipe |
|
600 to
1000 feet |
12 inch
Cast Iron Pipe |
400 to
800 feet |
24 inch
Cast Iron Pipe |
200 to
400 feet |
6 inch
AC Pipe |
400 to
800 feet |
12 inch
AC Pipe |
300 to
500 feet |
24 inch
AC Pipe |
100 to
300 feet |
6 inch
PVC Pipe |
200 to
300 feet |
12 inch
PVC Pipe |
100 to
200 feet |
24 inch
PVC Pipe |
50 to 100
feet |
Thus knowledge
of the pipe material and diameter is important to knowing how far
the leak sound may be transmitted along the pipe walls.
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How Do Leak Sounds
Travel Through Soil?
Soil absorbs
water leak sounds very quickly:
Soil absorbs
the high frequencies to a greater degree than the low frequencies.
For a leak in a
pipe 6 ft deep, the “Hiss” or
the “Whoosh” sound is weak and “muted,” i.e.
only the lower frequencies are heard. For a leak in a pipe
3 ft deep, the sound is louder and slightly higher in frequency.
Surveying
“Surveying” is
the term applied to listening for water leaks when there is no
obvious evidence, like water flowing on the street. Every hydrant,
valve, and service line is a possible location to hear the sounds
of water leaks:
Since the sounds
travel on the pipe walls better than through the soil, always
listen at the hydrants, valves, and meters first.
As you get closer to the leak, the sound gets louder. Finally,
decide which two of these locations are the loudest. Now you are
ready for “Water Leak Pinpointing.”
Surveying at a Hydrant
and a Service Line:
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Listening
for Leak Sounds at Hydrant |
|
Listening
for Leak Sounds at Meter |
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Pinpointing
“Water
Leak Pinpointing” is the term applied to the process of
pinpointing the exact leak location. For Acoustic Leak Detection,
the exact leak location is usually the spot where the leak sounds
are the loudest:
To find
this spot, the listener must carefully mark the location of
the
water line on the street after locating it exactly with a pipe
and cable locator. Usually, the piping between the valve or
hydrant
with the loudest sound and the valve or hydrant with the second
loudest sound is the section of the line that needs to be marked.
The section must be accurately located and marked on the street
in order for the listener to consistently listen directly over
the
pipe.
The listener
moves the ground microphone 3 to 4 feet each time in the direction
of the water line, listening, and moving closer to the water leak.
While the listener is moving, he does not adjust the volume
control,
since the volume control must be held constant in order to make
accurate comparisons. When the listener is very close to the
leak,
it may be impossible to decide based upon the users hearing
alone whether the leak is in one spot or in a spot 3 to 4 feet
away. When this occurs, the listener must study the visible display
(meter) to see if the signal is slightly stronger
at
one location than at another location.
Leak
Pinpointing Over a Hydrant Line
The loudness of a leak heard on an asphalt street or a concrete
slab depends upon the size of the leak, water pressure, and
depth of the pipe. Hard, dry materials like asphalt, concrete,
rock, and compacted soil transmit sounds better than wet clay,
sand, or loose soil. The sounds travel further on iron and
steel pipes than on PVC pipes or Poly pipes.
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