3
Guidance to Effective Acoustic Leak Detection
All acoustic leak detection methods are based on the premise that normal water
passage through pipelines takes place noiselessly. When the water passage is
disturbed a noise is created. Causes can include partial pipe blockages, sudden
changes in pipe diameter, abrupt changes in pipe direction, pumps or meters
installed in the pipeline, consumer usage or pipeline damage. Pipeline damage can
include holes, cracks or splits, complete pipeline rupture, leaking joints or leaking
valves.
Careful application of leak detection techniques will enable the operator to eliminate
detected noises generated by poor pipeline design or consumer usage and to
identify leakage due to pipe system damage.
The strength and clarity of noise generated by leaks will be affected by the water
pressure, the size and shape of the orifice allowing leakage, the type of ground
material around the pipeline, the type of ground cover over the pipe, the diameter,
wall thickness and material of the pipeline and the quantity of water leaking.
A small orifice or hole and high water pressure generally produces a higher
frequency noise. Often the noise level increases around valves, pipe elbows, T
connections, pipe ends, etc. since the partial obstruction increases pressure and
creates some further disturbance in the water path.
Leak noise is transmitted along the pipeline both through the water and the pipe wall
as well as into the ground around the pipe. The noise travels much better through
"hard" materials so that the noise travels much further along metallic pipes than
asbestos cement pipes which themselves are better than plastic pipes. Ground
material generally provides a poorer travel path than the pipeline itself. Soft sandy
ground provides a worse travel path than well compacted ground with a hard paved
surface covering.
The leak noise can change in strength and pitch as it travels along the pipe or
through the ground. The deeper the pipe is buried and the softer the ground the
more the noise will be dampened.
When a leak is produced in a metallic pipe the leak noise will transmit well through
the pipe. It does not travel so well through a plastic pipe. This means that a leak
noise can be heard further away on metal than on plastic. Also bear in mind that the
further you go from a leak noise source the more difficult it is to pinpoint that leak
noise accurately.
Background noise can interfere with leak detection. Traffic and machinery noises can
travel for considerable distances through both air and ground material and often
occur in the same frequency bands as leak noise. Sometimes it is necessary to use
leak detection techniques at night when interfering noises are less.
It is very important to adopt a methodical approach when using any instrument for
acoustic leak detection. It is necessary to practice the technique in order to
distinguish between different sounds, recognising background or interfering noises,
