LOKAL 400 Operating Manual
Page 30
16. Information for Use in the Field
16.1. Acoustic Leak Detection
16.1.1. How Sound is Created
When there is a leak in a high-pressure pipe system, water that leaks out generates sound that is caused by the friction that
occurs when the water escapes through the hole at high speed. This also leads to oscillations in the pipe itself. The sound
travels along the pipe and can be picked up at contact points such as valves, hydrants, fittings, etc. which can actually be quite
far from the actual leak itself and can be transformed into audible sound by structure-borne sound microphones.
16.1.1.1. Ground-borne Sound
When water that is escaping from a leak comes into contact with the ground, this causes the parts of the ground that it comes in
contact with to oscillate. The sound spreads out from the leak in circles and can be picked up by a ground microphone near the
actual leak. The frequency of these signals lies between 30 and 700Hz.
Frequencies whose wave lengths are smaller than the depth that the pipes have been laid in are strongly absorbed because of
the low-pass effect of the surrounding soil. This means that only the low frequencies normally reach the surface of the ground.
If the water has been escaping from the leak for a relatively long period of time, a water blister sometimes forms and continues
to contain the water that is leaking from the pipe. The sound that is transmitted through the ground is therefore barely audible
and is almost impossible to pick up with a ground microphone.
The sound caused by the leak can be made audible again if compressed air can be fed into the pipe system that is being
examined. In such a case, the compressed air is fed into the pipe system with a minimal amount of overpressure via a hydrant
or house connecting point. This combination of water and air generates a sound at the leak which is clearly audible.
The diagram below shows the influence of the ground formation on the distance that the ground-borne sound waves can travel
with reference to the leak frequency in metres. Low-frequency sounds spread out further than high-frequency sounds, and
compact ground conducts the sound to the surface better than ground which is not as compact.
16.1.1.2. Structure-borne Sound
Structure-borne sound oscillations occur when water escapes from a leak with high pressure and at a correspondingly high
speed which causes the pipe in question to oscillate.
The sound that is generated at the point where the water is escaping spreads out to all sides of the pipe. Small-diameter or thin
steel pipes will oscillate strongly and the sound the leak is causing can be picked up at a considerable distance from the leak
itself. Thick pipes or especially pipes made of plastic material, on the other hand, do not oscillate as strongly and the sound
generated by the leak does not spread out nearly as far.
The frequency and the material of the pipes play a major role as far as the distance the sound travels is concerned. As is the
case with ground-borne sound, low-frequency sounds travel further distances, and softer materials like PVC or PE pipes
absorb the energy caused by the leak more strongly than metallic pipes.
