Acoustic leak detection
| Acoustic leak detection | |
| Type | Detection method |
|---|---|
| Also known as | Leak noise detection, sounding |
| Detects | Sound of water escaping under pressure |
| Requires | Pressure in the pipe |
| Typical equipment | Ground microphone, listening stick |
Acoustic leak detection locates a leak by listening for the sound water makes as it escapes through a pipe wall under pressure. The escaping water produces a broadly continuous noise, which travels along the pipe and radiates into the surrounding ground or structure; an operator with a ground microphone or listening stick compares the sound at successive points and works towards the loudest. It is the oldest of the instrumented methods still in general use and remains the first method applied to most pressurised leaks. No statute or published standard defines the method, so the description below is of established industry practice and carries no citation; the leak detection code of practice that would be the natural reference is not available as a verifiable source.
Principle[edit]
The noise is generated by the pressure difference across the point of escape, so the method depends on the pipe being under pressure. A drained or depressurised pipe makes no leak noise, which is why tracer gas is used where a system cannot be pressurised.
Frequency content varies with pipe material and the size of the escape. Metallic pipes carry leak noise efficiently over long distances, so a leak can be audible far from where it actually is. Plastic pipes attenuate the signal much faster, which reduces the distance the sound carries and makes the loudest point a closer indication of the true position.
Equipment[edit]
A ground microphone couples a sensitive transducer to a hard surface and is used for buried pipes under paving, tarmac or concrete. A listening stick or contact probe is applied directly to a fitting, stop tap or exposed pipe. Both feed an amplifier with filtering that lets the operator attenuate traffic, plant and other ambient noise.
Interpretation is not automatic. The operator is comparing relative loudness across a series of points rather than reading a value, and a single reading in isolation carries little information.
Limitations[edit]
Soft ground absorbs sound and reduces the range at which a leak can be heard. Deeply buried pipes, plastic pipes and very small escapes all reduce the signal. High ambient noise limits the method in daytime urban conditions, which is why survey work on distribution networks is often carried out at night.
Because sound travels along the pipe, the loudest point on the surface is not necessarily above the leak. Where the pipe run is long or the material is metallic, leak noise correlation is used to resolve position from timing rather than from loudness.