The Science of Leak Noise: Some “Sound” Advice for Acoustic Leak Detection

sound waves

By Rob Meston

Effective water loss control (WLC) is one of the most critical activities every water system must face, particularly as droughts increase and conservation mandates become more stringent. Leak detection has quickly become a cornerstone of every WLC program. No matter what type of equipment is used, procedures and methods for acoustic leak detection should be adjusted to meet the specific system being surveyed, for the best results (detecting more leaks).

Leak sounds are directly related to the type of pipe, size of pipe and pressure inside the pipe. Typically, water flowing through a pipe does not make noise, unless disturbed. Disturbances can include (but are not limited to):

  • Sudden changes in diameter (typically with higher flows)
  • Sudden change in direction (typically with higher flows)
  • Flow through pumps
  • Flow through meters
  • Service draw
  • Throttled valves
  • Pressure Reducing Valves (PRV’s)

Leak sounds occur as turbulence is created at the leak location when the inside and outside pressure are trying to equalize. These disturbances range in frequencies. For example, leaks in metal pipes transmit sounds at a higher frequency. In cast iron, copper and steel pipes, you are most likely to hear leak sounds in the range of 500 Hz to 1,500 Hz, although under varying conditions, the frequencies may be higher or lower. In PVC pipe, leak sounds typically resonate in the range of 70 Hz to 850 Hz. Under certain conditions, some small leaks; such as pinholes on steel mains, can be much louder than a large leak on a cast iron main due to the restriction at the point of the leak.

The sound generated from leaks is transmitted through the pipe wall, surrounding soil structure as well as the water itself. Leaks sounds will typically travel farther in rocky soils and soils with a relatively low water table.


Leak sounds typically travel farther in metal lines, lines with higher pressures and smaller diameter lines. All of this should be considered when conducting an acoustic water leak survey. The following list illustrates good, fair and poor contacting practices:

1. Main Contact – GOOD
It’s rare to have direct main contact unless pot holes have been added or the lines are exposed at bridges, river crossings, etc., which tend to amplify ambient noise.

2. Gate Valves – GOOD
Other than the main line, this is the best contact point. If valve is dirty, clean or use aluminum rods not valve key, or steel probe, which tend to amplify ambient noise.

3. Hydrant Gate Valves – GOOD
As with Gate Valves, these are the best contact point besides direct main contact. However, these should not be relied on as the only point as hydrants are usually 300 ft or more apart.

4. Service Shutoff (Curb-Stops, Etc.) – GOOD
When the line is metal, these are GOOD. When the line is plastic, these are POOR.

5. Fire Hydrants – FAIR/POOR
Wet barrel fire hydrants are FAIR. Dry barrel fire hydrants are POOR. Both tend to amplify ambient noise, making it easier to miss low level (quieter) leak sounds.

6. Blow offs – POOR
As with hydrants, these tend to amplify ambient sounds. If the blow-off has a valve, the valve should be sounded.

7. Air Release or Vacuum Valves – POOR
These are usually found on transmission mains and mains with elevation changes.

8. PRV’s – POOR
These usually sounds like a leak as the pressure on one side is lowered as it passes through the valve. If flow is high and the pressure differential is significant, it is very easy to miss a leak near a PRV, particularly if the PRV can’t be by-passed.

9. Probe – FAIR/POOR
A probe is often used on lines with limited access when the line runs under soil and there is a lack of appurtenances. Probes tend to amplify ambient noise. Probes can be difficult to drive into hard soils.

10. Ground Mic – FAIR/POOR
A ground microphone is often used on lines under pavement or hard cover. Sound typically dissipates quickly as you move away from the leak, so the line location must be known. Ground microphones tend to amplify ambient noise.

11. Other – POOR
Hose Bibs, valve lids, service lids, etc., usually don’t have contact with the line itself and tend to amplify ambient noises.

It’s important to note that some systems have limited access and the access points (air vac, blow-off, etc.) are FAIR or POOR. These should be sounded if access is limited, regardless of the contact quality. For the best survey results, it is recommended that the following guidelines be used for an acoustic leak survey:

PVC Pipe – Every available appurtenance should be sounded (often called a “point-to-point” survey).

Cast or Ductile Pipe – Sound all valves and all hydrants and services where hydrants and valves exceed 300-400 ft. If system pressures are lower than 40 psi (as a rule of thumb), these distances should be shortened.

Metal Pipe with Plastic Services – It is highly recommended that a point-to-point survey be conducted. It is common to detect a leak on a plastic service line at one location but not be able to detect it at the next service.

Asbestos Cement – If system pressure is good, sound travels well on this pipe and survey distances should not exceed 200-300 ft where access is readily available.

A Few Key Points to Consider…

  1. Leaks don’t always make noise, or the level of noise is too low and can’t be detected with even the most sensitive and sophisticated equipment. This is often true when water pools around a leak site. This can act as insulation and muffle leak noise.
  2. Unless you’re looking for a large leak in a short period of time, avoid “hydrant to hydrant” surveys. As noted above, these are not good contacting points and while leaks nearby may be detected, it’s also very easy to miss leaks with this approach. This is a common practice for some companies who provide acoustic leak detection services. It allows them to quickly survey a system, reducing the time on site and ultimately lowering their price, particularly when presented in a competitive bid.
  3. Avoid companies that guarantee they will find every leak. This is impossible. As noted above, leaks don’t always make noise.
  4. Never take anything for granted. Make sure to isolate valves to eliminate flow through crosses, tees and throttled valves as these can sound like leakage.
  5. Get proper training from a reputable and highly experienced professional or a highly experienced service provider. Equipment manufactures often hire sales staff with little to no field leak detection experience. This makes investment in equipment a waste of capital. The equipment will often sit on the shelf.
  6. Get regular training to ensure staff is using the equipment correctly.

In conclusion, no two water systems are alike. Procedures and methods must be modified for each system to increase the likelihood of finding more leaks.

Rob Meston is president of Utility Services Associates. Meston has been providing acoustic water leak surveys since 1990. He has surveyed thousands of miles of pipe and found thousands of leaks since that time. He has extensive experience using all technologies. Meston also provides training to utilities throughout the country.

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