Acoustic Ground Microphone Technology

Basic Principles of Acoustic Water Leak Detection Technology

To understand how acoustic water leak detection works, one must understand the interplay of following four factors: the different types of sounds water leaks make, the various factors affecting the sounds, how the sounds travel down pipes, and how the sounds travel through the soil. Each one of these factors is explained below in full detail.

1. The Various Sounds Water Leaks Make

Pressurized water lines with a leak often make one or more of the following 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 subsurface material.

• Small “clinking” sounds of stones and pebbles bouncing off the pipe.

The “Hiss” or “Whoosh” sound, a constant static noise, is the only one of the four always present in pipes with a 30 psi or higher water pressure. The other sounds may or may not be present, and if they they are, they are usually not loud enough to detect. Therefore, when using a ground microphone to listen for a leak, one usually listens for the “Hiss” or “Whoosh" sound.

2. Factors Affecting the Sounds

The following six factors affect the loudness, or sound intensity, and frequency of the sounds:

a) Water Pressure - As shown on the graph below, the loudness of the leak is directly proportional to the water pressure inside the pipe (up to a certain limit):

b) Pipe Diameter - Large diameter pipes, regardless of whether or not they are composed of PVC, concrete, steel, or iron, transmit less sound from water leaks than do small diameter pipes, but they transmit lower frequency sounds better than small diameter pipes.

c) Pipe Depth - Soil absorbs the water leak sounds very quickly. Therefore, leaks in water lines that are only 3-4 feet deep are much easier to hear at the ground’s surface than leaks in water lines that are 7 - 8 feet deep. At this depth, only very large leaks with good water pressure will produce enough noise to be heard at the surface.

d) Pipe Material - Knowledge of the pipe material is important. Metal pipes, such as iron mains, copper services, and steel pipes, produce louder sounds and higher frequencies than do PVC or asbestos-cement pipes.

e) Soil Type and Compaction - Sandy soils, saturated soils (such as bogs and swamps), and very loose soils--specifically those covering a freshly buried pipeline--do not transmit water leak sounds that easily. Conversely, hard compacted soils transmit water leak sounds quite easily.

f) Surface Type - Finally, the surface type, whether it is an asphalt street, loose dirt, concrete slab, or grass lawn, also affects sound transmission. 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 because their surface variations make firm contact more difficult.

3. How Leak Sounds Travel Down Pipes

Metal pipes, particularly iron mains between 6 to 12 inches, copper services, and steel pipes, transmit water leak sounds for hundreds of feet in either direction, but asbestos-cement and PVC pipes do not transmit the sounds nearly as far. Distances transmitted for the sounds of water leaks are a function of pipe diameter and pipe material. The chart below summarizes this relationship. Hence, knowledge of the pipe material and diameter is needed in order to determine how far the leak sound may be transmitted along the walls of the pipe.

4. How Leak Sounds Travel Through the Soil

As shown on the figure below, soil absorbs water leak sounds very quickly. High frequency sound waves are absorbed by soil to a greater degree than low frequency sounds. For a water leak in a pipe that is 6 feet deep, the “Hiss” or the “Whoosh” sound is weak and muted, so only the lower frequencies can be heard. Conversely, for a leak in a pipe 3 feet deep, the sound is louder and slightly higher in frequency.

The Itron Digital Leak Detector (DLD)

We use the DLD for our basic leak surveys and leak pinpointing. This piece of equipment has a digital audio processor that uses dynamic range compression to accentuate leak sounds, reduce loud noises, hear leaks missed by other instruments, block ambient noise, and automatically reject electrical interference.

Surveying for a Water Leak with the DLD

“Water Leak Surveying” is the term applied to listening for water leaks when there is no obvious physical evidence of one. Every hydrant, valve, and service line is a possible location to hear the sounds of water leaks. Since the sounds travel along the pipe walls easier than they do through the soil, the technician first listens at hydrants, valves, and meters. As he or she gets closer to the leak, the sound gets louder. Once it has been determined at which of these locations the sound is the loudest, the technician is ready to commence “Water Leak Pinpointing" at this spot.

Pinpointing an Underground Water Leak with the DLD

“Water Leak Pinpointing” is the term applied to the process of pinpointing the exact location of a water leak. As shown below, when using an acoustic leak detection ground microphone, the exact leak location is usually the area where the leak sounds are the loudest. To perform this procedure, the technician moves the ground microphone across the water line in question and takes readings every 3 to 4 feet along its entire length, making sure not to adjust the volume control (since it must be held constant to make an accurate comparison of each increment). Once the technician moves close to the spot of the leak, it may be possible for he or she to pinpoint it by using hearing alone or, if listening is not sufficient, the visible display (meter) to find the area of peak response. Once the area of peak response is found, by either or both of these two methods, the location of the leak is marked out on the ground with pink paint and/or flags.

Advantages of Acoustic Ground Microphone Technology

• Confirms whether or not a leak is present.

• Quickest and easiest way to search for, detect, and pinpoint a leak.

• Water service is not interrupted.

• Works best on small diameter metallic pipes.

Limitations of Acoustic Ground Microphone Technology

• Background noise can interfere with leak sounds.

• Pressure must be high enough for the leak to produce a sound.

• Maximum depth for pinpointing is 5-8 feet for 1-3 gallon per minute (GPM) leak.

• Pinpointing over soft soil or grass cover is more difficult.

• Pinpointing is sometimes only possible at night.

• Difficult to find leaks in non-metallic piping.

• Takes years of experience to develop an ear for hearing and pinpointing leaks.