Sniffing by Sound

When an underground gas main springs a leak, the break may not be found before enough gas has escaped to explode with devastating force. All over the U.S., gas companies are constantly on the alert for any gadget capable of pinpointing a small leak before it balloons into a big blowup.

Last week engineers from Illinois Institute of Technology were field-testing the most sensitive leak seeker yet, a device that does its sniffing by sound. The Illinois scientists discovered that if sound waves of 40 cycles per second (a frequency just above the piano's middle G) are fed into a gas main, they will travel down the pipe, escaping along with the leaking gas.

The escaping sound is extremely weak, but while microphones set on the ground are checking a suspected pipe, a portable radio transmitter broadcasts a tone of the same frequency as the test note. An automatic electronic device compares the transmitted sound with sound picked up by the microphones. If the sound coming out of the ground proves to contain a significant amount of the tracer frequency, it cannot be blamed on leaking water or distant traffic or any other background noise. It can come from nothing but a nearby leak in the sound-filled gas pipe.

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