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How Underwater Sound Systems Work


Funky Sounds in Your Pool
Live underwater music? Too expensive.
Live underwater music? Too expensive.
Alan Band/Fox Photos/Getty Images

Can we really hear music on the bottom of a lake or a swimming pool? Sure we can.

Sound travels through both air and water. In fact, sound waves move 4.3 times faster underwater than they do through the atmosphere [source: NOAA]. Whales and dolphins use sound to navigate and communicate in the ocean, and humans employ sonar (sound navigation and ranging) technology to map undersea topography and the movements of both animals and ships.

Yet for all our submarines and snorkel gear, we humans are creatures of the air. We evolved to process sound data on land, so we simply don't hear as well at the bottom of a river -- despite the ability of some divers to have ultrasonic hearing underwater.

Just consider Parliament's hit 1976 single "P. Funk." As front man George Clinton point outs out in the lyrics, the song is in fact "doin' it to ya in the earhole." The sound waves vibrate the audio bones in our inner ear, an effect we call air conductivity hearing. When you're underwater, however, flooding in the outer ear prevents a lot of that necessary vibration and essentially nullifies your air conductivity hearing ability.

Fortunately, George Clinton and company don't just do it to us in our ear bones; they also do it to us in our skull bones. We call this bone conductivity, the conduction of sound to the inner ear through the bones of the skull. This mode of hearing is 40 percent less effective than air conductivity, but it's still the primary way we hear underwater [source: NOAA].

How does it sound? Well, the limits of bone conductivity make stereophonic sound impossible underwater. The skull provides only a single source of sound transmission, whereas air conductivity hearing provides two -- one in each ear. But that doesn't mean the sound itself is monophonic -- rather it's what Stanford University music researcher John A. Maurer IV referred to as omniphonic sound [source: Maurer].

In a swimming pool, fast-traveling sound waves leave the underwater speaker and bounce off the bottom of the pool, the surface of the water and each side of the pool. The sound reaches the listener from all directions, and the human brain simply can't process the original sound source.

The range of typical human hearing spans from 20 hertz to 20,000 hertz. Some varieties of underwater speakers, especially older models, can't produce wavelengths lower than around 1,000 hertz. To put that in perspective, the lowest and highest frequency notes on a typical 88-key piano are 27.5 hertz and 4,186 hertz. However, underwater speakers like the Clark Synthesis Diluvio employ voice coil technology -- essentially an electromagnet composed of coiled wire -- to reach wavelengths as low as 20 hertz.


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