How Underwater Sound Systems Work

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.

Normal speakers don't fare all that well at the bottom of a swimming pool, so it should come as no surprise to learn that underwater speakers come specially designed to not only deliver quality audio in a submerged environment, but also to avoid shorting out, electrocuting swimmers or turning to driftwood.

It's a rather niche corner of the audio market, so you can expect to drop anywhere between $300 and $2,000 on a quality underwater speaker. Typical models consist of a watertight, waterproof plastic speaker connected by a length of insulated speaker wire to an above-water sound system. You might choose to install the speaker inside an alcove in the pool -- in which case you might use an acoustic sound lens to direct the sound waves out into the pool -- or you might just attach the speaker to a rope and dangle it into the water.

A good underwater speaker won't sound all that great if you use it out in the air. In fact, it might sound downright awful. According to Clark Synthesis President Bill Phillips, a quality underwater speaker is more like a fine wooden instrument such as a cello or violin. The sound radiates off the entire surface area of the speaker rather than from a focused point.

The volume of water you can cover with a speaker varies. For instance, Clark Synthesis recommends one of its Diluvio pool speakers for every 20-foot-by-20-foot (6-meter-by-6-meter) section of the pool. The Lubell System 3300 Diver Recall System -- intended for use on dive boats -- can sound a siren heard across an entire 1-mile (1.6-kilometer) underwater radius, assuming optimal conditions. Dive speakers such as these enable instructors to communicate with other divers, but they can also blast funky beats for man and fish alike.

Jacques Cousteau may have dubbed it "the silent world," but evidently he never thought to crank "Yellow Submarine" from the bottom of the Calypso.

Each year, divers converge on Florida's Looe Key Reef for the Underwater Music Festival, and boat-mounted Lubell speakers crank the tunes for more than 300 divers and snorkelers [source: Lower Keys Chamber of Commerce].

But hey, it's not all classic rock and rowdy scuba divers. French composer Michel Redolfi is something of a specialist in underwater music. His albums combine natural ocean sounds with electronic ambience to create soundscapes specifically designed for underwater broadcast.

Redolfi's concerts, as you might imagine, typically take place underwater -- often in swimming pools, but also in the ocean itself. His 1984 album "Sonic Waters" includes hydrophone recordings of an original electronic piece broadcasted through the waters off the coast of La Jolla, Calif. In collaboration with music engineer Daniel Harris, Redolfi debuted multiple musical instruments designed exclusively for use in an underwater environment. These included the SOSNO underwater percussion instrument (named after its designer) and the electronic DUCS (Digital Underwater Controller System). His 1992 opera "Chrysalis" even involved a soprano submerged in a 2-ton plastic bubble.

And yet Redolfi isn't the only artist to dabble in the world of underwater opera. Back in 2009, Juliana Snapper debuted in an opera titled "You Who Will Emerge from the Flood." The performance took place in a Manchester, England, swimming pool, with Snapper using "mouth-to-water" singing that required intense research and "hours submerged in my bathtub and borrowed pools" [source: Helmreich].

Not to be outdone, German swimmer-turned-opera-singer Claudia Herr debuted her underwater opera "AquAria Palaoa" in Berlin on May 1, 2011. The finished piece is a combination of opera, underwater musical performance and synchronized swimming.

And yet all Harry Houdini would have heard inside his Chinese Water Torture Cell was the muddy sound of the outside audience and the clink of his chains.

I dearly wanted to work a bacta tank reference into this article, but alas there's only so much room for silliness in an article. This is especially true when the science involved is so fascinating. I have to admit, I wasn't expecting this one to stir my mind all that much, but I found the science of underwater acoustics really fascinating.

I'd like to thank Clark Synthesis President Bill Phillips for taking the time to chat with me about underwater sound system technology. He was nice enough to straighten me out about some of the finer points. When personally testing out speaker installations for the U.S. Synchronized Swimming Team, he likes to play some Vivaldi on both air and underwater speakers, dipping his head in and out of the water until the sound quality in the pool matches that at poolside. I love that commitment to sound quality.

Oh, and if you'd like to explore the underwater sounds of composer Michel Redolfi, I encourage you to explore the musician's official Web page. You can also sample his sounds on his YouTube page.

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