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An Algorithm That Makes Voices Clearer

A Denver company finds a good use for high-frequency harmonics.
November 28, 2007

As people age, they lose the ability to hear in the higher frequencies. Turning up the volume on the TV set or stereo is a stopgap measure, but in the long run it only aggravates the problem. Able Planet, a company located near Denver, has developed analog circuitry that makes the high-frequency components of speech clearer without increasing their loudness. The technology is built into a line of headsets, telephones, and assistive listening devices aimed not only at the elderly but also at younger people who are worried about hearing damage, and even at video gamers who want to hear each other over the din of virtual battles.

Spare your ears: A line of headphones from Colorado company Able Planet increases the clarity of high-frequency audio without increasing its volume. The technology is aimed not only at the elderly, but also at younger people who hope to avoid hearing damage and at video gamers who want to hear each other over the din of virtual battlefields.

The loss of high-frequency hearing is rooted in the inner ear, which consists of a hollow tube curled up into a snail shape. The tube is called the cochlea, the Latin word for “snail,” and it’s filled with thousands of tiny hair cells that resonate to sound. The eardrum’s vibrations are transmitted to the cochlea by three tiny bones; as the cochlear hair cells vibrate, they stimulate nerve endings, which send information to the brain.

The hair cells at the base of the cochlea are the ones that resonate to high-frequency sounds. Since sound enters the cochlea at its base, the high-frequency hair cells get the brunt of the incoming sound pressure. Over time, that causes the cells to deteriorate. High-pitched parts of speech such as the “s” and “sh” sounds become harder to hear; the word “cats” begins to sound like “cath.” Women’s voices become harder to hear than men’s; violins become harder to hear than bass guitars.

Turning up the stereo volume just makes the hair cells break down faster. For that matter, high volume also induces distortion, so it’s not a good solution even in the short run.

Able Planet’s technology offers an alternative. Called Linx Audio, it works by enhancing the harmonics of the high-frequency sounds. A harmonic is a sound spontaneously generated at a multiple of the vibrational frequency that caused it. For example, a piano string vibrating 100 times per second (that is, creating a 100-hertz tone) will also spontaneously generate tones of 200 hertz, 300 hertz, and higher. These additional tones are the harmonics. Virtually all complex sounds, speech and music included, consist of an original tone–called the fundamental frequency–and its harmonics.

Remarkably, the ear does not hear a fundamental frequency’s harmonics as separate tones, even though they are detected in different parts of the cochlea. The cochlea adds them together, or synthesizes them, so that the brain hears a single unified pitch. This is the idea behind Linx Audio. By enhancing the harmonics of the high-frequency sounds, it stimulates more areas of the cochlea. So if the user has lost a patch of hair cells in one area–and thus the ability to hear a particular tone–the ear can still reconstruct the sound if it is getting the tone’s harmonics. Thus, the “s” sound in “cats” is more likely to be heard, and speech will sound clearer. (If the user has lost all of the hair cells that pick up a fundamental frequency and each of its harmonics, this strategy won’t work. But hearing loss is almost always partial rather than complete; there is usually some degree of hair-cell survival throughout the cochlea.)

On its website, Able Planet likens the technology to replacing a single C note on a piano with a C chord created by playing several C keys together. If the listener can’t hear one of the C notes because of cochlear damage, she’ll still hear the chord because of the redundancy of having several “versions” of the note.

This focus on enhancing the harmonics distinguishes Able Planet’s products from devices that improve clarity by using noise-reduction algorithms that generate out-of-phase waveforms to cancel out undesired sounds. (Many of Able Planet’s devices offer active noise cancellation as well.)

Kevin Semcken, Able Planet’s CEO, pursued the technology because he was struggling with a high-frequency hearing loss himself. “To test the impact on intelligibility,” he says, “we put the technology into one of two identical headphones, and I could easily hear the difference. I switched between them and was able to hear high-frequency tones in the Linx Audio headphones that were not intelligible in the unretrofitted version.”

Able Planet is taking aim at the computer-gaming population with one of its lines of Linx Audio-enabled headsets, called LoSt-CaUzE, its nonstandard capitalization in emulation of the quirky typography favored by gamers. Many popular games are played in multiplayer mode over the Internet, with players talking to each other to strategize. A review of the LoSt-CaUzE headset noted that the Linx Audio technology made it especially easy to hear the in-game chatter of the other players amid the gunfire and footsteps.

It’s not every day that one finds a technology designed to make women’s voices and wanna-be soldiers’ voices easier to hear, but there it is: an innovative use of high-frequency harmonics.

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