Here’s a puzzle for you: what is it you can hear but cannot hear, creates noise to make you sleep, and is a key feature of some 35 iPhone / iPod Touch applications that represent some of the most high-tech snake oil ever invented?

The answer is Binaural Beats, an aural illusion created when you listen to two different tones, one in each ear, that your brain interprets as something else entirely — a beat.

The closest thing to which I can liken the sound is the wobbly twang of a single key being struck on an out-of-tune piano. It’s a useful comparison in that most piano keys (except in the lowest octaves) make sound by forcing a mallet to simultaneously strike two or three strings tuned to each other – two or three, instead of one, to amplify the volume and add richness to the sound. However, if the strings aren’t tuned closely enough, they generate a resonance between them that we hear as that ‘out-of-tune’ beat.

Now imagine listening to two slightly different electronic tones being played through headphones, but each tone in a different ear. Turn the speaker away from one ear; you only hear only one tone in the other ear. Listen to both, you hear the beat of them resonating with one another. The illusion is that the resonance — the beat — is only in your head. You are hearing what you cannot hear.

Okay, the world is full of cool, odd phenomena and illusions. What makes this one really off-beat is the belief, shared by many in and out of the scientific community, that Binaural Beats wave forms resonate with and influence brainwaves, and are thus capable of affecting our behavior.

A little science stuff: Scientists have determined that, for this beat to be perceived, the two tones must be below 1,000 to 1,500 Hz, and be separated by between 3 and 30 Hz (e.g., if one tone is 500 Hz, the other must be no more than 530 Hz or less than 470 Hz). Among those 3 Hz to 30 Hz frequency differences are the frequencies that scientists, measuring brainwave activity using Electroencephalography (EEG), have associated with certain states of mind (Beta, 12 to 38 Hz, wide awake; Alpha, 8 to 12 Hz, awake but relaxed; Theta, 3 to 8 Hz, light sleep or extreme relaxation; and Delta, .2 to 3 Hz, deep dreamless sleep). Many scientists think this is not a simple coincidence, and that, in fact, listening to Binaural Beats can induce (or “entrain,” as they say) the brain into the corresponding state of mind – in effect, creating an altered state of consciousness.

Until the past couple decades, interest in and access to this phenomenon was more or less academic and confined to the laboratory. What’s changed? Well, if you set out to design the perfect delivery system for providing this electronic drug to the masses – a Binaural Beats Bong, perhaps? — you couldn’t do much better than a portable music device with ear buds onto which you can load any sound that can be turned into an MP3 file.

In fact, if you go onto the Apple iTunes music store and search on “Binaural Beats,” you can find dozens of sound files (you couldn’t call them songs, exactly) that have Binaural Beats layered in with all manner of natural and synthetic sounds that are included to make listening to them more palatable. (A bare bones Binaural Beats sound loop is frankly pretty boring.)

Still, Binaural Beats sound files are not exactly “Top Forty with a bullet” material. The real buzz on Binaural Beats isn’t coming from the iTunes music store, at all, but from that latest iteration of the California Gold Rush, the Apple iPhone & iPod Touch, the Apple App Store, and the tens of thousands of software engineers hoping to discover the killer app that will make them millionaires. Using Binaural Beats as the basis for their applications, developers are adding bells and whistles, such as timers, fade outs, alarms, and layers of murmuring mantras, to target specific, supposedly suggestible states of mind. Where are they supposed to take you?

A search on “Binaural” in the iTunes App Store currently yields 35 apps, ranging from the cheerful “Be Happy” and “Easy Relax” to the more off-the-wall “Brainwave Altered States,” “mind Freek” and “iDoser,” to the pitchman’s plodding “SleepStream,” “Lose Weight” and “Quit Smoking Now,” to my personal favorites “Attractor” and “Zen Bound.”

Under the hood, you’ll find descriptions that promise everything from stress relief and power naps to lucid dreams, astral projection, out-of-body experiences and weightlessness (“mind Freek”).

So…do they work?

The Binaural Beats concept first caught my eye when I was browsing the Apps store on my iPhone and saw several apps, such as “SleepStream,” that said they used these sounds to create a drug-free sleep aid, entraining your brain to the Theta and Delta sleep states by producing sounds with similar frequencies.

Being a night owl in an early-bird world, I often have trouble getting to sleep before 11 pm, and since some of these were free, I thought I’d give them a try. So I downloaded “SleepStream” and “Ambiance,” both of which offer a layer of Binaural Beats against a wide choice of calming, environmental soundscapes, such as “Rain Shower,” “Desert Winds” or “Ocean Waves.” They also include a timer you can set to turn off the app after a spell, as well as the ability to change the volume of the soundscape relative to the Binaural Beats, allowing you to turn the beat up and the soundscape down or vice versa.

I tried a couple of different sounds and found them to be calming and even mesmerizing, but not usually sleep inducing, although I have to admit several times I did wake up in the middle of the night with earplugs still in my ears and did not remember the app turning itself off.

Then I found what seemed to be the ‘Cadillac’ of Binaural Beats apps, “Attractor,” which at $5.99 is targeted at the upmarket consumer. The maker of this app believes that to properly entrain one’s mind to a certain frequency, you have to start off at the waking state (around 20 Hz) and then gradually raise or lower the beat to the desired frequency. Attractor does precisely this, over a period of time you select (10, 20, 30 or 60 minutes). Like the others, it also offers background soundscapes, but is unique also in allowing you to record statements that it will embed in the sound as it is played back to you, thus creating your own, customized hypnotic suggestions.

While I found Attractor more mesmerizing than the other apps, it was no more successful, alas, than any of the other apps at putting me to sleep.

But perhaps I’m not the best candidate for Binaural Beats. Although most anyone should be able to perceive the magical beat, only those capable of clearing their minds and concentrating for extended periods on the sounds are said to be able to achieve the desired results.

It’s unfortunate, really, that I can’t seem to catch the Binaural Beats groove. If I could, I’d really be interested in seeing what would happen if I embedded in “Attractor” such hypnotic suggestions as “Your hair will grow back” or “You will win the lottery, buy a desert island and drink well-chilled Coronas on the beach for the rest of your days.”

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