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I'm looking at communicating digital data through the air as soundwaves.

To illustrate the problem, I generate a 10-cycle 2kHz wave, play it out through my MacBook's speakers and pick it up on my iPod Touch:

enter image description here

The echoes are about half the amplitude of the probing signal.

HotPaw pointed out to me in a comment on a recent question that there is a potential problem with dead-spots: regions of destructive interference.

enter image description here

If I use a particular carrier frequency, transmitting from one point in the room to another, there will be certain locations where acoustic cancellation / destructive interference occurs, resulting in a dead spot.

Surely other technologies such as Wi-Fi must encounter the same difficulties. How do engineers deal with this phenomenon?

The only solution I can think of is to simultaneously use say three different carriers, and have the listener select the strongest.

PS severely hampered on the tags: data transmission, communication, radio, information transfer, echo, Radio, rf, signal, bleh

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As you figured out, multipath causes distortion in the channel's frequency response. This can be combated using clever modulation techniques like OFDM and/or using equalization at the receiver. – Jason R Mar 19 '14 at 2:25

If the channel is static and you are in a deep fade -- then, yes, your only option is to increase the bandwidth or go multicarrier like you suggest.

In fact, modern WiFi systems use that exact technique: OFDM ("Orthogonal frequency division multiplexing" -- sometimes known as multi-tone or multicarrier transmission).

Alternatively, you could go with two receive antennas (microphones in your case) spaced far enough. If you use 2000Hz carrier, the wavelength is v/f = 300[m/s] / 2000 Hz = 0.15 m. So a distance of around 10cm should give you enough diversity. This can also be implemented at the transmitter.

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This article points out the problem of deep-fade, and offers a frequency sweep approach.

However, it also states that communicating in a low signal-to-noise environment of ultrasound is still an open problem.

This paper links at the bottom to several ultrasound projects.

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