Air pressure, defined by the popular perception of the meaning is the amount of air in your car tires.  A meteorologist would be inclined to refer to it as atmospheric pressure, which is measured with a barometer.  However, the physicist might say that the air molecules are repelling each other at a static rate until an outside force acts on them.  All of these groups are correct.

In short, air is a soup of various molecules of gas, liquid, and solids, proportionally in that order.  All molecules have electrical shells that attempt to maintain a neutral charge.  If a charge is applied to a group of molecules, they immediately repel each other.  This expansion creates a pressure that builds up against the surrounding molecules.  The pressure wave traverses space, expanding and diminishing.

That explanation is the basic principal behind the wind, flight, speech, sound, music, and concussion.

Variations in the speed at which sound pressure changes has been categorized, here is a list of them:

• barometric pressure – ~ to .2hz, The measurement of changes in atmospheric pressure.
• infrasound – .2hz to 35hz, sub audio sound
• audio frequency – 20hz to 20khz, the audible range of human hearing
• ultrasound – 20khz to 200mhz, used for ranging, imaging, and processing.

Variations in the intensity also change its characteristics.  The standards for sound are well documented, as the subject has been studied since recorded history.

With all that out of the way, now I can put this phenomenon to use with micro controllers as a communication method.

Early computers used modems to communicate.  These were acoustic coupled and this means they were wireless at distances shorter than 2cm.  Eventually, the audio function was removed and the audio frequency was transferred directly to the telephone wiring for more reliability.  This was from the demand for automation, since machines can operate more efficiently than a human dialing.

However, the earliest electromechanical device to communicate wirelessly was the telegraph using Morse Code.  This is still used today, however most of the functions are handled automaticlly.  Still the method is proven and its usefulness demonstrated time and again.

Back to the modem, since this addresses the inherent bandwidth limitation of Morse Code.  The audio characteristics of Morse Code makes is prone to noise and interference.  Several methods are available that use a variance of frequency shift keying, such as MFSK, GFSK, and CPM to name a few.  This allowed modems to communicate at faster rates never attainable with Morse Code systems.

The subject of using audio as a means for micro controller communication is vast, I’m sure I’ll be revisiting this subject later.  In the meanwhile, here is some more details about various communication methods used with smart phones.  Enjoy.