Silent Sound Relication

Spelling: relocation

An indication of the damage that all the ISPs that dropped their NNTP servers altogether or even just their binary groups.

Anyone on one of the big paid for NEWS providers would not notice. Duh.

The non-linearity in the propagation of sound in an ideal gas is caused by the heating that occurs with adiabatic (no heat transfer) compression. (This was first figured out by Laplace in 1816.) The adiabatic ideal gas model works well for air within the range of human hearing (but not including shock waves). If you look at the pressure vs volume function for an ideal gas undergoing adiabatic compression you will see that it is not linear; the slope increases with increasing pressure (and therefore temperature). Therefore the heating in the high pressure part of a sound wave (and cooling in the low pressure part) causes the high pressure parts to have greater deviation from mean pressure than the low pressure parts. (It also causes the high pressure parts to travel faster than the low pressure parts, producing second harmonic distortion, but that and many other effects are irrelevant for this application).

So now you mix two ultrasonic sound beams and where phase matches you get more positive pressure offset than where it cancels, causing sound at the beat frequency - but not very much if safe levels of ultrasonic sound are used, you can only hear it because it is being produced right at your ear, continuing even into the ear canal if the ultrasonic transducers are aimed there.

See Blackstock for the complete analysis.

And trash that notion that sound in air is linear, this approximation only works well at low frequencies and levels, symphony orchestra and below more or less. (You would think that something figured out in 1816 would be a little better known by now!)

Glen

Feeding one tone into one ear, and another into the other, spaced at (say) 10Hz apart will also result in a 10Hz modulation being heard. However, the interference is then in the neural tissue, not eardrum or soundwave.

Quite an effective method for entraining brainwaves.