Physically, this wave is characterized as a non-linear, discontinuous wave front that features infinite amplitude and accompanying acoustic pressure wave that may generate a pressure as high as 100MPa in a time as short as one microsecond.
22.
If the input is from a microphone, and not a CV mask or its equivalent, the inverse filter also must have a pole at zero frequency ( an integration operation ) to account for the radiation characteristic that connects volume velocity with acoustic pressure.
23.
The ratio between two acoustic pressures in deciBels is expressed by the equation dB = 20log ( p1 / p2 ), so for every doubling of distance from the point source p1 = 1 and p2 = 2, thus there is a sound pressure decrease of approximately 6 dB.
24.
In inverse filtering, the speech sound ( the radiated acoustic pressure waveform, as obtained from a microphone ) or the oral airflow waveform from a circumferentially vented ( CV ) mask is recorded outside the mouth and then filtered by a mathematical method to remove the effects of the vocal tract.
25.
The acoustic power ( energy per second ) crossing unit area is known as the intensity of the wave and for a plane wave the average intensity is given by I = q ^ 2 / ( \ rho c ) \,, where q \, is the root mean square acoustic pressure.
26.
An alternative definition is possible in terms of pressure instead of intensity, giving PL = 20 log ( p _ s / p _ r ), where p _ s is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and p _ r is the RMS pressure at the receiver position.
