The effect of the evanescent wave upon acoustic measurements in the human ear canal.

When making acoustic measurements in a human ear canal, it is often necessary to monitor the output of a sound source with a microphone positioned within a few millimeters of that sound source. This microphone will not only measure the pressure due to the propagated acoustic wave, which we wish to measure, but also the pressure due to the evanescent wave. The pressure due to the evanescent wave can be viewed as a source of error in the measurement of the propagating acoustic wave. This paper attempts to quantify the magnitude of this error. Theoretical predictions are made of the relative level of the evanescent sound pressure in a number of source and microphone arrangements applicable to ear canal measurements. It is shown that these theoretical predictions represent an upper limit of evanescent sound pressure that can be measured experimentally. The maximum measurement error due to the presence of the evanescent wave in human ear canals below 10 kHz is predicted to be 3 dB in adults and 1.3 dB in 1 month old infants, when the loudspeaker and microphone ports are spaced more than 2 mm apart.