Question

A parabolic reflector antenna having a circular opening of radius 0.35 m is used to focus sound
waves. Assume that the intensity level of a whisper is 20 dB at 1 m from a speaker, and that the
threshold of hearing is 0 dB for which �C = 106K: W/m:. (i) Determine the power of the acoustic
wave generated by a whisper. (ii) If the energy of a whispered conversation is collected by the
parabolic antenna at a distance R from the speaker, find an expression for the received power as a
function of R. (iii) If such power is delivered from the focus of the reflector to the ear of a
listening detective through a tube of diameter 1.2 cm with 10% efficiency. How far can a
whispered conversation be comprehended?

Answer 1