Television sets particularly older models produce a whistle

Television sets (particularly older models) produce a \"whistle\" sound at about 15600 Hz (this is the frequency with which the electron beam sweeps across the face of the screen). This sound is audible to most youngsters and adults who haven\'t lost their high-frequency hearing. Imagine that a TV set is on (with the \"mute\" on) in your sister\'s bedroom as you walk by the hallway. If your sister\'s door is ajar, leaving an opening 5 cm wide, and your ear is 1.5 m from the door as you walk by, for how many meters can you hear the TV \"whistle\" ?

Solution

frequency=f=15600 Hz and the speed of sound=343 m/s

hence, the wavelength is,

wavelength = speed of sound/frequency

= 343/15600

= 0.021987 m

for first minima, the distance is,

x = wavelength*D/a

= (0.021987)(1.5 )/(0.05)

= 0.66 m