Question

The lecturer with a monotone voice of 440 Hz and you are both 2m away from the long, smooth and tall wall. You will hear sound that comes directly to you from the lecturer along with sound that comes back from the wall on it’s way to reaching you. You need the path here needs to have the same angle onto the wall as it has after reflection. How far from the lecturer will you pick your seat so you can’t hear the voice at all and find your well deserved rest?
-I drew the problem out with the isosceles triangle and altitude of 2m and solved for wavelength but I don't know what to do next.

Answer 1

Ms Nina, You must not be hearing any voice at all which means you must be hearing the super position of the reflected wave and the original wave at a place where destructive interference happens.For destructive interference, the condition is
path difference = (n+1/2) wavelength of the wave. As the reflected wave is at a rigid boundary an additional path difference of lambda/2 must also be taken into account. Iam not able to put in any diagrams / equations here. Please mail exact question to usha_kasi at rediffmail dot com

will help you to the best of my knowledge.