Sound and other waves undergo diffraction just as light does. Suppose a loudspea

Question

Sound and other waves undergo diffraction just as light does. Suppose a loudspeaker in a 20 °C room is emitting a steady tone of 1100 Hz. A 1.2m wide doorway in front of the speaker diffracts the sound wave. A person on the other side walks parallel to the wall in which the door is set, staying 16m from the wall. When he is directly in front of the doorway, he can hear the sound clearly and loudly. As he continues walking, the sound intensity decreases.

How far must he walk from the point where he was directly in front of the door until he reaches the first quiet spot?

Explanation / Answer

   Minimum sound would be observed when       y   =   m * * D / a    where   y   =   distance from central maximum                m   =   order of diffraction   =   1   (for first minima)                   =   wavelength   =   v / f   =   343 / 1100                      =   0.312   m                D   =   distance between source and door   =   16   m                a   =   width of door       y   =   1 * 0.312 * 16 / 1.2             =   4.16   m    hence first quiet spot would be at 4.16   m.

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