# Sound reflected from wall

Hello

When a mechanical wave gets reflected from a rigid surface,say a fixed end of a rope ,it undergoes a phase shift of ∏ ,but when a sound wave gets reflected from a hard surface ,say from a wall,it doesnt undergo any phase shift .

There is confusion regarding phase shift of sound wave from a hard surface,like a wall.At some places,I have read it doesnt undergo any phase change .At other places I have read ,sound undergoes a phase shift of ∏.

What is the correct explaination ?

Thanks

CWatters
Homework Helper
Gold Member
It depends on the relative impedance of the object that it's travelling in and reflected from....

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reflec.html

When sound waves in air (pressure waves) encounter a hard surface, there is no phase change upon reflection. That is, when the high pressure part of a sound wave hits the wall, it will be reflected as a high pressure, not a reversed phase which would be a low pressure. Keep in mind that when we talk about the pressure associated with a sound wave, a positive or "high" pressure is one that is above the ambient atmospheric pressure and a negative or "low" pressure is just one that is below atmospheric pressure. A wall is described as having a higher "acoustic impedance" than the air, and when a wave encounters a medium of higher acoustic impedance there is no phase change upon reflection.

On the other hand, if a sound wave in a solid strikes an air boundary, the pressure wave which reflects back into the solid from the air boundary will experience a phase reversal - a high-pressure part reflecting as a low-pressure region. That is, reflections off a lower impedance medium will be reversed in phase.

Continues..
Electrical waves travelling down a transmission line are also phase changed (or not) according to the impedance of the end. For example a pulse arriving at a short circuit (=low impedance) is reflected inverted.