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While studying the fundamentals of sound waves in organ pipe, I noted that the fact about phase of reflected waves is contradicting while referring multiple sources

This book of mine describes the reflection from a rigid surface/closed end to be in phase

Reflected waves in phase

Whereas this one describes the reflection from a closed end to be 180° out of phase

Reflected waves out of phase

I found the same issue while referring some online portals on this topic. Why are they contradicting each other?

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2 Answers 2

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Its a bit weird to compare the phase of a wave and its reflection, since their directions are different. Both the sources you have put up are saying the same thing: Compressions are reflected as compressions, and rarefactions are reflected as rarefactions.

Now because their directions are different, the phase difference is continuously changing, so I dont really know what the books mean when they say there is no phase difference, or there is $180^ \circ$ phase difference. The important thing to understand is the bold statement above.

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  • $\begingroup$ Yeah, what you've shown in bold is same in all reference sources. It is only the phase shift they are describing differently and I don't know why it is so $\endgroup$
    – Vivek karunakaran
    Commented Oct 21, 2020 at 18:47
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It isn't a contradiction. It is two different situations with different behavior. In the first, a sound wave reflects off a hard surface. A example is reflection from the closed end of an organ pipe. The second is reflection from the open end of an organ pipe.

A similar difference can be seen waves in springs. Here is a video from UCLA physics.

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  • $\begingroup$ Sorry, I had attached the wrong image then. I've changed it now. Please take a look at the Second one, it describes closed end to be reflecting waves out of phase $\endgroup$
    – Vivek karunakaran
    Commented Oct 21, 2020 at 13:51
  • $\begingroup$ The confusion is that sound is a pressure wave, but also a displacement wave. Here is a link that explains it. $\endgroup$
    – mmesser314
    Commented Oct 21, 2020 at 14:07
  • $\begingroup$ As you said, let's consider one of the books were speaking about displacement wave and another one about pressure wave, even in such case shouldn't both these waves be in phase while reflecting off a rigid surface? I am aware of the fact that there is a 90° phase difference between displacement and pressure waves in general and that's why their graph looks different in the link you gave me. But which kind of wave is this 180° shift associated with? $\endgroup$
    – Vivek karunakaran
    Commented Oct 21, 2020 at 14:23

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