Do we hear sound at pressure or displacement antinodes?

Question

I have read from Young & Geller (2007), College Physics 8th Edition, Pearson Education Inc. (pg 385) that we hear sound at pressure anti-nodes rather than displacement anti-nodes as microphones sense pressure variations.

When we have 2 speakers facing each other to form a standing wave between them, it forms a standing wave with open ends at both speakers. This means that the ends closest to the speaker are displacement antinodes and thus pressure nodes. However, going by the above fact that we detect loudest sounds at pressures antinodes, this would mean that when we place a microphone closest to the speakers, it should detect very soft sounds. Yet, many experiments online showing standing wave formation between 2 speakers seem to show that the loudest sound will occur closest to the speakers.

So where exactly do we hear/ detect sound?

Thank you!

Answer 1

If you realise that our ears are pressure sensors you can easily reach the answer that we perceive the sound to be louder at pressure anti-nodes.

One more thing to consider is that no speaker or any other physical system is a good candidate for the idealised (and rather simplified) models used at an introductory textbook. I won't say always (although maybe I could when referring to every day physical systems such as speakers), but most often there are evanescent waves close to most (if not all) vibrating bodies (related to acoustic radiation of course). This, as well as the fact that an "open end" (in the ideal sense) does not exist (this means that the terminating impedance cannot become infinite) you will never get perfect cancellation (nodes).

Hope this helps somehow...

Answer 2

I was visiting a science center which had an acoustic standing wave set up in a long tube through which people could crawl. The standing wave pattern was drawn on the side of the tube and it looked to me that they had made a mistake be showing nodes at the two open ends however, when I did crawl through the tube, I found the ends were quiet and there regularly spaced loud and quiet places along the tube. So I can confirm that my ears at least detect pressure nodes and antinodes rather than displacement ones.