Can someone please help explain an experiment which I am struggling to understand. You have a speaker without any of the outer casing (just the small disk part) and secure it onto a clamp stand. You then have two microphones; one above the speaker and one below it which are both the same distance from the speaker. The microphones are connected to a CRO which display both waveform at the same time. You drive the speaker with an signal generator at 500Hz. Both the waveforms on the CRO destructively interfere perfectly on the screen. This is what I don't understand. Doesn't sound radiate in all directions from the source so therefore shouldn't the sound reach each microphone in the same phase seeming as though they are located the same distance from the speaker? The other thing I don't quite get is that this experiment is suppose to show why you need to mount a speaker onto a board (baffle). Mounting it onto a board cuts out the sound going backwards out of the speaker that would normally destructively interfere. Why does it normally interfere? How can the sound that is going backwards interfere with the sound that is going forwards? Many thanks in advance for any help!