Understanding Interference of Sound Experiment

[Jimmy87]

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!

 

[berkeman]

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!

If the microphones are the same distance away from the speaker, they are indeed receiving the signals with the same phase. If you display each microphone waveform separately as 2 traces on the oscilloscope, you will see identical sine waveforms. You must be using an "A-B" feature of the o'scope, if you are seeing a single flat trace.

The speaker enclosure is more meant to enhance the sound of the speaker, by resonating at certain frequencies. It also can help prevent desctructive interference if the backward moving sound wave reflects off of a wall and becomes a forward moving sound wave. Depending on the distance to the reflecting wall, you could get partial cancellation of the total forward moving sound wave.

 

[AlephZero]

If the microphones are the same distance away from the speaker, they are indeed receiving the signals with the same phase.

The OP's description of the experiment is a bit ambiguous. Since the mikes are "above" and "below" the speaker, I guess the speaker was pointing vertically not horizontally?

The sound wave radiated from the back of the speaker is 180 degrees out of phase with the sound radiated from the front. And of course if you rotate the microphone through 180 degrees, you also change the phase of the electrical signal by 180 degrees! So the result would be different it both mikes were pointing in the same direction, or if they were both pointing towards the speaker (i.e. the mike above was pointing down and the mike below was pointing up).

So there are plenty of reason why you might get phase cancellation, but without a more precise description of the experiment we don't know the actual reason in your experiment.

 

[berkeman]

Oh, good point! I missed that interpretation. Thanks

 

[sophiecentaur]

I always understood that a baffle acts as a matching device. The comparatively small cone has comparatively large amplitude oscillations and the radiation resistance approaches zero where the cone is substantially smaller then the wavelength. See this Hyperphysics link.. (Just like a radio antenna.)
Using a baffle has the effect of increasing the radius of the speaker cone and raising the radiation resistance - improving the match. You could look upon it as simply disturbing more air around the drive unit.
This is not to disagree with the interference-based argument but the impedance based approach translates nicely to explaining the action of speaker cavities.

The measurement with the microphone in the OP doesn't necessarily depend upon the microphone directivity as small (probe) microphones work largely on the local pressure and are pretty much omnidirectional. This is unlike an RF antenna, which cannot be isotropic.