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Re: What about microphone diaphragms?

209.86.4.30

Posted by Charles on August 7, 1999 at 15:47:35

In Reply to: What about microphone diaphragms? posted by David Spear on August 7, 1999 at 07:11:04:

Hi David,

Let me begin by saying that I am reading and responding to this thread from top to bottom, and I reserve the right to change my mind as I go along. If these issues were simple they would have been resolved long ago.

It's certainly true that high frequencies are modulated by lower frequencies "at the mic", but I think this happens in the air before the sound ever reaches the mic. The picture is very complex however. Tones that come from a common sounding board modulate each other as the sounding board vibrates. As the sounds from different instruments mix in the same air, more such modulation takes place. It's pretty clear that what is happening in the sounding board is a "Doppler effect", but I'm less sure about what is happening in the air. It looks like a Doppler shift and the same numbers seem to apply, but the cause of the effect seems quite different to me. Perhaps this is only a matter of perspective.

Recording studios use close-micing to eleminate much of the interactions between instruments in most recordings. Many people see this as a "bad" technique (and I was in this group at one time), but it's really not at all clear that there is anything wrong with this approach. If your audio system actually CONTROLLED the pressure waves in the listening room I'd argue that you should simply reproduce the same pressure waves in your listening environment that existed in the original space where the preformance ocurred. If this were possible, accurate audio would be a product rather than a hobby. One major obstacle is the fact that the speakers only EXCITE the pressure waves. We can control how the waves are excited and what they do after they are excited, but only to a limited degree. Absolute fidelity would require absolute control.

But, given such limitations, it sure seems reasonable to me to say that a speaker should operate just like a microphone in reverse. This case has been made often in support of full-range single-driver systems, and I think it's pretty generally acknowledged. Those who design systems employing multipal drivers go to great lengths to insure that all the drivers "appear" to the air as a single driver. The degree to which this goal is achieved and the difficulty involved is the source of all driver blending problems. Obvious, I guess.

One of the responses last night indicated that this issue may revolve around the way sound is seperated into components by the crossover and then reblended in the air after it leaves the drivers (If you didn't read this in the thread, it's only because I'm reading a lot INTO what has been said). If that is indeed the case I think this is becomming a driver blending issue, and I am starting to suspect that the use of the term "Doppler" to describe it may simply be misleading (as it's not the classic Doppler effect at all). If this is what these guys are looking into they may be on to something that will bring on the next big improvement in accuracy with multi-driver systems, and if accuracy matters, we should all be paying attention to what's going on. Like you though, I am very skeptical that simple changes to the crossover will correct the problem (if it exists at all).

Now I'll go and read the rest of the responses and we'll see how this thing progresses.

Thanks for the reply,

Charles

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