From: DPierce@world.std.com (Richard d Pierce)
Newsgroups: rec.audio
Subject: Re: Designing Crossovers
Date: 1 Oct 92 02:09:06 GMT
Organization: The World Public Access UNIX, Brookline, MA

In article <1992Sep30.230858.6934@cbfsb.cb.att.com> rprice@cbnewsg.cb.att.com (rodney.price) writes:
>
>I saw this post today, but I missed the discussion that lead up to 
>it.  I'm interested because I did a master's degree in acoustics, and 
>my thesis involved making acoustical impulse response measurements in 
>rooms, among other things.  A room is going to do horrible things to 
>the spectrum of the sound coming from a loudspeaker, and I'm curious 
>to see how this test equipment might fix the problem.  What sort of 
>test equipment are you talking about?  "Open field" presumably means 
>that you want to place the loudspeaker away from walls, the floor, 
>etc, and that you use a room that is as dead as possible (or perhaps 
>you take the speaker outside).  You play pink noise through it, use a 
>microphone that, we hope, is better than the hoped-for response of 
>the loudspeaker, and look at the result on a spectrum analyzer.
>
>To give an idea of what the room can do to the sound received at the 
>microphone, suppose that we have an ideal speaker that perfectly 
>reproduces the signal driving it.  You play a perfectly sharp pulse, 
>a delta function, through it.  If this were the only sound received 
>by the microphone, your spectrum analyzer would show a flat frequency 
>response.  Assume, however, that you get only one early reflection 
>off the floor, so that T milliseconds later a second pulse arrives at 
>the microphone, attenuated from the first pulse by some factor A.  
>Your spectrum analyzer shows the Fourier transform of this signal, 
>which (magnitude only) is
>
>H(f) = 1 + A^2 + 2 A cos(2 pi f T).
>
>If A is reasonably large, as it usually is in real life, the spectrum 
>you see is far from flat, even though the loudspeaker is perfect.  A 
>real room is going to give you all kinds of reflections.  How do you 
>fix this sort of problem? 
>

Well, the relevant literature has been pretty ripe with solutions, and
there are a variety of commercial products that deal with this issue quite
successfully.

Let's take your received impulse (which, by the way, is a pretty terrible
way to measure real loudspeakers because of its ridiculously high crest
factor which makes the speaker less than happy and tweeters less than
working :-) and you truncate the data before that first reflection, then
do your FFT, Voila! anechoic measurements in a room!. 

Sure there are problems associated with the technique. Clearly, for
example, truncating the data effectively adds a high-pass function to it.
But the low frequency part of loudspeakers is the easy part, in many ways.
It's the only part that we seem to be close to fully understanding.

Commercial products that effectively give you anechoic measurement
capability in a room include the DRA MLSSA system (which uses mls stimulus
and cross correlation techniques to derive an impulse response, then it's
FFT all the way), Crown TEF systems (which use swept sine and tracking,
delayed spectrum analyzer), B&K (which is a conventional and wildly
overpriced impulse-FFT system), Rapid Systems, Ariel/SysID, etc. etc.


-- 
|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
|       (508) 433-9183 (Voice and FAX)      |
From: javiers@hpmwtd.sr.hp.com (Javier Sanchez)
Date: Mon, 5 Oct 1992 15:28:35 GMT
Subject: Help!! "POP" on car stereo
Organization: HP Santa Rosa Site (SRSD MWTD)