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Old 15th December 2011, 10:33 PM   #19311
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Ed,

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Originally Posted by simon7000 View Post
Time aligned is a really big issue when you build big arrays.
It is one even in the fairly small arrays we used two decades ago.

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The "Acoustic Center" moves with frequency and to a lesser extent level.
That depends on he drivers and various of their parameters...

When AMR's Speaker came out I had a whitepaper that covered everything I considered really relevant to say about why it is what it is. It came to around 30 Pages and covered less than halve I wanted. Marketing said "too long" and condensed it to one page...

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Some folks consider the center to be at the voice coil.
Not quite, BUT it depends on driver construction. In the Eckmiller driver it is at the crossover frequency very close to the voice coil for both drivers. In the TH315 it needs LC Delay lines to make it work. In AMR's speaker I measured the acoustic centre with pulses...

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Then there is the issue of a standard cone. It is a cone!
Hopefully not. Cones are nice for IScream...

My ideal cone driver (for bass or bass low mid in a multiway system) has a shallow NAWI cone and non-spherical section dustcap that are mirror symmetrical (kind of like a big ring radiator) with equal distance (but not surface) from centre to voice coil and from voice coil to edge of cone. Cone termination is still a major PITA, but with a 4" Voice Coil for an 8" cone it is possible to push the trouble out of band. The acoustic centre in this case is reasonably constant as the whole driver is easier to make behave inunison.

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The air pushed by the back of the cone moves slower than that. So the air from the front of the cone arrives a little bit ahead of the dust cap air. For combinatorial calculations the barycenter is a better choice.
But the barycentre moves with frequency for most drivers. However, there is a saving grace. If the driver does not work as ideal piston parts of the diaphragm decouple. And long wavelength have more "tolerance" than short ones...

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Then we can talk about horn loads.
Never really encountered any, outside musical instruments.

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Originally Posted by simon7000 View Post
At low frequencies the air movement conforms to the horn shape and behaves as if the outer edge of the horn where maximum pressure change occurs is the combinatorial center. At higher frequencies the center moves back as the air motion lifts earlier.
I have found the view of "horn" of little use. I have found the "waveguide" concept more in line with observations I have made, but I notice that badly designed waveguides show "bad" behaviour normally considered to be a functional characteristic of "horns".

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Now for the case of a single full range loudspeaker, these issues are generally most important at crossover frequencies between drivers.
So we are not talking about a single full range loudspeaker, but a multiple loudspeaker, multiway system.

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In a multiple loudspeaker system you have many drivers covering the same frequency range but different seating areas. The combinations of adjacent drivers can narrow the coverage patterns, so the ability to predict that is a wee bit important. The we get to worry about how drivers in different frequency ranges combine!
Ed, I used to work with guys that your kind of installs in the 80's and sat at mixing desks for bands. I know that. But the concepts and challenges do not directly translate and scale to domestic systems.

If you have the time, look up the Eckmiller Driver. It gives you an idea of my background in serious audio. It was like really ancient news when I was wet behind the ears...

In the 80's we used column speaker designs for PA (as in public address, not sound reinforcement) with a very defined directivity across the whole usable bandwidth (< 100Hz) and as we could not really deal well with the delays (multiple output digital delays where still years in the future for generic public installs and what existed was not available in the eastern block) we had to use things very differently...

Anyway, here we are not normally talking about massive stadium or hall systems, but home audio... So some problems disappear, others raise their ugly head...

Ciao T
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Old 15th December 2011, 10:48 PM   #19312
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So Scott if you need to borrow something to do a calibration don't be afraid to ask.

I already said I have access to traceable standards, afterall those cell phone mics need .2dB calibration.

Maybe you forgot, I'm trying to help DIY'ers gratis (my wife figures Jan is paying me about $1 an hour for my time). There is also the challenge factor how good can you do on $50 worth of scavenged parts. I already know you can equal an AP with a $200 sound card, free software, and some cleverness but the plots don't say AP in the corner. Sorry Bruce ignore that statement it's merely rhetorical.
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Old 16th December 2011, 12:40 AM   #19313
SY is offline SY  United States
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Well, Scott, keep it up and one of these days, you'll be able to hang with the real pros.
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Old 16th December 2011, 12:47 AM   #19314
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Darn, they never give me enough money!
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Old 16th December 2011, 12:58 AM   #19315
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Darn, they never give me enough money!
Cut out the middleman, go solo, set your own price, and keep it all.

se
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Old 16th December 2011, 12:59 AM   #19316
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Good for you guys you want money... I want this migraine to stop ASAP...
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Old 16th December 2011, 01:04 AM   #19317
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Your migraine can stop at any time. Just don't look! '-)
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Old 16th December 2011, 01:40 AM   #19318
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Your migraine can stop at any time. Just don't look! '-)
It does not, actually... Our office went to Circus, but I could not go... I took already too much of Excedrin. Does not help.

Edit: sorry for off-topic... It hurts...
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Last edited by Wavebourn; 16th December 2011 at 01:55 AM.
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Old 16th December 2011, 01:49 AM   #19319
wayne is offline wayne  United States
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Darn, they never give me enough money!
John it's not about the money. It's all about the money thats why more was invented and get it up front like you said.
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Old 16th December 2011, 04:28 AM   #19320
FrankWW is offline FrankWW  Canada
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It does not, actually... Our office went to Circus, but I could not go... I took already too much of Excedrin. Does not help.

Edit: sorry for off-topic... It hurts...
My sysmpathy, fellow sufferer.

I found pseudoephidrine hydrochloride 60mg helps but I take ASA with it.
It works if I take them early enough.

Your results might be different
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