posted
So I've read alot of posts about the 1/4 and 1/2 wave concept, but if you choose not to adhere to it, are you selling yourself short of dB's when using a wall? Can the design with the subs in front of the sensor and the port on the left be reversed (ie: port on right, subs on left) if you still adhere to 1/4 wave specs? I figure someone's already played with this idea, and I'd prefer not to waste my time, if I can.
posted
1/4 wave theory is just a good idea to put you in the right place when building a wall and working from there. it just gives you a really good starting place... really you should take into account of how much space you need for your equipment and enclosure, then do a little yin & yang with it.
quote:Originally posted by ShadowStar: Analysis of the car's transfer function once the wall is mounted would be a good idea.
ShadowStar
And do you do that?
-------------------- Dbdragracing is not a sport... IT'S A CURSE! 2009 WF 5th place Posts: 434 | From: ATHENS GREECE | Registered: Oct 2003
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quote: Analysis of the car's transfer function once the wall is mounted would be a good idea.
Ok, can you elaborate on that? Are you saying build a wall without a reflective baffle? Then test the in-car response? And base your baffle depth on the response?
posted
"Ok, can you elaborate on that? Are you saying build a wall without a reflective baffle? Then test the in-car response? And base your baffle depth on the response?"
As far as 1/4 wave theory goes, I'm not a "true" believer in it. If you space your baffle such that the center of your radiating area (center of one cone for a sealed box) is exactly 1/4 the wavelength of your peak frequency away from the microphone point, you have no way of knowing if complex (reflective) interactions around the car are killing your SPL score.
Obviously, for multiple subs, and ports, this becomes so much more complex that basing your wall on a simple calculation is sheer madness. (Usually )
However, 9/10ths of the time, your wall is dictated by physical factors in the car, and ends up in roughly the same location almost every time. What I've done in the past is to build an "approximation" of the final wall (a port-hole, a wall in place, and a sub mounted) and put a baffle around the back of the sub such that it represents a sealed box of known volume (albeit mounted TO the wall)
The ease of this testing method is based on the fact that your testing at low SPL's should yield frequency response data about exactly the same as for high SPLs, so you don't have to brace the crap out of the wall, you can perform some low wattage testing, to keep labour time down.
Now, having factored in some of the response effects of your wall (by putting an approximation in the car,) you can take some in-car SPL readings (or hook up an LMS if you got it, which I do, its great, woo hoo go Linear X) with the mic in the final SPL micing location.
Now, take the same sub, and the same box volume and shape, and do some ground plane measurements in your bay (or your driveway, or some local basketball court, or whatever.) Don't know how to do this? It's fairly simple, I'm sure there is a post here already about it, or on the internet, or in the loudspeaker cookbook
When you divide the response of the car by the response of the ground plane, the similar box effects factor out (your sealed box response) and you get a very close estimate of the car response effects (the car transfer function.) This is the same for 99% percent of the cars I've dealth with, a low frequency gain starting at about 45 and rising and freq. goes down, and a response "bump" up in the 60-80hz region. The middle of that bump is your note, for that wall, in that car, or very closely thereabouts. Plus, with your temporary wall, you can move it about a bit, angle it, mess it around, and see if you get some better effects.
1/4 wave theory is.. a very hopeful construct, but, given the fact that usually there are boundary locations closer to the radiators than the mic is... Stuff gets pretty complicated.
And cancellation has an almost limitless ability to kill your SPL score.
ShadowStar
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It's all about knowledge, love and respect. Posts: 2579 | From: Somewhere In the Northeast | Registered: May 1999
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posted
I did the same as ShadowStar suggested for testing when building my last wall... It was for MECA with open door and proved my bandpass raped the ported and sealed I was playing around with, as it worked quite well in full scale, but the testing method would apply to a sealed vehicle just the same.
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