From: kmr4@po.CWRU.Edu (Keith M. Ryan)
Newsgroups: rec.audio.car
Subject: Re: 2 subs vs. 4
Date: 4 Oct 1992 20:35:20 GMT
Organization: Case Western Reserve University, Cleveland, OH (USA)
Reply-To: kmr4@po.CWRU.Edu (Keith M. Ryan)
NNTP-Posting-Host: slc12.ins.cwru.edu


In a previous article, gt0869a@prism.gatech.EDU (WATERS,CLYDE GORDON) says:


>No, the output from the 4 10" woofers will in theory be 3 dB louder. 
>This is from mutual coupling- every time you double the # of drivers,
>sensitivity should go up 3 dB (double the acoustic output for the same
>poewr) This is one reason small amp systems with 130+ dB output exist-
>get enough drivers next to each other, and they will get LOUD! 

Okay, I was working with the impression that the respective total powers would
be similar [ ie 100 watts to each SVC 10", and 100 watts to each voice
coil of the DVC 10" [ in essence 200 watts total in each case ] If you 
assume 100 watts total to each speaker  [ ie 100 watts each to each SVC
10", but only 50 watts to each voice coil in the DVC 10" [ in essence
200 watts total versus 100 watts total ], then there will be about a 3dB
increase ( twice the power ) .
	When you add additional speakers, if the total power in remains
constant, then the total acoustical power out will remain fairly constant.
( you will have some lobing effects which will increase volume in certain
areas though ) However, if when two speakers are hooked up, the amplifier
delivers more total power ( let's say double the power into 2 ohms ), then
adding speakers will increase accousitcal power output.
	This, however, is extremely simplified. You will have localized
increases, as well as decreases, etc. However, the idea that the more
woofers you add ( with the same power in ) the louder it'll play, is 
a wrong conclusion. Look at it this way: you drive one woofer with
1 watt and you get 90 db's. If you add another woofer, with 1 watt
total in, you do not get 93 db's out. [ argh, that really is not the
right terminology, one should deal with the total acoustical energy ]
if that were so, then we could add another woofer, and get 96 dB's for
only 1 watt. But then we could add another one.... till finally we
have 12 woofers, 1 watt in with a total output of 133 dB's! Come on, 
you don't get more power out, with out putting more power in! 
	Experimental evidence seems to contradict this, but be careful.
If you add another woofer in parallel, it'll be louder. But, the amplifier
is seeing a lower impedance, and is producing more power!
	( note: it is possible to increase a systems efficiency at certain
points thou, such as adding woofers in a box to alter the resonance freq-
ency, etc which may alter the output at a particular frequency. )

	Also, as I said earlier, the superpositioning of the magnetic fields
in the DVC are not 100% in phase, so you'll loose some output. ( the
exact amount varies on the quality of the  woofer )


>While this doesn't always work exactly to theory (What does ?!) due 
>to losses, bad placement, etc- it does work to more or less extent in
>all multiple driver systems. I have both calculated (I wish I could find
>the class notes) and observed this in practice.

	Probably was a localized increase ( and a subsequence decrease in
another local ) or the power input was not held constant.
-- 

	" It's time for Congress to tell the president to shove his 
	veto pen up his deficit! "  
					- Rep. Jim Traficant.