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From: bill@vrdxhq.verdix.com (William Spencer)
Newsgroups: rec.audio.high-end
Subject: Re: crossover design
Message-ID: <9227@uwm.edu>
Date: 30 Jan 91 13:44:05 GMT
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in article <9136@uwm.edu>, jroth@allvax.enet.dec.com (Jim Roth) says:
>  For example, if you use 3'rd order Butterworth filters, then they should
>  be 3 dB down at the crossover frequency so that the summed power response
>  on axis will be flat 

>From what I see in the common literature, this is not right in two ways.
It's the summed pressure response (=voltage response), not the power. That
does _seem_ wierd that power doesn't add. But the other problem is that 
"power response on axis" seems to be a non-concept. Power response
controls the total energy in the room -- the response in a reverberant
field. (Is there an acoustical law for this?) 

> - the signals will be 90 degrees out to the drivers
>  at the crossover point.

Yes. Therefore at some angle relative to "on axis" the difference in delay 
between drivers results in in-phase operation and a 3 dB peak. The 
"polar tilt". 

>  On the other hand, a Linkwitz-Riley (cascaded Butterworth filter) crossover
>  keeps the signals sent to the drivers in phase at all frequencies, so the
>  -6 dB figure is correct.

You're presuming even order, which is of course the reason for the L-R.
The combination of both drivers produces constant pressure on axis using
half the power. But where does this increased "efficiency" come from?
Yet we see the total energy in the room is the summed power response. 
Evidently this "increased efficiency" is not really that, it's a 
directional thing.

The even-order can not be constant power and constant pressure at once,
but the odd-order can. How? That polar peak off-axis adds more energy to 
the room.

My understanding of all this is sketchy, but it's the result of other 
experts avoiding the subject entirely of trying to put together the pieces
and explain what's really going on. Comment or clarifications?


dlin@prodigal.psych.rochester.edu (Daniel Lin) writes...
	
>	The literature suggests that equations used to determine low and high
	pass filter componenets cannot be applied to design the bandpass
	filter due to interactions between components. What kinds of
	calculations are required to determine the necessary adjustments?
	Are any adjustments needed for woofer's low pass or the tweeter's
	high pass crossover values? 

Try Bullock's article in Speaker Builder 2/85 or the Loudspeaker Design 
Cookbook. The SB article may give you more options.

From: jj@alice.att.com (jj, like it or not):
>	Anyone seriously considering "real" bi-amping
>(which means a low-level crossover in front of the amps)
>should probably look into various asymetric designs for
>crossovers.

Is there a way of explaining how these result in flat response other 
than the equations?
Obviously I can see the equations sum. So what? That's _not_ understanding.

>Which, you note, adds up to 1, with no phase-problems,
>and no amplitude problems.

What's the off-axis response? (Genuine interest, I don't know)
Total phase shift? (see below)

-----

Okay, with odd order crossovers you can connect drivers in either 
polarity. Does anyone want to volunteer which they think is better?
I can see a few things. You can try both and see which compensates
better for nonperfect drivers. The delay between drivers and the preferred
up or down polar tilt can be considered. 

If the reverse polarity connection is chosen, which driver gets reverse 
absolute polarity? Seems that this depends on crossover frequency. 
However, also seems wierd that subwoofers could end up out of phase by this
logic, I know they are in some designs. (This question applies to second order
xovers also).

In phase connection avoids this previous question. However, out phase suits
typical quasi-first order designs due to the extra phase shift of the driver
(not in all cases). With third order things get more complicated. In 
phase connection results in 360 degrees of phase shift going through crossover,
not zero. So, a very complicated decision results, at least on principle.
Luckily (?), most peaple claim the difference is inaudible. Due to the 
presence of the other considerations listed above, it's hard to truly 
test this factor in isolation.

So, some answers and some questions. Let's get a discussion going.

Bill Spencer