From: Dave Dal Farra <gpz750@bnr.ca>
Subject: Re: Active Crossovers
X-Xxdate: Fri, 8 Jan 93 15:45:38 GMT
Organization: BNR Ltd.
X-Useragent: Nuntius v1.1.1d9
Date: Fri, 8 Jan 1993 15:37:13 GMT


In article <bill.726453637@sarge> William Spencer,
bill@verdix.com writes:
>Yeah, like they're not near as much of a problem. You
missed his big error
>though -- a Butterworth x-over won't have lobing error
either. It *will*
>have a 3 dB on-axis peak however.

This is only true of certain Butt filter orders.

>>[...]
>>>Second order L-R's also invert tweeter phase and are
not recommended.
>
>Is this really grounds for non-recommendation? 3rd
through 5th order normal
>polarity connection crossovers have a full 360 degree
phase shift through 
>x-over, while 1st, 2nd & 3rd order reverse-polarity
x-overs have only 180
>degree shifts but reverse polarity in the passband of one
driver. I (again)
>ask the question -- which may be of only academic
interest but could make
>for a hell of an argument -- which is really better or
worse?

Phase shift through x-over depends on filter order. You
can't lump them
together.

Without a doubt, the 180 deg phase shift in the high pass
pass-band
caused by a second order Butt (or L-R) is subjectively
disastrous.  
Harmonics in the upper frequencies will be in reversed
"polarity" with
regards to the fundamental reproduced by the woofer/mid.
Give a listen 
for yourself.  My ears tell me this is the basis for a non-
recommendation.

>There may also be some confusion about the merit and
disadvantage of the 
>tilted polar peak (yes, peak, just like the even-order
Butterworth case).
>The peak occurs at the angle at which the driver
contributions come together 
>in phase. At some other angle there will be some
cancellation compared
>to on-axis. 

"POLAR PEAK?"  Response is flat on axis with a properly
executed L-R.
Power response is not flat, though, but still
only shows a minor dip around x-over.  The non-flat power
response is
hinted at with with your comment that "At some other angle
there will be
cancellation compared to on axis".

The higher the filter order, the smaller the power dip
(though the filters
transient behaviour worsens as order increases). 
Occasionally, it is 
worthwhile to take a hit in on-axis response to improve
power response, 
especially with lower order filters.

With higher order filters (i.e. 4th) the subjective effect
of changes in
power response due to the x-over takes a back seat to the
subjective
effect of on-axis response due to x-over.  Again, listen
for yourself.

>Flat power response implies a constant total energy put
into the 
>room, therefore the same response in dry and reverberant
environments
>(assuming a LOT of that idealized world stuff). Since you
know there will be
>cancellation at some angle, and flat response on axis,
you also know there
>must be a peak at some other angle or the integral sum of
energies at all 
>angles can not be constant. 

OK so far...

>The polar peak can be seen as good thing
>if you want flat on-axis pressure response and a flat
power response, because
>otherwise you just can't do it!

Maybe I'm missing something here, but this comment seems
to contradict
itself.  I'm not exactly sure what you're trying to say...

Dave
Newsgroups: rec.audio