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Mono house and comb filtering question

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Anonymous
June 24, 2005 2:59:38 PM

Archived from groups: rec.audio.pro (More info?)

Some of the other discussion reminded me of this old question...

'Way back when, we did a massive overhaul of a coffeehouse speaker
system. It started as a plan to fly the mains, then scope creep set in
and we wound up converting them from two-way open cabs to three-way
closed with optional tri-amped mode.

Of course in the process of the conversion, everything got rewired for
external crossover (passive after amps, or active before amps). And
therein lies the question.

When we first fired up the new system, we noticed we were getting
midrange comb filtering in the center of the hall. The system designer
insisted that he had done impulse tests and checked phase on everything,
and that this (therefore?) was normal and expected.

My understanding of the physics, my experience with other mono halls
(and with center-panned stereo), and the fact that reversing phase on
one of the mids cured the problem without introducing any other
artifacts we could hear, all seemed to disagree with him.

But I'm not claiming to be an expert in speaker system design, and he
theoretically was a pro. So: *Could* he have had everything set up
correctly and still produced this interference? Or did he just have
trouble admitting he'd blown something that obvious?
Anonymous
June 24, 2005 3:21:31 PM

Archived from groups: rec.audio.pro (More info?)

Joe Kesselman <keshlam-nospam@comcast.net> wrote:
>
>When we first fired up the new system, we noticed we were getting
>midrange comb filtering in the center of the hall. The system designer
>insisted that he had done impulse tests and checked phase on everything,
>and that this (therefore?) was normal and expected.

What does this mean? He did impulse tests on individual drivers, or
on the system as a whole?

Remember the crossover phase response is going to be all over the
place.

>My understanding of the physics, my experience with other mono halls
>(and with center-panned stereo), and the fact that reversing phase on
>one of the mids cured the problem without introducing any other
>artifacts we could hear, all seemed to disagree with him.

If the crossover was like most crossovers and it had a radical change
in phase response over the crossover region, putting the midrange
drivers out of phase may have compensated for that perfectly.

>But I'm not claiming to be an expert in speaker system design, and he
>theoretically was a pro. So: *Could* he have had everything set up
>correctly and still produced this interference? Or did he just have
>trouble admitting he'd blown something that obvious?

I don't know. For all I know, it could have been a room problem that
was being compensated for. But it's easy enough to measure the whole
system response on-axis in the parking lot and then to measure it
on-axis installed in place and see what is going on. If the midrange
driver phase is incorrect, there will be huge dips in the crossover
regions where the drivers are cancelling one another out rather than
reinforcing one another.

If he _didn't_ do this sort of testing when you had a complaint, he
was falling down on the job. He should have done just a simple sweep
test, if only in order to quiet down the customer.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
June 24, 2005 7:56:57 PM

Archived from groups: rec.audio.pro (More info?)

On Fri, 24 Jun 2005 10:59:38 -0400, Joe Kesselman wrote:

> Some of the other discussion reminded me of this old question...
>
> 'Way back when, we did a massive overhaul of a coffeehouse speaker system.
> It started as a plan to fly the mains, then scope creep set in and we
> wound up converting them from two-way open cabs to three-way closed with
> optional tri-amped mode.

That must've been one helluva coffeehouse.
Related resources
Anonymous
June 24, 2005 8:18:50 PM

Archived from groups: rec.audio.pro (More info?)

In article <CeqdnWRVc4tJgiHfRVn-1g@comcast.com> keshlam-nospam@comcast.net writes:

> When we first fired up the new system, we noticed we were getting
> midrange comb filtering in the center of the hall. The system designer
> insisted that he had done impulse tests and checked phase on everything,
> and that this (therefore?) was normal and expected.
>
> My understanding of the physics, my experience with other mono halls
> (and with center-panned stereo), and the fact that reversing phase on
> one of the mids cured the problem without introducing any other
> artifacts we could hear, all seemed to disagree with him.

In practice, the mid-range speaker polarity is often inverted because
that's the way it works best. Perhaps modern digital crossovers and
the contemporary knowledge of time-aligned cabinet design (instead of
just mounting all the speakers on a board with holes in it) have
changed this.



--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
Anonymous
June 28, 2005 3:47:24 AM

Archived from groups: rec.audio.pro (More info?)

> I don't know. For all I know, it could have been a room problem that
> was being compensated for.

Good enough. I'd rather give the guy the benefit of the doubt as far as
I can... but I also wanted to make sure I was learning the right lesson
from the experience.

> If he _didn't_ do this sort of testing when you had a complaint, he
> was falling down on the job. He should have done just a simple sweep
> test, if only in order to quiet down the customer.

The downside of accepting spare-time/volunteer assistance, even from
someone knowledgable, is that you generally aren't their top priority.

You'll notice I'm trying to avoid names. I DON'T want to tar the guy
here; I still like him and respect his skills. He *did* accomplish a
difficult and mostly thankless task, despite the scope creep, and after
we got it all dialed back in the system worked quite respectably. I'm
just trying to make sure I understand what happened so I know which
direction to jump next time. <smile/>
Anonymous
June 28, 2005 6:57:21 AM

Archived from groups: rec.audio.pro (More info?)

On 24 Jun 2005 16:18:50 -0400, mrivers@d-and-d.com (Mike Rivers)
wrote:

>In practice, the mid-range speaker polarity is often inverted because
>that's the way it works best. Perhaps modern digital crossovers and
>the contemporary knowledge of time-aligned cabinet design (instead of
>just mounting all the speakers on a board with holes in it) have
>changed this.

Certainly for the case of time-aligned odd-order Butterworth
crossovers, ideal in several ways, the lowest change in
group delay through the crossover region happens with the
drivers in opposite "DC" polarity.

But the OP's case sounds like he's possibly a victim of
expert-itis, often fatal, incurable except by radical
scortched-earth methods that would contravene the Geneva
Convention. Let us pray for our fallen brother.


Chris Hornbeck
"I can build you a test that will show either one. Which
would you prefer me to demonstrate?"
--scott
!