Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Mono house and comb filtering question
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? |
#2
|
|||
|
|||
Joe Kesselman 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." |
#3
|
|||
|
|||
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. |
#4
|
|||
|
|||
|
#6
|
|||
|
|||
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/ |