Twelve 10" NHT Subwoofer Build. - Page 10 - AVS Forum
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post #271 of 274 Old 01-17-2009, 04:31 PM
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Erich,

This makes perfect sense. This is a perfect example of why you should NEVER have different drivers/subwoofers covering the same frequency range in a sound system. You are guaranteed to get comb filtering as a result. There are systems which are intentionally designed to use that comb filtering on purpose, but that is way beyond this thread.

The reason is that the phase responses of each system are different. Every system has a phase response which is primarily a function of the shape of its frequency response. Since you changed the shape of the frequency response of the 15" subs to be different from that of the 10" subs, they also now have a different phase response also. At some frequencie(s) the two systems are going to be 180 degrees out of phase with each other and completely cancel. This is the problem that you had.

The changes in frequency response with listening position, in your room, are not unusual. They are completely typical. If you want to learn more about them, download the Floyd Toole surround system papers from the Harman website. I think there are five of them total.

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post #272 of 274 Old 01-17-2009, 06:20 PM - Thread Starter
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Thanks for the info Jack.

I haven't remeasured the difference between the front and back since finding the issue. I have a pretty good feeling that it won't be nearly as bad as it was yesterday.


I'm still a little confused about the exact reason why this all happened, I'll have to do some reading about comb filtering I guess.


I didn't do any phase changes, only a lower crossover to the back subs. I didn't know that it could still cause phase issues at some frequencies.

I just assume that from 0-50hz the 10's and 15's would be working together to boost that end up, then the 15's stop at around 50, and the 10's keep going up to the 90hz range.



So what you're saying is that for my idea to have worked, I would send 0-50hz to the 15's, and ONLY 50hz and up to the 10's?
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post #273 of 274 Old 01-17-2009, 07:03 PM
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Erich,

Comb filtering is the generic term used to describe the notches in the resulting frequency response that happens when you have two or more sound sources producing the same sound with some phase shift or group delay difference between them.

The fourth post down in this link has a graph that shows what comb filtering looks like in the frequency domain.

http://forums.mackie.com/scripts/for...c;f=7;t=002253

If you go here:

http://en.wikipedia.org/wiki/Nonminimum_phase

Look at the section under "Relationship of magnitude response to phase response" The math here describes the relationship between the frequency response and the phase response of a minimum phase system. We are dealing with a minimum phase system here. In that type of system, the frequency response and the phase response are linked to one another. If you change one, the other must change.

When you turned the crossover down to 50Hz, you changed the phase response of the system, because you changed the frequency response of the system. If your box modeling software is any good it will show you the phase response as one of the available graphs. Model two identical systems, one with a 50Hz LP filter and the other with a 100Hz LP filter. You will see that the phase responses are significantly different above 25Hz or so.

For you to get maximum output at all frequencies, with no comb filtering (no notching) you need to make sure that the frequency response of each subwoofer system is identical. This may mean setting both of their crossovers to the same LP frequency or not. What you care about is the frequency response of the complete system (driver+box+port, and any HP or LP filtering, this could be from the crossover or power amp of whatever). If the 15" roll off a little lower in frequency than the 10", you may need to set the LP filter on the 10" to compensate, so that each system has the same frequency response.

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post #274 of 274 Old 01-17-2009, 08:23 PM - Thread Starter
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Jack, I really appreciate the lesson, and it does make sense now.

After switching things around and sending the same frequency to all subs, I got the following graph (no EQ applied).

BLUE is the 10" subs only. For music, I only turn these on.

GREEN is the 15" subs only. Never used by themselves.

RED is with all subs running at the same time. Used for movies only.





I think the red looks pretty good. Eventually, I may do away with the 15's and EQ the lower end for the 10's.


For what it's worth, in reference to just the 10's and the blue line; the drop from 25-35 is mostly due to that seating position. The 2nd row has more in that range and less around 50.

I feel a lot better about this and I'm now going to finish up the room and actually start enjoying it. A few more acoustic panels, and a slight change in seating should bring me more into the flatter curve, then I'll use the EQ if needed.




Finished product: This also includes my VR-3 main speakers that I made.

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