# Large Null around 90Hz



## Stylus (Aug 6, 2009)

I have been dealing with an issue for a while and would like some input. I have a large null at around 90Hz from my main speakers. I can deal with it a bit by increasing the crossover point of the sub, but I start to be able to locate the sub this way. Any input appreciated.

Here are some quick points about the system/measurements:

2-channel system
Integrated amp with no internal crossover
Mains run full range
Sub crossover in second pick is set to about 90Hz
Measurements taken with both speakers running from the main listening spot
Room is approximately 22'x15'x8'
Speakers are ~3' from front short wall
Listening position is ~12' from front wall, 10' from back wall
4 2'x4'x4" acoustic panels in room

Mains w/o SUB








W/ Sub XO ~ 90Hz


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## laser188139 (Sep 19, 2009)

Stylus said:


> ... I have a large null at around 90Hz from my main speakers. I can deal with it a bit by increasing the crossover point of the sub, but I start to be able to locate the sub this way.
> ...
> Speakers are ~3' from front short wall ...


I've seen nulls from my mains, not quite as sharp as yours. It is probably no coincidence that your 3' distance from the front wall is 1/4 wavelength at 94Hz, which could give you a 1/2 wave cancellation reflection from the front wall. You can verify this by moving the front speakers farther from the front wall, and seeing if that lowers the frequency of the null. If it does, that gives you an method to lower the sub crossover, at the cost of having the speakers more in the room. 

Bill


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## Moonfly (Aug 1, 2008)

Have you applied 1/3 octave smoothing to that graph. If no then I would as it it more representative of what your ear actually hear IMO. Generally, very deep narrow dips are hard to hear, if at all. Its also usually very difficult to fix them as such a deep dip is usually a true null that cant be boosted out with eq.

Being at 90hz though, its also possible its a phase issue, so I would take separate speaker and sub graphs, then compare them to this combined graph. That will tell you if its a phase issue or not. If you need help determining the graphs then post them up and we can help. If you have variable phase on your sub, then slight adjustments, measuring after each, will also help determine if its a phase issue.


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## Stylus (Aug 6, 2009)

Thanks for the replies. Bill, I will continue to play around with placement, that calc is interesting, could be a big part of the problem.

@moonfly, here is a pic of the mains with 1/3 smoothing. Not sure I understand the phase issue. The first graph I posted was with mains only, no sub. I don't have them handy but I do have sub only measurements and they do not have the same null issue. The problem is the null is above where I would like to run the sub.


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## Moonfly (Aug 1, 2008)

Hi stylus. My initial thoughts were in relation this this being a sub or sub with speakers issue, which is what my comments on phase were about. Its clear its entirely down to the speakers (just double checking :T). As Bill says, it looks like a placement issue above all else, and moving things around is the key to improving things.

That said, the smoothed graph more or less completely removes the deep narrow dip, which is likely how your ears hear the material. Its basically showing you that you probably cant actually hear that dip, and might not need to worry about it that much. Of course, even the smoothed graph shows there is room for improvement. Also, have you tried one speaker at a time, and compared those results?


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## lsiberian (Mar 24, 2009)

I'd definitely try moving the speakers back and forth to see if it changes the null.


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## Stylus (Aug 6, 2009)

I'll play around with placement over the weekend and let you know how it goes. Thanks for the help. :T


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## Stylus (Aug 6, 2009)

I moved the speakers out about 6" and it decreased the null some. I also measured both speakers individually as well and averaged them and it is much better! So what does this tell me if anything??? 

Green Line = Both Speakers measured in stereo from listening position
Black Line = Speakers measured individually and then averaged. Also measured from the listening position.


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## laser188139 (Sep 19, 2009)

Before you averaged them to give the black line, what did the individual left and right speaker curves look like?

Bill


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## Moonfly (Aug 1, 2008)

laser188139 said:


> Before you averaged them to give the black line, what did the individual left and right speaker curves look like?
> 
> Bill


Can you post these graphs?


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## Stylus (Aug 6, 2009)

Here you go:


LEFT








RIGHT


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## laser188139 (Sep 19, 2009)

Isn't that fascinating! Neither the left nor the right speakers alone shows the sharp null near 90Hz. Together they do. 

The obvious hypothesis, then, is that you are getting negative reflections from one that are subtracting from the other. In particular, when you calculate the direct reflections from the opposite walls, can you come up with a case where the reflected distance from speaker #2 to the opposite wall and back to the listener is ~6' longer than the direct distance from speaker #1. (I know, it's hard to see how this happens if the speakers are symmetrically placed in a symmetric room, and the listener is on midline, but as the left and right curves are not identical, the room appears not to be symmetric.)

Do you have your four acoustic panels covering the first reflection points? 

Here's another idea. Using REW v5, if you loopback the left channel on the soundcard, you can use it to establish an absolute reference to time zero. The phase graphs from the two speakers, measured separately, can then be compared to verify that the overall signal from one is out-of-phase with the other. This might even give a definitive answer to which signal is arriving later, and so is a longer effective distance.

Bill


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## Stylus (Aug 6, 2009)

laser188139 said:


> Isn't that fascinating! Neither the left nor the right speakers alone shows the sharp null near 90Hz. Together they do.
> 
> The obvious hypothesis, then, is that you are getting negative reflections from one that are subtracting from the other. In particular, when you calculate the direct reflections from the opposite walls, can you come up with a case where the reflected distance from speaker #2 to the opposite wall and back to the listener is ~6' longer than the direct distance from speaker #1. (I know, it's hard to see how this happens if the speakers are symmetrically placed in a symmetric room, and the listener is on midline, but as the left and right curves are not identical, the room appears not to be symmetric.)
> 
> ...


I'm not sure, here is a rough estimate of the layout. You are right that it is not symmetrical. The left speaker is ~5' from the left wall, the right is ~3' from the right wall. The speakers are ~8' apart and the listening position is between the speakers ~9' away. The left wall also notches in where the listening position is about 2'. So the listening position is roughly centered between the 2 walls ~6.5' on each side.

I do not have the first reflections covered, I have one panel on the front wall between the speakers and 3 on the back wall. I also forgot about 2 2'x2'x4" panels that I have on the floor behind each speaker and a cube thing I made for an alcove above a built in cabinet.

This is terrible, but here is a quick layout of the room.










Can you explain the loopback thing more, not sure I understand. :dontknow:


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## laser188139 (Sep 19, 2009)

Stylus said:


> ... Can you explain the loopback thing more, not sure I understand.


In its default mode, à la REW v4, REW has no absolute reference for what is time zero. So it starts time zero in its analysis of the impulse curve with the arrival of the first sound at the mic/computer. It doesn't matter if speakers are at difference distances, each speaker is analyzed starting at its own start point. So the phase graph shows changes across the speaker's response, relative to itself, but the phase graph gives you no information to compare one speaker to another. 

If you wire the left channel from the computer output back to its input, similar to the soundcard calibration wiring but this time on the left channel instead of the right, you can go into the REW Preferences -> Analysis and tell it to use the left channel as the reference for time zero. Now, REW evaluates the impulse curve relative to the looped-back impulse on the left channel, so it can give you absolute time measures to each speaker (via the group delay graph) and absolute phase measures that include the distance to the speaker. So you can see whether the phase of the signal from one front speaker at a specific frequency matches the phase of the signal from the other front speaker at the at the same frequency. (I used this with REW v5 to determine if my sub distance was correct. I described the Preferences dialog here.)

If you measure each front speaker this way, and you see that the phase difference between the two is near 180°, you can be satisfied that this is the explanation of your dip. I think it would be a reasonable inference that whichever speaker is giving you the later time in the group delay at this frequency is the one whose reflection is subtracting from the other. (This inference might be a stretch; I've not tried this myself.) The phase graph for each might also be interesting on this question: if one speaker's phase curve appears smooth around 90Hz and the other changes behavior near 90Hz, the one with the anomalous behavior should be the one whose reflection is causing the dip. 

Bill


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