Advice on these graphs

**Fatawan** · **07-02-08, 12:25 AM**

Hello--I wonder if I could get some advice on the graphs made with my first go at REW. My room is a heavily treated HT, about 12' x 26' x 8'. I have two MFW-15 subs in the front, and an SVS PB12-Ultra on the riser in back. The SVS was in the back of the room, but the graphs showed a huge dip at 60Hz, and that was fixed by moving it up onto the riser. However, there was then a big boost at 58Hz. That was fixed by moving one of the MFW-15's off the stage behind the screen and onto the floor . So here is where I am at at the moment--this is a graph with only the MFW-15s being used:

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Here is the SVS being used by itself:

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...and when I combine all three, I get this:

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The graph is pretty smooth until that huge suckout at 25Hz. How did I get that with the combination of the other two graphs? If I play with the phase of either the MFW-15s or the SVS, I can greatly improve that 25 Hz dip, but then I cause a dip up higher.

Any and all advice greatly appreciated. Will moving subs around help?

**Wayne A. Pflughaupt** · **07-02-08, 12:35 AM**

Can you rescale your graphs so we can better see what's going on with them? See here.

Regards,
Wayne

**Fatawan** · **07-02-08, 01:02 AM**

I'll try again

MFW's only
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SVS only

Combination of all three subs
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**DrWho** · **07-02-08, 01:54 AM**

Are you using any smoothing on these graphs?

How large is your room? It looks to me like there are signs of a dip at 25Hz in all the measurements - it's just more pronounced in the combined response.

What are the dimensions of your acoustical space?

And which tuning have you chosen for the SVS? Does changing the tuning point get rid of the dip?

**Fatawan** · **07-02-08, 08:16 AM**

No smoothing

Dimensions of room are in the first post(12' x 26' x 8')

20Hz tune on the SVS, and changing that made no difference on the graph.

**Fatawan** · **07-02-08, 03:22 PM**

This is a bit better--cutting the paired MFW's at 25Hz helped a lot, with a 20Hz boost on the SVS.
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**Wayne A. Pflughaupt** · **07-02-08, 09:35 PM**

Well something just doesn't look right to me. Both these subs have extension down to 18 Hz or lower, yet here they can't even make it to 25 Hz before falling off the map. More oft than not a sub performs best in or near a corner, while locations like center-of-a-wall can exact a penalty in extension and output.

Regards,
Wayne

**thewire** · **07-03-08, 06:45 AM**

The many dips look to me like maybe we are trying to accomplish something that is with absorption where diffusion/absorption may be necessary.

**DrWho** · **07-03-08, 11:32 AM**

Quote:

Fatawan wrote:

Dimensions of room are in the first post(12' x 26' x 8')

Oops, I missed that the first time around. Sorry about that.

The wavelength of 25Hz is ~45 feet. Is your listening position ~11ft from the back wall?
http://www.peavey.com/support/techno...ncellation.cfm
(11ft is a quarter wavelength)

**DrWho** · **07-03-08, 11:35 AM**

Quote:

thewire wrote:

The many dips look to me like maybe we are trying to accomplish something that is with absorption where diffusion/absorption may be necessary.

In order for diffusion to be effective at any frequency, the depth of the panel needs to be at least 1/4 wavelength. So at 100Hz, you're looking at a minimum panel depth of 3ft (yikes!)...and it only gets bigger as you go lower.

**Fatawan** · **07-03-08, 02:22 PM**

Here's an interesting turn of events. I ended up cutting a very narrow slice at 25Hz on the MFWs by 15db. That greatly improved that 25Hz dip. Then, I ran Audyssey on my Onkyo pre/pro. I am impressed at what it accomplished, to say the least!

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**Fatawan** · **07-03-08, 05:06 PM**

Quote:

DrWho wrote:

Oops, I missed that the first time around. Sorry about that.

The wavelength of 25Hz is ~45 feet. Is your listening position ~11ft from the back wall?
http://www.peavey.com/support/techno...ncellation.cfm
(11ft is a quarter wavelength)

My head would be about 9 feet or less from the back wall actually. What is counter-intuitive(at least in my mind) yet very effective was cutting the 25Hz level on the front-of-room MFW's by 15db, which greatly reduced that dip. There must be some interaction between the MFWs and the SVS going on there. I tried cutting the 25Hz area on the SVS and it just made the problem worse. Not sure what is going on there!

**thewire** · **07-03-08, 08:16 PM**

Quote:

DrWho wrote:

In order for diffusion to be effective at any frequency, the depth of the panel needs to be at least 1/4 wavelength. So at 100Hz, you're looking at a minimum panel depth of 3ft (yikes!)...and it only gets bigger as you go lower.

Yes been reading about this in my book. Not suggesting that to treat an area bellow for the subwoofer would help, just maybe some treatments installed prior may be harming the lower frequencies. For example room modes are often reinforced by reflection which in the case of a absorber that has poor diffusion will also reflect reinforcing room modes. If so many room modes are reinforcing other areas but there is point at which room modes are not (area where room modes have large spacing) these reinforced ones will be greater in amplitude and the dips will have lower amplitude than usual as a result. If perhaps diffusion, such as on the back wall, even sides, may help to bring these areas back to life. It is difficult to say without seeing a full range measurement but I see that dip around the 100Hz area and it gets me to thinking there may be more to this than simply cut and boost.

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