Do I have a room null problem?

[Jonnyozero3]

See here...

http://community.webshots.com/photo/...51378261Caxogn (open it full size and expand it to make it clearer).

I'll have to run some freq response graphs when I have time. I think I may have to move my couch back. Dangit.

Edit - yeah I think I have too much time on my hands lately.

[curtis]

One thing that I know is not good is the fact that your listening position is equidistant from the front and rear walls.

[Jonnyozero3]

One thing that I know is not good is the fact that your listening position is equidistant from the front and rear walls.

I remembered reading that, but then I read another paper that gave me the impression that I was okay for some reason. I can't remember why I thought that. Edit - I think I was under the impression that if the speaker wasn't there, it didn't "activate" the null. Hmmm.

I guess the question is, do I move my seating forward or backward. Either has tradeoffs for the video side, and messes up my speaker spacing (.73 ratio from Ascend manual) as well. Right now, I'm pretty much set for video viewing distance, but I guess I can adjust. After all this work, it kind of stinks to find something like this out...well okay I've been in denial for about a month and resisted posting

If I move backward by about 12 inches do you think that'll make an appreciable difference?

[BradJudy]

I have a very similar layout and distances (but more open to other areas). While you can likely expect a null because of the 1/2 way position, it may or may not be that bad (depends on the massive number of other variables at work).

Here's my room:

http://www.bradjudy.com/pics/sub_room/basement_new.gif

and the response across different positions on the sofa

http://www.bradjudy.com/audioblog/wp...05_notraps.jpg

No massively evil nulls, although there is one that gets deep toward one end of the sofa.

If you have a means to measure, I would measure the response before adjusting positions. Measure the existing position to see if it's that bad and measure at your alternate location before moving furniture. If you can't measure, it's hard to say how much improvement one foot will give. The lines in my chart are one foot from each other, so you can see what amount of change you might encounter.

[Jonnyozero3]

Thanks for posting that info, very neat.

I can measure, but I have to do it manually :P SPL meter, tripod, paper, pencil, Rives audio test cd, or also a custom-by-1hz mp3 cd from the snapbug BFD site. I have six seat positions as well so I'll try to check them all.

I moved my couch forward about a foot plus and it looks decent. I'm worried I'm too close to the PJ screen, but I guess I can reduce the size if I need to. We'll see. Maybe I'll get around to charting the response this weekend.

I want to get set up so I can do freq sweeps using my computer (audigy 2), but I'm not sure what I need yet. I'll work on that. It'll be a must when I start using the BFD I think.

[BradJudy]

You have everything you'll need for sweep measurement except perhaps the right cables. You just need the RS SPL meter, cables, a full-duplex sound card and RoomEQ Wizard (free software). You can use the software to do sweeps and configure a BFD. WAY better than measuring by hand and would only cost you a few dollars in cables/adapters.

[Jonnyozero3]

Sweet - that makes me happy I'll get ontop of that soon.

Now, I've found what made me think my sofa spacing was okay:

http://www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf

Very interesting white paper. I'd like to hear your thoughts (Curtis' as well) on it; pages 11-16 are the meat of it I think. To my eyes it seems to argue that sitting at a null is good because it means you have an "equal" freq response, where as sitting in a mode you experience a boost in that particular frequency. Hmmm.

It refers to that phenomenon as either activating or not activating a room mode, as opossed to a high dB loss at a certain freq. Am I interpreting this correctly?

[BradJudy]

The whitepaper does a decent job covering the idea of positioning the woofer and listener to reduce the excited and preceived nulls and modes. I did a series of measurements of my sub in the corner and then 1/2/3ft along the wall. I'll see if I can find them.

Something I haven't tried yet is putting the sub at the mid-point next to the sofa which should eliminate any nulls due to that dimension for the listener.

Keep in mind a couple of things:

1. This is a three dimensional issue and you're unlikely to change things like the sub height or to move the sub away from either wall (it's rare to see a sub not against at least one wall).

2. Whitepapers like this can only really cover the ideal rectangular room. Open areas and odd shapes can make the response harder to predict. It looks like you have no opennings, so yours will be easier to predict.

[Jonnyozero3]

Okay, so if I place a sub directly behind the listener it essentially nullifies the null because it is a "near-field" position? Heh, nullifies the null?

I guess the moral of the story is that I need to bust out the SPL meter, get the right cables, and start running freq response curves. That will answer all my questions.

What is nice is that I can be really flexible as far as placement is concerned. If I have to pu the sub 4 feet from the front left corner of the room out in the open...I'll do it! This is the MAN room damnit! I'll come back and post some more if anything interesting happens.

[BradJudy]

I'm going to try putting my sub mid-room next to the sofa. If it works well, I'll make a riser for the Hsu and put a nice wood top on - end table conversion.