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View Full Version : Do I have a room null problem?



Jonnyozero3
09-14-2005, 01:48 PM
See here... :(

http://community.webshots.com/photo/451378261/451378261Caxogn (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
09-14-2005, 02:36 PM
One thing that I know is not good is the fact that your listening position is equidistant from the front and rear walls.

Jonnyozero3
09-14-2005, 03:04 PM
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 :rolleyes:

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

BradJudy
09-14-2005, 03:23 PM
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-content/8_4_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
09-14-2005, 07:48 PM
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
09-14-2005, 08:12 PM
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
09-14-2005, 08:20 PM
Sweet - that makes me happy :D 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
09-14-2005, 09:28 PM
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
09-16-2005, 01:38 PM
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
09-16-2005, 01:46 PM
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. :)

Jonnyozero3
09-16-2005, 02:19 PM
OOh sounds like a good project. I'm downloading RoomEQ now after getting my RS SPL meter to record me impersonating some sort of weird canadian/midwestern "helllooooo" voice. But hey, it worked.

tamuct
09-17-2005, 09:28 AM
This is the MAN room damnit!
Ah, it would be nice to have such a room in my house. I think a requirement for our next house is that it have a media room. I had to beg and plead with my wife to get the big black Ascends in my house. Something about not matching the decor...

Jonnyozero3
09-17-2005, 12:41 PM
That's why you get them custom painted in a color of your choice ;)

Edit: Color of her choice :)

I have to say that having a room like this is great. It's a lot of fun. I need to buy a dartboard though.

MikeAndAnnie
10-21-2005, 09:11 AM
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.

Room Eq will have a new version out very soon, perhaps as early as Monday. John Mulcahy is putting the finishing touches on it.
If you don't have a sound card with line in/out for use with it (just headphone and microphone jacks, like me), the ADStech RDX-150 will work as an external USB connection (so it works with a laptop as well), and its about $49.99 at CompUSA:
http://www.adstech.com/products/RDX-150/intro/RDX-150_intro1.asp?pid=RDX-150

The box/instructions don't say it is full duplex, and there tech service has not responded to my emails about that. BUT I can confirm it did work fine for me.

tamuct
10-21-2005, 09:14 AM
The Griffin iMic is a pretty good USB soundcard (for those doing Room EQ with their laptop). My laptop also only has Mic and Line out (no line-in). It's usually found in the Mac section with all the other Griffin products, but it's compatible with Windows and Linux as well. :D

The iMic has Line out and Mic in, but an external switch changes the Mic input to stereo line-in.

MikeAndAnnie
10-21-2005, 09:42 AM
The Griffin iMic is a pretty good USB soundcard (for those doing Room EQ with their laptop). My laptop also only has Mic and Line out (no line-in). It's usually found in the Mac section with all the other Griffin products, but it's compatible with Windows and Linux as well. :D

The iMic has Line out and Mic in, but an external switch changes the Mic input to stereo line-in.

...but make sure your Windows version is compatible. Mine is still Windows 98, and alas, it is not. Here is the company's blurb on PC compatibility:
http://www.griffintechnology.com/support/imic/IMIC-PC-002.html

Jonnyozero3
10-23-2005, 03:38 PM
So, I was playing around with Room Eq Wizard and I'm worried I've hurt my speakers.

I tried following all the steps as best I could to set up this program to do a frequency sweep. When I tried to do the calibrate step I got tons of loud nasty feedback. My ears still hurt :(

I have an Audigy 2 soundcard. I had the RS Spl meter connected to "Line-In", and then the "Left Line-Out" connected to "Mic-In" I thought this was okay but now I'm worried that I mis-read. Page 15 here shows my soundcard:
http://files.americas.creative.com/manualdn/Manuals/TSD/797/English.pdf

Okay after thinking about it, I think I had the "line-in" unmuted in the playback section. It looks like that caused the feedback. I still haven't tried the program again because I'm afraid :o

The tweeters on my CBM-170s smelled a little odd so I'm very worried that I damaged them. I listened to some music at moderate levels and they seem okay, but my paranoid self thinks I'm hearing a slight difference. I just don't know. I'm only partially sure it's psychosomatic. Should I be really worried or should they be okay? :confused:

bikeman
10-23-2005, 03:55 PM
Should I be really worried or should they be okay? :confused:
I vote for OK. If you fried the tweeter, it'd be real apparent. These things don't generally die a little bit at time. Have a glass or two of your favorite beverage and then go back and listen. I'll bet it's fixed. ;)

David

Jonnyozero3
10-23-2005, 04:10 PM
Thanks David. I think I'll go for a run to relax and then have a beverage and enjoy the system tonight like you say. No more tweaking for today. :p

Also, I guess I was being dumb because I don't think I can get this to work on my Audigy 2 since it doesn't have separate line-ins/outs for left AND right. Am I right? I was under the impression that set as I had it, it should work, but re-reading the help files I'm thinking no. I dunno, I need to go do something else for awhile and come back another day.

sensibull
10-23-2005, 06:39 PM
Also, I guess I was being dumb because I don't think I can get this to work on my Audigy 2 since it doesn't have separate line-ins/outs for left AND right.

See my reply to your post over on AVS. I thought the same thing when I first tried Room EQ, then someone kindly pointed out to me those single jacks carry both left and right signals (or else you wouldn't get a stereo signal when you plugged headphones or speakers into them), so you just need a 1/8" > 2 RCA splitter.

Jonnyozero3
10-23-2005, 07:34 PM
Ahh, thank you for the clarification. You'd think they'd make that more clear in the help files since the Audigy-type cards are very common.

I'll pick some up tomorrow and hopefully not destroy anything :o

My ears hurt for well over an hour after that happened. Ouch is all I have to say.