Tuning speaker / room response?

OK, take two of the experiment will thus involve pink noise and a spectrum analyser. I have done a bit of research on linear mics, PC Sound Cards et all, and as it turns out, it is not really economical to turn my PC in to a spectrum analyser. (See other thread on PC Spectrum Analyser).

What I found is that using an osciloscope, I got a nice 1kHz sine wave from a test CD to look as such. When my sound card was done with it, looking at the representation on the PC, there were tons of harmonics. So, I have a spectrum analyser coming to do this right.

So let me describe my room, here is a quick picture:


The black boxes are the speakers, they are actually toed in about 50 degrees. The blue box is roughly where I took the meassurements, and where I normally listen. The room (narrow part) is about 14' wide, about 20' long. Ceiling at about 9'.

The left wall is almost exclusively glass, about 90%. Glass is framed in hardwood. The floor is hand-made tile over concrete. Front and rear wall are drywall. Right hand side wall on narrow part is brick. Front part right side wall open to higher level floor. Ceiling is hardwood boards with beams hanging down at 8' and 16' from the front of the room; beams are hardwood, 4" wide, 8" high.

There is a couch against the back wall, two recliners (one where blue square is, one to the left). There are rugs inbetween the speakers and listening spot, from about 1' left of left to about 1' right of right speaker, another rug between recliners and couch.

The plan for this weekend is to repeat the experiment using pink noise and spectrum analyser. I will perform the test at several points in the room to see what results I get.

Finally, I suspect some people are going to suggest changing speaker placement radically. That is not really an option due to the traffic pattern in the room. I am sure it would be better to move the speakers 3' more out of the wall, but it aint gonna happen. *sigh*.

More data later...

Niels.

Neils can you place the sound system on the back wall where the couch and recliners are? I see you realize that where the speakers are placed now, that they should be around 60in off the wall behind your speakers. Looks like you might get better results reversing the setup. If your auctual room size is 14 wide X 20 long, placing the speakers (woofer center to walls and opposite speaker) 42in from side window wall and 82in to opposite speaker and 60in from wall behind speaker and seating position 10ft from same wall. This could be a starting point to take measurments from. This is if you can reverse the room if possibe. Also that little nook at the bottom right, if you can turn that into a continuous wall (closet maybe with a solid door) might help with the reversal. Just for fun try this arrangment and see if there is an improvment. Good-nite, i'll ck in the morning.

I have a similar question to Njonker's. I have plotted a graph using the test tones from Stereophile test cd 3. I used a Radio Shack spl from the listening position. My results are much flatter than Njonker's, staying within +-4 db of 80db at 1 kHz, with the exception of a 10 db spike at 6000-8000 kHz. What room characteristic would cause this? how does this compare to others who have tried this? Njonkers, I think you may have started something with your graphs. You have inspired me to get up to speed with my computer skills.

The methods described so far really only give rough approximations to the actual response at the sweet spot. This is because of the multitude of reinforcements and nulls that are caused by the multiple reflections. Your actual response is actually much worse than the curves shown. Impluse methods, such as MLSSA or those used by Sigtech, or specialized sweep tones should be used for room tuning. The asc tube trap site has an extensive explaination on their MATT test page (http://www.tubetrap.com/. I use both ASC tubtraps and a Sigtech DSP to tune my room. They make a huge difference and my room is good to begin with.

I think I may have missed the thrust of this discussion, and I'm a month late to boot, but here's some related info:

It looks as though the analog RS SPL meter is capable of providing data sufficiently accurate to be useful in addressing speaker/room frequency response. By applying the published correction values you are "calibrating" your $35 test instrument! The overall response of the meter is far from flat, but apparently pretty consistent from unit to unit, so the corrections should apply to your meter as well as mine. The analog version is preferred over the digital version for its superior high frequency response, and it is the one the corrections were established with and for. If you are so inclined, you can modify the meter with some capacitor changes and a better electret mic capsule (cheap and obtainable from Digi-Key) and improve the meter's response so that the corrections are no longer needed. Running repeated tests and charting the results is mighty tedious, and I'd recommend wearing earplugs to protect your hearing and preserve your sanity. Considering the HUGE frequency response deviations exhibited by virtually all speakers/rooms, the testing is worth the effort if you are able to make constructive changes based on the measurements. But, if there's nothing you can do about a mountain-range response, you might be happier not having it staring you in the face! Personally, I think that correcting (or at least minimizing) these gross frequency response errors caused by speaker-to-room interactions yields a more significant improvement to a system than most of the stuff we spend our time and money on.

Here are some sites/pages to check out:

(Sorry, but I don't even know how to make these active links, much less generate and post beautiful graphics! I guess you'll have to cut and paste...)

1) http://216.150.71.139/audioinnovation/rsmeter.html

2) http://www.hometheaterforum.com/uub/Archives/Archive-000001/HTML/19990806-17-000048.html

3) http://www.audiophilia.com/hardware/spl.htm

4) http://www.smr-archive.com/forum_3/messages/1220.shtml

5) http://www.gti.net/wallin/audio/audio.html