Axiom Home Page Forums General Discussion Technical Questions My room's frequency response graph

I'll dig up some links later, but here's the quick answer.

Speed of sound is about 1130 feet per second. You can determine the "wavelength" of any particular frequency by dividing 1130 by the frequency, ie the wavelength of a 250 Hz signal is 1130/250 or about 4.5 feet.

If you have two signals of the same frequency but "out of phase", ie 1/2 wavelength apart, you get cancellation. If the signals are in phase, they add. You can actually simulate this pretty well by tying a rope to a doorknob and wiggling it to make waves, then tying another rope at right angles near the doorknob, having someone else wiggle that rope, and watching what happens when the waves combine. Or you could trust us

So... if you have a dip at 250 Hz, you're looking for 1/2 wavelength at 250 Hz or 2.25 feet. This can either be a round trip distance (eg. center of the speaker is 1.125 feet from the side wall) or the difference between two paths.

Once you understand this and start looking at all the different combinations of signals you can get, it's natural to panic and cower on your couch convinced that you can never get decent sound with all those signals interfering and reinforcing

EDIT -- forgot to mention that one of the most common causes of a dip like that is cancellation between main and subs at the crossover frequency. You didn't say what your crossover frequency was and I imagine it is closer to 80 Hz than 250 Hz, but some receivers *do* have very high crossover frequencies and I just thought I would ask to be safe.

So Rich, as John B. discussed, one possible factor causing that 250Hz dip is the center of the woofer cones being a quarter-wavelength distance(about 14-15")from room surfaces, especially if two or more surfaces(e.g. ceiling and back wall)are involved.

For the record, I learned this from JohnK so *listen* to him

Thanks for the follow up. Hey, I remember the rope wave demonstration from school (was that middle school?).

I've seen posts on that calculation several time, but it didn't occur to me apply it like that. I guess I'll be measuring some distances this weekend! But given the configuration of the room, I'm not so sure I'm going to identify where that 2.25 ft (or 1.125 ft) difference is.

Might adjusting the toe-in of the speakers help, or is that barking up the wrong tree? (or would I just be shifting the null to another frequency)

The crossover is definitely set @ 80Hz so that's not the issue. I had noted that there are some anomolies on the graph around the crossover point, but that looked like an issue I could address after fixing the bigger issues!

Thanks so much for your assistance. This has been a great learning experience.

Rich

This is a black art, not a science, unfortunately. The best we can do with theory is stop you from blundering around completely in the dark... it's still trial and error but at least you will know what to try...

JohnK--

Holy Cr*p. Or should I say, DUH! I ran over to the tape measure and guess what? You didn't even really need me to measure, you already knew, didn't you?

Almost exactly 14" from the ceiling to the center of the woofer cones, and a little closer to 13" to the back wall.

Since I drilled nice big fat holes to get my FMB into the wall, I'm not so sure my wife is going to be pleased if I want to move them. Actually, given the constraints of the location, I'm not sure I could move them anyhow.

So any other thoughts on how I might improve the situation at the prime spot (since this issue seems to be isolated to this one position, though I guess the other spots may have other issues)?

Thanks guys.
Rich

Yeah, Rich, with your speakers that close to the ceiling, that's what I guessed.

Freakin' genius I tells you!!!

I was so impressed I ran over to my wife to tell her the whole story. Thanks again to you and bridgman for taking the time to come up with this. I sincerely appreciate it.

I'll report back results of any experiments I run this weekend, and am open to any suggestions.

Regards,
Rich

Wow... it doesn't usually work out that nicely...

I don't think there are a lot of alternatives to moving the speakers, unfortunately. If the frequencies were higher you could just put something absorbent on the wall, but AFAIK you need at least a few inches of fuzzy stuff to absorb as low as 250 Hz.

>>So Rich, as John B. discussed, one possible factor causing that 250Hz dip is the center of the woofer cones being a quarter-wavelength distance(about 14-15")from room surfaces, especially if two or more surfaces(e.g. ceiling and back wall)are involved.

>>Holy Cr*p. Or should I say, DUH! I ran over to the tape measure and guess what? You didn't even really need me to measure, you already knew, didn't you? Almost exactly 14" from the ceiling to the center of the woofer cones, and a little closer to 13" to the back wall.

OK, that was creepy. At minimum, I'm impressed