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

Thought some of you might be interested in seeing my room's FR graph. This was a project I did with my boys. It was pretty darn enlightening, though I'm also confused by a couple of things.

The Radio Shack SPL meter was on a tripod right at my usual listening position. The speakers are Axiom m22ti, mounted horizontally (tweeters in) about 7' high, toed in and pointing down slightly towards the listening area. They are approx 12 feet apart (need to check that again), and are about 16-17 feet from the primary listening spot.

The graph (all measurements were adjusted for the known RS deficiencies and are not raw measurements):




Couple of my takeaways:
1) My room (really my listening position!) is HOSED! Makes me think of Mark Johnson, whose room sucks. Well, my room seems to be sucking right at 250 Hz, a fairly important frequency. [Note: when I moved the meter several feet to the left or right, that frequency was greatly improved, so it is something specifically in my alleged sweet spot]

2) I'm running my sub a little hot, and I didn't think I was, so I need to make some adjustments

3) My sub (SVS 20-39pci) performs a better than I thought it did. I confess to some recent sub envy with everything I've been reading, but it appears to be performing pretty well. It sounds good on music & HT, but I still feel like I'm missing some slam and pressurization at the bottom end. The room is pretty darn large, and open in a way I can't close off. I probably need a multi-sub set-up if I want to achieve a stronger chest-thumping effect.

4) Man, stuff sure does rattle in my living room! 22.5 Hz was exciting one whole wall in particular. The inserts in the fireplace were buzzing/vibrating like crazy in the upper 30s.

Any other thoughts or observations? Suggestions, particularly with respect to the suckout @ 250Hz at my listening position?

Thanks,
Rich

Let me start with the obvious...your room acoustics suck but you are halfway to better sound now that you have taken the time to understand where the nodes are. First thing to do is spend a couple of weeks reading up on acoustical treatments. Once you have developed your understanding I would suggest you concentrate on basstraps in the corners which will have the most impact on smoothing your peaks and nulls. Next experiment with panels athe prime and secondary reflection points from your main speakers. Finally consider a good equalizer. The road to improved room acoustics is a long one but the most significant thing you can do to get better sound...moreso than buying new equipment.

Well, I am trying to find out what are those deficiencies on the RS meter...

I have the digital SPL from Ratshack, and I didnt know that I had to correct for the measurements made with it..

Please enlighten me about that matter...
Thanks

There is a "sticky" post over at AVSForum that contains several links for Radio Shack SPL Correction Tables. Most of the links also explain anything else you need to know about that.

Most of the links also focus on the range <200Hz for sub measurements. There is one (I forget which) which shows the corrections all the way up to 20Khz.

Hope that helps! Take care,
Rich

If I remember correctly, a 250 Hz dip means you have two different paths from speaker to seating position which differ by a couple of feet. Could be something as simple as having the speakers a foot or so from a side or back wall, but more likely the difference between "direct from speaker to ear" and "from speaker to ear including a bounce off the side wall or" is a couple of feet.

If the former, move the speakers further away from the wall and the dip will move. If the latter, treat the first reflection points as others have suggested.

My major problem right now with my room has to do with WAF.

The stockings are STILL hung by the chimney with care in hopes that St Nickolas....will continue to block the reflections off of the fireplace.

Yeah....she wants me to take the Christmas stockings down

This is why I use the Rives Audio Test CD 2. It has 20hz-20K hz tracks that are calibrated to compensate for the inaccuracies of the Radio Shack meter.

Rich:
You need really big stockings filled with fiberglass....

I oughta know something about stockings!

Mark,

Santa is right!!

We made up for not having fiberglass filled large stockings by adding lots of smaller ones!! All those grandkids!!

It was amazing what a difference it made!

The Wife IS looking at producing some kind of decorative drape we can hang from the mantle to help control the reflections, so I may not get good sound "only" at Christmastime!!

Yipee!!

Rich.

bridgman--

*That's* the kind of info I'm talking about! I love this place!

On one side, there is a wall that angles back before turning into the side wall, and on the other side there is just a side wall. So the side walls are not equidistant from the speakers. Unfortunately, with the wall mounting I have, I don't have flexibility to move the speakers. Even if I did, I'm not sure I could fix the problem by moving speakers because of the room's configuration. I think this may call for room treatments.

Do you have any references for where I can read more about why that particular frequency manifests this issue?

Or I could just sacrifice the alleged "sweet spot" to my wife and/or kids in an act of immense generosity, and move on over to where the suckout ain't!

Thanks!
Rich

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

Hmmm, a few inches of fuzzy stuff? Would a cat work? (I am just kidding) But that idea would probably go over almost as well as asking if I can tack up some sheepskins or something. Oh well.

I'll play with a few things this weekend, and see if anything else helps. If I can direct that null to another part of the room, that might suffice.

And thanks again for your assistance.
Rich

Here's some interesting reading :

http://www.ethanwiner.com/acoustics.html

Owens-Corning 703 rigid fiberglass seems to be pretty absorbent at 250 Hz; might be the best option. Comes in 2'x4' panels, 1" thick...

John, there seems to be a break in the action at the moment, so I'll again raise a point about room size and power requirements. There seems to be an exaggerated concern about "filling the room" whenever it's bigger than average, and thoughts go to 500 watt amps. Room size and listener distance are certainly both factors, but sometimes the inverse square law is misapplied and a 6dB drop per doubling of distance is applied as if it was the only determinant. As has been pointed out before, the reverberent field becomes equal to the direct sound field at a relatively short distance(3-4')in typical-sized listening rooms(see the discussion and especially fig. 10.4.1 here , with the result that the drop in total sound pressure level with distance is far less than the 6dB per doubling figure. Linkwitz also has a very interesting discussion on room acoustics (especially scroll down to reverberation distance and amplifier power sections)which indicates why, as a practical matter, the more ordinary power levels that we use are fine for doing the job.

JohnK & Bridgman--

How about the almost as large dip at 2.5Khz? Is that because it is a multiple of the 250Hz drop? But wouldn't I then expect to see drops @ 500Hz, 750Hz, and at lesser & lesser impacts as I get further away from the fundamental wave?

Doing the math at that frequency (1130ft / 2500) says the wave is ~.45 ft, or around 5.5 inches. So am I looking at multiples of the 1/2 or 1/4 of that? And since I'm right around crossover point (I think! I'm working from memory) it could be either tweeter or woofer, right? (Though the centers of both are aligned).

Any additional insights (& predictions! ) would again be appreciated.

Thanks,
Rich

That http://www.ethanwiner.com/acoustics.html link is awesome!

Thanks.

Rich

Good to see you are reading up. Here are several more for you to review with lots of info on DIY treatments .

http://www.audioholics.com/techtips/roomacoustics/roomacoustictreatments.php

http://www.audioholics.com/techtips/roomacoustics/DIYabsorbersacoustics.php

http://avsforum.com/avs-vb/showthread.php?t=255432

link 1

link 2

link 3

Link

Link

Link

Ya beat me to it, you type to fast

Good linking guys. I was wondering why they don't link automatically like at other sites after cutting and pasting the links.

[_url=http://www.audioholics.com/techtips/roomacoustics/roomacoustictreatments.php_]Link Name[_/url_]

Post your links inside "url" "/url" as above but without any space "_" before or after url /url

Thanks Bug. Much appreciated.

Samandnoah -- you are right that the 2500 hz dip is probably unrelated to the 250 Hz dip. Best guess is that you have the microphone positioned somewhere other than midway between the woofer and tweeter, so that around the crossover frequency you are getting signal from both woofer and tweeter and they happen to be out of phase at the speaker-to-mic distance you are using. One thing the Axiom folks mentioned is that mic placement is one of the critical things to get useful frequency response graphs, even in their anechoic chamber.

JohnK - thanks for the links. They explain some interesting things, particularly why distance seems to be more of a factor in a "wide" room or in a room with treated reflection points than it is in a narrow room.

I'm not sure how to fudge the equations to take into account a less reflective room of the same volume... maybe in the morning when I'm smarter

EDIT -- actually the tvhandbook link does go into some detail there; at least there is a reflection factor in the relevent diagrams.

Okay, Rich, I did the previous analysis in my head, but this one is much more complex, so I used a special computer program which I'd written a couple of years ago to solve room response problems. I fed all data in and noted that the analysis was taking an unusually long time, but the result finally popped up on the screen: "This room sucks, although not as badly as that of the unit Mark S. Johnson".

Well, so much for what passes(conceivably)for incisive humor on audio boards. You've got the math down cold for the surface bounce cancellation problem, but this would appear to be a different category of interference. As you mentioned, this falls in the crossover region(Alan has reported it as 2700Hz)and a phenomenon referred to as "lobing" between the drivers results in an off-axis dip( received directly and/or by way of reflection from room surfaces)in that region. This is especially the case when speakers which would do well when positioned vertically are instead aligned horizontally. Test reports online which show off-axis curves for horizontal centers often illustrate such a crossover dip. The solution here, as in the previous problem, is repositioning(vertical here), but that may not be acceptable.

John--
I thought that was hysterical -- particularly since I sent a note to Mark saying almost the exact same thing!

Funny how things you've read about and thought you understood (like lobing) suddenly take on a whole new depth of meaning when you actually experience it. Guess that's why I loved Chem labs in college! Or perhaps why I'm not in chemistry 25 years later.

Anyhow, I am stuck with my speakers' orientation. Next steps will be to play with:
1) The degree of toe-in, which should hopefully improve the dip @ 250Hz *at* the listening position (though likely just move it somewhere else)
2) The angle up/down. I'm thinking that changing it relative to the listening position *might* make it more on-axis?

We'll have to see how the changes impact the overall sound experience with respect to imaging and coherence. Life is full of trade-offs, so why should this be any different?!

It'll be interesting to see what, if anything, sounds better to my ears, and then doing the measurements to see what my biases really are. I'll report back.

Regards,
Rich

Up/down adjustment should help with the 2500 Hz dip. You will either have to move the speaker or get some seriously absorbent material to fix the 250 Hz dip.

Rich, maybe your note to Mark made him feel a bit better(at least until he reads this). Yeah, my undergraduate degree is a B.S. in Chemistry, so I still fool around a bit with the scientific stuff that got pounded into me.

The tilting and toeing-in would more likely have some effect on the 2500Hz crossover dip, in that getting a little more on-axis sound(which shouldn't have the dip)would help, although the off-axis sound which also reaches the on-axis listener from reflections(which actually makes up the majority of the sound in a room)would still show the dip. The 250Hz dip results from the distance situation that we discussed and changing the distances, rather than toe-in or tilt would be attempted remedies.

I hear you and JohnK regarding the major 250 Hz issue. But here's why I'm wondering if changing the toe-in would help:
1) remember, the speakers are horizontal rather than vertical
2) when I measured just a couple of feet away from the (alleged) sweet spot, that 250 Hz dip disappeared completely. (I did not measure the 2.5 Khz response away from the sweet spot)

So I'm assuming, perhaps incorrectly, that since the entire area doesn't exhibit this 250 Hz dip, that I can make the sweet spot better by not having both speakers directed at the sweet spot (and hence concentrating/focusing that 250 Hz killing reflection off the ceiling). I know I'm just moving the problem rather than truly fixing it, but that may need to be the interim solution.

I know that this 250 Hz issue is the key problem to fix. But there is no way I'm going to be able to put 2" - 4" of material on our 7'6" ceiling! Forget WAF, I don't think *I* can deal with that! This is too public of an area, not a dedicated HT/Stereo space.

If I'd stop being so lazy I'd take some photos of the area and speaker locations, and you'd understand the limitations! Again, I really appreciate your efforts on my behalf!

Rich

This is fun. We should do an "Extreme Listening Room Makeover" for Mark's place.

Hey- I was going to suggest that about a year ago when I first realized I had problems, but it seemed so...well...self-serving.

But honestly, I DID think it would be fun and end up an educational process in acoustics for many...