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

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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...