Axiom Home Page Forums General Discussion Home Theater VP150 Calibration Problems

Tony,

My VP150 is mounted on a shelf directly above my 65" Mits. It is 5.5' above the floor, 12' in front of the main listening area. I have the speaker angled down to compensate for it's relatively high placement. It is angled down so the tweeters are pointed at ear level.

After reading your post, I used my RS analog sound meter to make some measurements. Using my receiver's built-in white noise tones, with the sound meter pointed at the ceiling, I adjusted the volume of the receiver until the meter showed 75 dB (C-weighted, slow response) while I was in the middle of the listening area. I've got a 7' couch, and I moved the meter to the left and right edges of the couch while playing the white noise. The meter dropped 1 dB at the left and right edges. The drop-off was linear, in other words it seems to drop off steadily. I would estimate it reaches the 1 dB drop-off at approx 15 degrees off-center. I've got a chair that is quite a bit off-center (I would say it's about 45 degrees), about 10' away from the speaker. At this chair, there was a 3 dB drop-off.

I was curious to see if the angle of the speaker made any difference. So I placed the speaker flat on the shelf, so it was no longer angled down to the listening area. I noticed a 2 db drop-off in the center of the listening area and a 3 dB drop-off at the edges of the couch, so the net effect was the same as before.

HTH,
Jason

Thanks for the response guys. It kind of confirms what I measured. Whether it is audible on voices, it's hard to tell. I haven't seen any NRC measurements of the VP150. I think it would be interesting to see some. Since Ian got my VP150 back, I don't think I'm going to have him send it back to me. I guess I'll just move on. Now what's interesting is that Ian said in his original response to my query, that no way should I measure the differences I measured. One db is all it should differ out to 45 degrees, way beyond my angle. Now, folks are telling me it's normal to measure these differences.

Chess, if what I'm hearing are the artifacts of pink noise, I wonder why Ian had me send the VP150 back instead of giving me that explanation.

Call me confused. Thanks again, guys.

Tony

Tony, the easiest explanation for the differences in people's experience is apples to apples comparisons. Using sound test tones i measure no significant differences until a more extreme off-axis angle with the VP150 is reached. Ian may have also used a range of actual frequency tones. I don't know and hence i cannot explain his results although they do coincide with mine, but of course not with pink noise (apples to apples).
As i said previously, pink noise is part of the problem but all speakers have an off-axis response and this is something normal to expect. It is not a flaw with the VP150 and other centre channels may very well be even worse. Using the M2 as a centre still has an off axis response as well but keep in mind you cannot compare a horizontal vs. a vertical design speaker (apples to apples again). Its off axis vertical sound may be more noticeable or measurable with pink noise compared to its horizontal response. Axiom does ALL these measurements and incorporates this in their designs.

I will be over a friend's place this weekend and he has a Tannoy Saturn centre. I will measure the pink noise tones from his Denon receiver and see if it is any better or worse than the VP150. My guess though is that his sweet spot will be alot smaller but i could be wrong. The Tannoy dual concentric drivers really have a wide, even dispersion themselves.

Wow, that's one fugly center channel! I hope it makes up in sound what it lacks in looks.

How do you get those links to work. I haven't been able to get one to work yet, even when I copy the URL exactly.

Oh, and thanks for the explanation (again).

Take care,

TG

Re "folks are telling me it's normal to measure these differences", all I measured was -2db at 15 deg off axis, whereas you measured -5db. Some of this could be room effects, such as furniture, carpet, reflective walls, etc.

Also if you were off-axis vertically (i.e. VP150 points too high or too low), as you move laterally you'd get faster drop off. Just a possibility.

The off-axis problems of any stereo system (and the center channel is merely augmented stereo) are so well known Dr. Floyd Toole refers to it as an "antisocial" system -- it's optimized for a narrow sweet spot. IOW you can't have a large lateral audience who get equal sonic imaging, so it encourages solo listening. Floyd Toole White Paper

To me issues like off-axis behavior are minor next to the huge differences of room effects, good vs poorly mixed material, sub calibration problems, etc.

Joe:

True re room effects. But interestingly enough, neither the M2i nor the NHT M5 I had on hand exhibited these effects. The reason I was in a sense looking at this issue rather closely is that the layout of the VP150 is very unusual (in fact no other manufacturer has picked up on it) and seems prone to comb filtering which is what I think I was hearing and measuring. But I'm sure Ian knows a lot more than I do about speaker design!

I would just ask Axiom to send you a new VP150, since many of us are not experiencing your problem with our VP150's.

Jason

tonygeno,
Regarding the comb filtering.
Read this post by Ian and this thread.

I had seen those threads, Chess, which is why I ordered the VP150. However, in my environment, I noticed a very significant drop-off in output as I moved off axis, one that I heard with my ears. The meter just confirmed it. I have been emailing a couple of guys who notice this as a "muffling" of the center, but neither of them has calibrated their systems, so I don't know if it's what they're hearing. If this were such a great center channel layout, I wonder why others haven't picked up on it, though.