Question for OB experts regarding imaging

[Jed]

Sometimes it makes it worse.

More so if the tweeter is mounted off center. Each has its pros and cons. I'm certain though that in many average width enclosures that a tweeter mounted with symmetry on a baffle does not improve ON AXIS ripple as Dan claimed. Just a simple test in BDS will prove that the on axis is easily smoothed at the expense of the horizontal polar response as pointed out by Zaph in the below text.

Also, back to the baffle width thing. I found that in this same post by Zaph, he sums it up pretty well (please note it was in regards to monopole applications and not dipole so keep it all in relative context):

...

In my eyes, cabinet width affects 2 main issues for the woofer: 2pi to 4pi space corner frequency and diffraction ripple magnitude. The 4pi space corner frequency is not an issue to me, it's easily resolved in the crossover wherever it happens to fall. It's the diffraction ripple magnitude that's an issue, and I don't really see a lot of people being as concerned about it as I am. The magnitude of the ripple is essentially based on the ratio of piston diameter to enclosure width. The closer the piston diameter is to the enclosure width, the smoother the response.

For example, take a look at the raw woofer response of the Scan Speak 15W8530K00 in my ZD5 design. It's a 7" wide enclosure, relatively narrow for this woofer size.



There's hardly any diffraction ripple at all in the woofer response. And of course, on an infinite baffle, the woofer's response has no ripple other than it's own linear distortion issues. Now, put that driver in a 12-14" wide cabinet and see what the response looks like. It's pretty bad.

Of course, this is just in reference to a woofer / lower mid. Mids or tweets will always have ripple problems, some of which can be resolved on axis by offsetting. This comes at the cost of non symmetrical horizontal polar response. But mostly, I like to keep a lot of ripple out of the 500-1500 band because that's where I think it's the most audible. In an average 2-way, that's always going to be in the woofer's area. In an average 3-way, you're screwed with the mid unless you do what Dave does, separate tweeter mid enclosures. Or a tapered/mitered Avalon style box, or some other solution that isn't a big monkey coffin.

I prefer narrow baffles myself, though if I did a wide baffle, it would be really wide and probably curved back too. So in summary, I generally recommend as narrow as possible or as wide as possible, but nowhere in between. Considering most people think wide enclosures are ugly, that leaves the slim option.

I have a 13" wide 3 way tower enclosure sitting in my basement for 5 years now, unused. It was for a largish woofer (the cabinet was set up for 8 to 10" woofers) with a 4 inch midrange. That presented the worst case enclosure - midrange ripple was horrible, and offsetting the mid only helped on axis.

I've really got no new ideas to present here. Everything I mention can be seen for yourself by playing with a diffraction simulator.

[dlneubec]

I'm certain though that in many average width enclosures that a tweeter mounted with symmetry on a baffle does not improve ON AXIS ripple as Dan claimed. Just a simple test in BDS will prove that the on axis is easily smoothed at the expense of the horizontal polar response as pointed out by Zaph in the below text.

Just to clairfy what I meant: I was talking about a narrow baffle improving dipole ripple and when using a centered driver, a tapered baffle (trapazoid in my case) might improve on axis difraction ripple as compared to a parallel sided baffle. The latter was speculation on my part, assuming the trapazoid shape would spread out the diffraction artifacts more than a parallel sided baffle. I never did a sim to confirm that since I had already decided to do a tapered baffle for other reasons.

Ok, I just ran a test in BDS where I used a parallel baffle width (at the mid driver) of 10" and then one with a trapezoid with the same 10" width at the mid driver, but with an 8º taper on the sides, comparing the tweeter response on each. It appears to me that the trapezoid baffle is indeed better than the parallel sided baffle for on axis, 15º and 30º off axis, at least with these assumptions . To be clear, this is not a dipole comparison, but an edge diffraction comparison of the monople tweeter response on a parallel sided and trapezoid baffle, same edge effects treatment on both.

Parallel sided response 0-15-30º top to bottom:



Trapezoid response 0-15-30 top to bottom:

[Jed]

Thank you for the clarification, Dan.

[Paul W]

Dan,
No controlled studies that I know of.

Since you don´t get symmetrical wall reflections from the inside and outside radiations of a speaker in most rooms, a symmetrical polar response would hardly reach your ear anyway. I would concentrate on the constant directivity of the inside radiation instead of symmetry. If your off-axis radiation pattern does not "mimic the on-axis response, but with decreased amplitude as the angle is increased" at least to one side, left-right symmetry is useless.

Hi Rudolf,
Without specifying the room, we don't know which wall reflections "mimic the on-axis response...". IIRC, Toole indicates the front wall is the most likely candidate for an absorber. If we optimize to the inside at the expense of the outside, and place the speakers in a room with a front wall absorber, we absorb the "good" off-axis signal while bouncing the "bad" signal off the sidewalls. As I mentioned before, I personally believe the decision to offset is situation specific...but, just my opinion
Paul

[Dennis H]

Here's what The Edge predicts for a 4" cone centered on a 6" wide OB. Approx 0, 15 and 30 degrees - mic at 2m and then moving .5m and 1m to the side. Looks pretty easy to fit to a bandbass filter centered at 1200 or so.

[Winter]

Is there a difference between the terms "open baffle" (OB) loudspeaker, and dipole loudspeaker? Another acronym/buzzword to remember....

[Rudolf]

Hi Rudolf,
Without specifying the room, we don't know which wall reflections "mimic the on-axis response...". IIRC, Toole indicates the front wall is the most likely candidate for an absorber. If we optimize to the inside at the expense of the outside, and place the speakers in a room with a front wall absorber, we absorb the "good" off-axis signal while bouncing the "bad" signal off the sidewalls. As I mentioned before, I personally believe the decision to offset is situation specific...but, just my opinion

Paul,
you are probably right to keep the individual room out of this discussion. It only would open another can of worms and each and every solution would be different because rooms are different.
I would be happy, if this discussion only helps people to see that baffle width has to be discussed with driver size and driver passband in mind and that off-axis radiation should be as much in line with on-axis radiation as possible.

[Biff]

I am at the prototype testing stage of my original design OB mids/tweets section of a projected 3 way system. I would appreciate input from anyone experienced with OB implementation.

Two 4" mids and a Peerless HDS tweeter are mounted MTM. Spacing between drivers is very close, with mids slightly overlapping tweeter flange. Baffle width is 15", with the outer 3" of both side edges curved into a 3" radius (essentially "wings" that curve toward the rear). The drivers are offset from centerline by about 2". Xovers sound reasonable; I have no method of testing actual FR, but a CD with test tones sounds pretty smooth from 400 on up, as designed.

My question is about imaging. Mono recordings stay very well centered and have nice 'depth", but on certain stereo recordings, the image seems to jump from one side to the other, depending on frequency. Is this the "problem" that some builders report with OB designs? Is it due to baffle width, driver orientation, or could it be Xover anomalies or inequalities? I did not carefully match components between channels, as this is only the prototype stage, but all components were within 5% or so of label values.

I really like the tonality, dynamics, and spatial qualities of the OB format, but I could never live with this wandering image. Please help me solve this problem.

Peace
Tom E

I followed this thread with interest, but I wonder if something is being overlooked.OP said the image jumps from side to side, FR dependent. Two things I could think of, one being what has already been said, differences in reflections behind the speakers, which would certanly do it, and the other being there is some discrepancy such as a difference in drivers, XO as assembled, etc. Has each channel been swept, or just one, or the two together, etc.? Any inadequacy in the design would (I would think) cause similar behavior on both channels. A small difference in FRs would do exactly as described. Just a thought.

[Paul W]

I would be happy, if this discussion only helps people to see that baffle width has to be discussed with driver size and driver passband in mind and that off-axis radiation should be as much in line with on-axis radiation as possible.

Yes, you and I fully agree on these points. :T

[ThomasW]

+2..