TangBand 75-1558SE on a Waveguide with Measurements

[Pete Schumacher]

Re: TangBand 75-1558SE on a Waveguide with Measurements

Since this driver is really best used no higher than 3KHz, this waveguide may not be such a bad thing after all.

The response in the usable band, 500Hz - 2500Hz is improved over the non-waveguide version, and you get a bit of a sensitivity boost as well. The off axis response looks good too.

I wouldn't throw this one out yet . . .

[augerpro]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I does do some things well, the gain across its usable passband, and the 1.5khz dip is smoothed out. I think I see what Dave is saying. I had planned on these large roundovers to get it to flare away from the dome as quickly as possible but if we think of the walls of waveguide as a radiator-and out of phase from the dome- maybe the answer is to get the walls nearer the dome. Is this what you were referring to Dave by recessing it more? What I'm thinking is it will probably always have that dip, but maybe I can push it up in frequency. Maybe get lucky and knock out that 7khz peak acoustically.

[Pete Schumacher]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I does do some things well, the gain across its usable passband, and the 1.5khz dip is smoothed out. I think I see what Dave is saying. I had planned on these large roundovers to get it to flare away from the dome as quickly as possible but if we think of the walls of waveguide as a radiator-and out of phase from the dome- maybe the answer is to get the walls nearer the dome. Is this what you were referring to Dave by recessing it more? What I'm thinking is it will probably always have that dip, but maybe I can push it up in frequency. Maybe get lucky and knock out that 7khz peak acoustically.

Push it too far back and you lose the AC offset you were attempting to "correct." You'd then have to also offset the tweeter, which I suppose isn't too bad as it could go in a waveguide as well.

Still, I kind of like what you have there already. I was planning on crossing lower, simply because of CTC with the tweeter.

I'm looking forward to your further experiments!! :D

[Mayhem13]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I'm curious as to what happened to HD with the guide? I bet F2 and F3 are way down from 1khz to 400hz.

[dlr]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I does do some things well, the gain across its usable passband, and the 1.5khz dip is smoothed out. I think I see what Dave is saying. I had planned on these large roundovers to get it to flare away from the dome as quickly as possible but if we think of the walls of waveguide as a radiator-and out of phase from the dome- maybe the answer is to get the walls nearer the dome. Is this what you were referring to Dave by recessing it more? What I'm thinking is it will probably always have that dip, but maybe I can push it up in frequency. Maybe get lucky and knock out that 7khz peak acoustically.

In looking at pictures of the ATC (I haven't seen it up close for years) it seems to be a bit more recessed (could easily be wrong there) as well as the walls having a bit less curvature. The latter actually seems counter-intuitive at first. The problem may have more to do with the damping in the dome. The ATC always looked to be extremely damped. This limits its top end extension, but at the same time it reduces the output higher up the dome where the reflections they create are more exacerbated. Damping that reduces the amount of reflected energy. They're down 3db at, what, about 5K? My guess (and that's all it is) is that they roll off the dome output early to minimize radiation in the top area for this reason.

Were it to be recessed more, it would be closer to a horn loading, but it may push the reflections/dips higher in frequency. The closer the baffle rollover is, the higher they'll be, as you suggested. This kind of problem is the major problem for tweeters in coaxials. I hesitate to call it a waveguide, I don't see it as such in this application, especially given the provenance of that descriptive in audio. As Pete said, keep the lowpass at 2.5K or so and it should work reasonably well there.

If you spread out the opening, you may just push the interference down in frequency, though I would guess that it will be a bit lower in amplitude as well. It's a tough call.

I'd consider trying to sketch it out in profile. See what the geometry looks like from the side. In the end I can really only make guesses as to what might work.

dlr

[dlneubec]

Re: TangBand 75-1558SE on a Waveguide with Measurements

FWIW, based only on experience with my DIY waveguide tweeter mountings, if there was one thing I would try first, it would be to keep the depth the same and try a smaller roundover, say 3/4". As several of us have said, what you have is really an improvement, up to a point, and just maybe you can push the dip up a little higher with a smaller radius or alternatively a deeper WG with the same radius. In all cases, I would keep the throat as close to the dome as you can get it.

You really should expect to do a bit of experimentation to find the ideal solution, so you may want to make a baffle, if you have not already, that you can mount your waveguides into, so you just have to fashion each waveguide the same way and mount it into the baffle. I made a bunch of blanks up when I was doing my testing and then was able to quickly route new versions and swap them in and out for testing.

One thing I didn't do and was mentioned above, is take distortion measurements to see what effect the WG has on them. If you have to pad down the mid due to the boost, do you indeed lower distortion levels by some amount over the flush mount version?

[dlr]

Re: TangBand 75-1558SE on a Waveguide with Measurements

One thing I didn't do and was mentioned above, is take distortion measurements to see what effect the WG has on them. If you have to pad down the mid due to the boost, do you indeed lower distortion levels by some amount over the flush mount version?

It will be interesting to see the results of a smaller radius. You'll be trading off low end gain with (possibly) some higher end smoothness.

I would say that it's almost guaranteed that any non-linear distortion is reduced due to the increased sensitivity (the gain) in the low end. However, there is no guarantee that the linear distortion will not change for the worse. That may actually increase in some ways if the response is not as smooth. This would have the caveat that the change is prior to implementation with a crossover.

I'm still in the camp that says that any definitive distortion results require a crossover be in place to know what the real conditions are, both for linear and non-linear distortion. What matters is what results are obtained in a final design.

dlr

[dlneubec]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I agree Dave, especially since the WG will probably require a different crossover implementation than the surface mount. It would certainly have to padded down more, if an equivalent sensitivity were desired and the change in driver AC should require changes to the mid low pass and tweeter high pass as compared to the flush mount. Of course, unless you take the time to develop a crossover and test for both the standard mount and WG mount, we will never know how implemented distortion levels compare.

That said, I think it would be valuable to see how raw distortion measurements change, good or bad. It would be great to see how CSD changes as well. I wish I had done this with my Peerless HDS WG implementation.

[augerpro]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I had asked someone who owned an ATC dome and he said it was maybe 5mm from dome tip to the baffle plane.

I will be trying a smaller roundover also. I just figured that the biggest, most gentle slope would be the least detrimental. But if I think of the dome and roundover as two radiating sources seperated by some distance, with a null at the frequency of twice that distance, than I can see how smaller is better.

I've got two blanks cut right now for my test baffle. My current one is still valuable in that I can change the depth both deeper and shallower and so I can get a handle on that variable. Depending the results I may have a better idea what to do with the new ones.

Dan good question on the CSD. I did not plot it, but when I ran the test teh IR was clearly worse for the WG baffle. There was an additional peak just after the first +/- peaks.

[benchtester]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I will be trying a smaller roundover also. I just figured that the biggest, most gentle slope would be the least detrimental. But if I think of the dome and roundover as two radiating sources seperated by some distance, with a null at the frequency of twice that distance, than I can see how smaller is better.

I think you may be correct. The dip looks like a "waveguide dip to me". I see them a lot at 8 to 10 khz on 1" tweeters. I describe in my blog (click on the 2 on the right) how I used clay to form a waveguide. (Future plans to machine the shape in hardwood.) I ended up with a small radius near the surround and increasing as it went out, looking like the end of a trumpet horn. If I added clay near the outside making it closer to a full radius, the dip deepened.

My $0.02

[benchtester]

Re: TangBand 75-1558SE on a Waveguide with Measurements

than I can see how smaller is better.

Could be, in Zaph's alternate mounting of the D26NC55, he used a .5" radius and the dip was small and over 20 khz.

http://www.zaphaudio.com/tidbits/D26NC55-halfinchhorn-stock.gif

[Jonathan Anspach]

Re: TangBand 75-1558SE on a Waveguide with Measurements

It's great to see someone doing these types of experiments. Please keep it coming.

Have you read the article here? It might help guide you in what to try next.

http://sound.westhost.com/articles/waveguides1.htm

[Pete Schumacher]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I had a thought, and it's probably just a shot in the dark, but the change in response looked familiar to me.

Check out the waveguide response of the TB 3" dome



And now look at the RS28.



Is that dip in the RS28 response waveguide related?

[augerpro]

Re: TangBand 75-1558SE on a Waveguide with Measurements

I had figured it was interference due to not having a phase shield as most metal domes do, but it could be some kind of reflection off the faceplate yeah...what's half a wavelength of 15khz?

[Guest]

Re: TangBand 75-1558SE on a Waveguide with Measurements

A bit under 1/2". Why?