TangBand 75-1558SE on a Waveguide with Measurements

[benchtester]

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?

I am proposing that the TB 3" dome dip is waveguide related because the 4 kHz dip was not seen in it's stock FR.

I am assuming the RS28 is stock; and I am not very familar with the construction. I doubt there is a "waveguide" as such, but at such a high frequency it could be a similar reflection from the mounting plate. But I am more inclined to think that the dip shown here is a common effect of metal domes. Often you see a phase plate added to suppress this effect. (I don't think the RS28 has a phase shield, although sometimes they are clear plastic under the screen grille.)

I am a novice understanding the phase shield purpose and would welcome clarification.

As a wild conjecture, what if the waveguide brings out the phase issue with the 3" dome. It does have a pretty big "nipple". But the contrary argument would be that the graph shown is already includes Augerpro's phase shield.

edit: you guys are quick, two posts before I could compose mine (or I could be slow). 1/2" would correlate to a reflection off the mounting plate.

[joe7414]

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

looking at a cut away of a atc sm75 i estimate the wg is 25 mm deep and meats the dome at an angle of 45' ,there is a 8mm gap betwen the dome and wg
hope this helps v interested to see how it turns out .

[augerpro]

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

I should be getting more done on this soon, just been so busy with my PA and horn measurements I haven't had a chance to do the woodwork.

[Audio Quest]

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

Bravo for your effort and special thanks to John Krutke for all the testing and measurment data he supplied recently on the TB 75-1558SE 3" Dome Midrange. I can't say enough good things about John as he has helped so many DIY's by supplying information that has helped so many get past their loudspeaker design and building obstacles.

Horn loading of the TB 75-1558SE was something I had thought about as I would love to have a pair of the ATC SM-75 Dome Midranges. I'm not sure what your next step should be, but I am confident with this group of colleagues that resolving this horn loaded design is within reach.

Thanks again for serving on the front line and taking the bold steps forward. The work that you and John have done may make a horn loaded TB 75-1558SE come into fruition.

[augerpro]

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

So finally had some time to try out some new waveguides. Made an improvement, but still nothing usable. This is getting to be a tough nut to crack. Just to recap the first waveguide was basically a 1" roundover, but truncated as the baffle was only 3/4" thick, and it terminated quite close to the dome where it meets the surround.

First I tried to add a 3/8" spacer on WG1 just to see what increasing depth would do. Here is the original WG1 (red) versus the deeper WG1 (green):



Interesting but not an improvement. Next is WG2 which is a simple 3/4" roundover, again 3/4" deep (so a full roundover), but a larger throat terminating near the outside of the surround instead of near the inside. Red is a the stock driver and green is waveguide 2:



Finally getting somewhere! Now I thought I'd try a 45 degree chamfer instead of a roundover. I just reused this waveguide which means the chamfer is not full depth since I needed to leave a little edge for the bearing to ride on. Original roundover in red and chamfer in green:



Meh, not much change, maybe a slight improvement. One thing that bothered me about all these is that since the driver flange is not flat, there is an 1/8" gap between the baffle and the area by the surround underneath where the throat terminates. I geeked out in the first waveguide trying to leave that little 1/8" overhang to help this but its effect was swamped by the poor waveguide. For the hell of it just added some tape around the inside of the mouth so the waveguide would sort extend deeper almost touching the surround. Here is what that did:



So this means doing it right either involves making the little PITA overhang, or removing the driver flange so the baffle edge/ throat termination ends as close to the surround as possible. Which I'm a bit hesitant to do since it looks glued and screwed on, and may hold down the diaphragm.

So I have one blank baffle insert to try something new, what do you guys think? Moving the waveguide wall away from the diaphragm has seemed to help. Strange though...I didn't think it would actually raise the dip frequency as the distance from the wall to the dome got longer. I like the chamfer, as it's a small improvement and if would probably be a hair better if it was the full 3/4" deep instead being about 5/8" deep. Maybe an even wider throat? And an overhang to get the termination close to the surround?

[Mayhem13]

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

While it's not going to help driver to driver spacing at all, a large WG is needed to boost the low end response. To get rid of the dip at 4khz, try 1" of spacing bewteen the edge of the guide and the driver surround. I have a hunch there's a relationship between the wavelength of the dip and the circumfrence of the dome.

[dlneubec]

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

Brandon,

You are making good headway, it looks like to me. Depending on where you crossover, your last implementation could be very usable as is.

My first tip is that you might find that the dips you see on axis disappear off axis, so one thing you should try before anything else, as you do these, is to take a couple off axis shots on each and compare them to the flush mount. for positives, it looks like you have gained a very flat respone out to about 3.5khz and boosted the sensitivity about 2-3db, even some at the low end, which are all good things, IMO. I think you are getting closer.

That said, the problem is that you changed 3 parameters at once with the second implementation and you really should change only one if you want to figure out what is happening. You changed WG depth, throat dia. and roundover radius. You did get a better result, but it is hard to know which of those three were the main reason, which are positive changes and which are negative. You should stick to one throat dia and roundover, for example and try various depths. Then try a different roundover radius at the same depth. Then try a couple wider throat diameters with the other factors remianing the same. Hopefully you will see a trend from each and then have a better idea of which to combine to get the ideal response. It looks to me like you will have fill in that gap in the face plat or remove the faceplate. I wonder if 1/4"w x 1/8" thick weather stripping would work for your testing? I picked up some O-rings to try with my waveguide experiments, but decided it was easier to remove the face plate, though that may not be an option for you. I guess you could try sending an email to Tangband to see if they would tell you if the face plate is removable.

Much depends on what your goals are. My guess is that shallower will get you less boost but a flatter response, so perhaps a 1/2" deep WG with 1/2" roundover would be better, if you don't mind sacrificing the increased sensitivity. If you want to keep the sensitivity, then try the same depth and throat dia. with the larger radius to see how that compares with the chamfer and then keep those the same and try a wider throat dia.

BTW, one trick I employed to do roundovers or chanfers deeper than the baffle was to make a piece for the bearing to ride on and mount it behind the baffle. I simply used old driver cutouts and bored a hole through them of the proper diameter for the throat size I wanted and then stuck it to the baffle using double sided carpet tape. It worked like a charm for me.

There are enough variables that you have to have a lot of patience and a clear methodology to be able to figure out the ideal solution.

[dlneubec]

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

Going back to the original post, I see that the dip didn't really disappear off axis, so perhaps it won't in these new tests, but I would still do a couple off axis shots to be sure, especially on you last implementation.

[augerpro]

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

My first tip is that you might find that the dips you see on axis disappear off axis,

Measurements at 20 degrees still showed the dip :(

That said, the problem is that you changed 3 parameters at once with the second implementation and you really should change only one if you want to figure out what is happening.

I only have so much wood and time to route it man! Yes it's definitely not the scientific method, but I do think we can tease out some observations. The baffle depth was the same, 3/4". The roundover did change from 3/4" to partial 1", but I don't think that had much effect-just look at how little going to the chamfer changed things. I think the real improvement came by making this a larger throated waveguide.

I wonder if 1/4"w x 1/8" thick weather stripping would work for your testing?

I'll try that, good idea. It should work if a thin piece of tape worked.

Much depends on what your goals are. My guess is that shallower will get you less boost but a flatter response, so perhaps a 1/2" deep WG with 1/2" roundover would be better, if you don't mind sacrificing the increased sensitivity. If you want to keep the sensitivity, then try the same depth and throat dia. with the larger radius to see how that compares with the chamfer and then keep those the same and try a wider throat dia.

Right now the priorities are proper depth for a workable AC, smooth response in the passband, and gain, in that order. I may have to try a 1/2" depth to help smooth the response but I wonder how much it's going to hurt where I want the AC? Gain I'm only marginally concerned with compared to the first two. But yeah, I'll have to look into those options you mention.

Another thing Dan mentioned in another forum, which we are on the same wavelength for, is getting rid of the dip. I don't think I really need to get rid of it. I'd just like to push it up in frequency so the typical passband is smooth but with the dip placed over the peak of the natural driver response. An acoustical notch filter.

[Pete Schumacher]

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

I wonder if you could load it with an acoustic bandpass? What if there were a small damped chamber around the dome, with a small opening to the waveguide?

I'll shut up now . . . .

[augerpro]

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

Tried Dan's idea of using some foam strip to fill the gap between the driver flange and underside of the baffle. This is with the third iteration of the waveguide, green is with the foam:



I think we're almost there Looks like the little overhang (and some foam for good measure?) that I showed in the walkthrough of the first post is required. Kind of a PITA but if I can do it anyone can. My woodworking skills are pretty weak. The good thing with all this experimenting is finding out that many typical methods do not work at all. Should prevent someone else the headache of wasted attempts with this driver in the future.

[WayneN]

[augerpro]

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

So maybe ATC and Volt know what they are doing...Just tried a 3 7/8" cutout which is basically at the outer edge of the surround. 3/4" deep and with a 45 degree chamfer. This is probably the best so far, and pretty close to the last one posted above, but a little better off axis. One more experiment and I'll post the results.

[critofur]

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

Awesome, now, what we're really itching to see: some blind A/B listening between the TB and Morel versions of the speaker :D

[AdamFelix]

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

So maybe ATC and Volt know what they are doing...Just tried a 3 7/8" cutout which is basically at the outer edge of the surround. 3/4" deep and with a 45 degree chamfer. This is probably the best so far, and pretty close to the last one posted above, but a little better off axis. One more experiment and I'll post the results.

Hi,
Thank you for your hard work. I'm considering to use the Tangband-75-1558SE in my speaker mainly based on the good distortion measurements done by Zaph. Did you found a way to solve the acoustic center issues with this driver?

Best regards
Adam
Denmark