Diffraction Waveguide (DXT) Impulse

[Tenson]

I've been building a speaker for my sister-in-law recently as a present and used some drivers I had spare. The tweeter is the Seas
magnesium dome with DXT waveguide. On paper these look superb - the distortion is really very low. They basically take the motor of a very good tweeter (the 27TBFC) and use a stiffer dome and add a waveguide. The waveguide is special because it is the first to implement controlled diffraction to widen the top end dispersion. It works well, the waveguide narrows the low end (~3KHz) dispersion to better match mid-woofers and the diffraction rings widen the top end to give an even more airy and wider sweet-spot than a regular dome.

To make a comparison here is the frequency response on and off-axis of the standard dome 27TBFC

Here is the same measurments for the DXT - You need to ignore the rise in the middle of the range which would be made flat by the crossover. Just look at the relative levels of the off-axis responses. The top end doesn't roll away so fast off-axis and the bottom end narrows off-axis more like a woofer would.

I must say though I am not too impressed with the sound. How can that be!? It has really low distortion and good dispersion characteristics! It sounds a little rough and not focused. Certainly not bad, but not as good as the planar in the Kensai or Rob's ESLs.

I know what you're thinking, it must be the decay plot, right? The waveguide must be adding some horrible ringing. Nope! (Well not <25KHz)



Well, as always there is a good reason for subjective sound quality. It's just a case of needing to look a little closer. In a way it is the decay where this tweeter is odd, but it is over a very short time period. Lets have a look at the Impulse response.



Ahh now that is unusual! All those little spikes are the reflections from the diffraction rings. This is an aspect of the DXT waveguide I have never seen mentioned before so I thought I'd share it here. The subjective difference in sound between this tweeter and the Kensai one is indeed similar as when you add a sheet of felt around the baffle of a speaker and everything becomes more focused, intimate and calm.

By way of comparison here is the impulse response for the direct radiator tweeter in the AS3/6 speaker.



Okay now I admit, if I wanted to really confirm my assertion I need to take the same tweeter and put it in a non-DXT waveguide and do an a/b comparison. The impulse response is, as far as I can tell, the only place these tweeter are less than superb though. Anyway, they are going to my sister!

Happy listening over the holidays chaps

 

[Reffc]

That's really interesting and my own experience is that the standard TF27 sounded better than the waveguide version! I tried several tweeters recently for a re-build of a customer's speakers and was looking for exactly the same thing as you, and settled on the Monacor DT300 with Waveguide.

This is a similar arrangement but subjectively, it doesn't have any of the roughness of sound of the DXT. It doesn't use the diffraction rings and it does retain fairly wide dispersion from good off-axis response but I was using it in a 3-way so this was less of a blending issue. I note also that the impedance plot above doesn't seem to show any impedance changes which I'd normally ascribe to wave-guide loading? (ie caused by the change in acoustic loading)

I must admit that I have been very impressed with the DT300. Is this worth considering for your application?

Another I've been working with recently is the SEAS TF29. That has been superb. Very linear off axis and very smooth.

 

[RobHolt]

Intermod from the huge spike at 27khz beating with audible frequencies?

Perhaps try measuring the distortion from the tweeter but drive it with pink noise extending to 30khz and see if anything 'folds back'.

 

[Tenson]

I note also that the impedance plot above doesn't seem to show any impedance changes which I'd normally ascribe to wave-guide loading? (ie caused by the change in acoustic loading)

I think the non-symmetrical resonance peak is like that because of the waveguide. It would be symmetrical otherwise.

Did you use the DXT, or a normal smooth waveguide attached to the TF27?

I'll have a look at the DT300. I won't change the tweeter though, remember this was built to use bits from my shelf. The sound is certianly not bad. I just expected something special from all the great measurements.

Intermod from the huge spike at 27khz beating with audible frequencies?

Perhaps try measuring the distortion from the tweeter but drive it with pink noise extending to 30khz and see if anything 'folds back'.

I don't think it would be that because I've just been listening to standard CD rips and high-bitrate Spotify. My measurements with the Rohde & Schwarz show a very steep digital filter kicks in on the DAC directly after the upper limit of the sampling range.

If we meet up at one of our places later this week you can have a listen Rob. I've got the passive Xovers made I just need to wire them up inside the speaker.

The mid-woofer in use is an Adire Extremis 6.8.

 

[Reffc]

I think the non-symmetrical resonance peak is like that because of the waveguide. It would be symmetrical otherwise.

Did you use the DXT, or a normal smooth waveguide attached to the TF27?

I'll have a look at the DT300. I won't change the tweeter though, remember this was built to use bits from my shelf. The sound is certianly not bad. I just expected something special from all the great measurements

It was the DXT....I'm a big fan of the TF27 though...it's a great tweeter. I'm using its sibling, the T29 in a current speaker project and it's very good indeed.

Some finishing work to do on rounding the front baffle, but almost there with these now.

Yes, now you come to mention it, the asymmetrical plot would account for the wave-guide.

The DT is a fabulous unit for the money and easily on a par with the TF27 imho. It works really well with the smooth waveguide

 

[Tenson]

I'm curious, how did you go about joining the timber edging at the tri-corners?

Looks like quite a nice traditional speaker. Decent size box and baffle

 

[Reffc]

Thanks

The jointing was a real headache. In the end, the solid walnut edging met each panel with an almost full length tongue joint (both directions). The rear panel contains recesses which mate with ridges on the back of the internal bracing and there's a complex lipping arrangement where the three corners meet.

It took a lot of working out for something so seemingly straight forwards. The end result is a supremely strong and rigid cabinet though so it was worth the effort.

It's fairly traditional...just under 2 cubic feet net volume with a decent front baffle. Golden ratio used throughout.

 

[Tenson]

Ahh yes, now I look closely I can see a tenon and mortise join on each left and right hand side edging strips. I guess you might have then done another tenon and mortise for the top and bottom strips after the first ones were fully glued and set? Time consuming work!

Will you veneer over it, or leave the contrasting edges?

Have you considered modelling the baffle for diffraction effects? I can't help but notice the central alignment of the two drivers. 'The Edge' is a quick and effective program for this. http://www.tolvan.com/edge/help.htm