Here Is the design , a basic 2-way monitor using the Seas ER18RNX paper woofer ( same as In zaph's SR71 ) and the Seas 27 DXT tweeter .
The Seas ER18DXT ported two way
It's a rather new design but whats special about it is the Xover network using a second order on the tweeter , makes him play really low ... but it works .
Thing is I was going to build zaph's SR71 until I came upon this , on a meeting of fellow DIY'ers mark brought these things and they were best in show , You can see the link there .
even on zaph's own blog he considers this to be an upgrade from the SR71 , L18 builds ...
costs slightly more then the SR71 ... but probably outperforms it in any way.
your thoughts ? im seriously wondering about building these now
The Seas ER18DXT ported two way
It's a rather new design but whats special about it is the Xover network using a second order on the tweeter , makes him play really low ... but it works .
Thing is I was going to build zaph's SR71 until I came upon this , on a meeting of fellow DIY'ers mark brought these things and they were best in show , You can see the link there .
even on zaph's own blog he considers this to be an upgrade from the SR71 , L18 builds ...
costs slightly more then the SR71 ... but probably outperforms it in any way.
your thoughts ? im seriously wondering about building these now
Well, the powerrespons of Marks design is just awesome and that just isn't possible with a regular tweeter. Besides, both Zaph and JonMarsh are recommending Marks design so for me the choice is simple.
I was comparing the Waveguide mounted TDFC to the DXT. Here's the HD plots with a single cap and WG mounted. Naybe you thought i was comparing the SR71?
Besides recommendations, the graphs pretty much tell the whole story. This configuration could cross well to a lot of 8" woofers as well.
Hi guys,
A couple of comments.
The distortion profile is very good. I suspect that the DXT motor is very similar/identical to the TDFC motor. No doubt a larger waveguide will have less distortion. However, the distortion of the system, ER18+DXT is excellent and below a certain level, arbitrary decreases in distortion are probably not audible. The idea was to keep distortion well within a certain standard of excellence while optimizing the power response as best as possible.
I'd love it if Seas put out a larger DXT-the diy MCM is an excellent choice, but a pain in the rear to cut, mount, etc. This is a great way to go if you're up to the challenge and want a larger box. Jon over at HTguide is working on exactly this. John's MCM TMM is also an option, but big!
As far as dampening-if you want to pad the walls, this is fine. I've never found this that effective, but it will not hurt. I think that stuffing the interior aggressively is more effective lowering internal resonances. You might have to skip the batting and just use the polyfill if the wall treatment is thick.
As far as the order-it's second order acoustic 2 octaves on either side of the xover point, though, as noted the tweeter probably does have a 3rd order asymptote.
Again, the main benefit of this two way, as opposed to any other well executed two way, is that it does have much a much smoother power response. This requires a waveguide and a lower order xover.
Mark K
A couple of comments.
The distortion profile is very good. I suspect that the DXT motor is very similar/identical to the TDFC motor. No doubt a larger waveguide will have less distortion. However, the distortion of the system, ER18+DXT is excellent and below a certain level, arbitrary decreases in distortion are probably not audible. The idea was to keep distortion well within a certain standard of excellence while optimizing the power response as best as possible.
I'd love it if Seas put out a larger DXT-the diy MCM is an excellent choice, but a pain in the rear to cut, mount, etc. This is a great way to go if you're up to the challenge and want a larger box. Jon over at HTguide is working on exactly this. John's MCM TMM is also an option, but big!
As far as dampening-if you want to pad the walls, this is fine. I've never found this that effective, but it will not hurt. I think that stuffing the interior aggressively is more effective lowering internal resonances. You might have to skip the batting and just use the polyfill if the wall treatment is thick.
As far as the order-it's second order acoustic 2 octaves on either side of the xover point, though, as noted the tweeter probably does have a 3rd order asymptote.
Again, the main benefit of this two way, as opposed to any other well executed two way, is that it does have much a much smoother power response. This requires a waveguide and a lower order xover.
Mark K
Thanx for the Reply Mark...it just so happens that i've got everything including a spare set of MCM guides so i was thinking to forge ahead with my TDFC's instead of the outlay for the DXt tweeters. I've got a pretty good method of adapting the tdfcs to the waveguide that involves gluing wellnuts into the guide with epoxy and then screwing the tweeter on to the backside of the guide. Subsiquently i fill all of the voids with epoxy which increases the density of the guide. Well i'm sure you can understand my hesitance to spend more $$$ as this is DIY..Wadda you think?
Thank you for the reply mark. I think i'll pass on the wall padding then.. But i still find it wierd how you came to that conclusion ,while most people leave the poly and aggresively pad the walls instead ,like in most zaph's designs ,The sr71 also use the er18rnx and he does not mention poly fill ...Wish i could know why . On the side note ,Mark you kinda miscalculated the total cost on your site ,the woofers and tweeters are 259.40 $ while at the total cost they are 159.40 $ so total cost without cabs is less then 500$ not 400$
. On the side note ,Mark you kinda miscalculated the total cost on your site ,the woofers and tweeters are 259.40 $ while at the total cost they are 159.40 $ so total cost without cabs is less then 500$ not 400$ If you have the MCM waveguide and don't mind mating it to the tdfc, this is a good way to go. That would be sort of a cross between Zaph's waveguide and Jon's ER18MCM project-
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It would have the potential to be very good.
Thank you for the reply mark. I think i'll pass on the wall padding then.. But i still find it wierd how you came to that conclusion ,while most people leave the poly and aggresively pad the walls instead ,like in most zaph's designs ,The sr71 also use the er18rnx and he does not mention poly fill ...Wish i could know why
Ooops! I did chisel off $100. Oh well, it's worth it
As far as the stuffing-The rationale for the stuffing the way it is-
Most folks just follow the standard convention of stuffing a sealed box and lining a ported one. It works.
But I've had good success using Martin King's mathcad sheets to model stuffed ported boxes. I wouldn't call the box a TL, but the reality is that TL and ported boxes are along the same spectrum and martin's software can model stuffing a ported box very well. The net result of stuffing a ported box is some slight reduction in bass output which martin's software lets you model nicely and compensate for, and the flip side is markedly decreased internal reflections which can affect midrange/low treble FR significantly.
All my ported boxes over the last couple of years have been with Martin's software and I've always been satisfied with the balance of bass output and midrange clarity.
But, yea, people think it's really weird to stuff a ported box significantly...
Says the 2'' precision port is being used. It does not say how much should I cut it and where ... Or what is the final tuning freq ...
I had a number of different lengths I was trying. I can't recall off the top of my head what I settled on. I'll double check this over the weekend and let you know.
I'll try to post a sim of Martin's nearfield low end response also when I get a chance. I'm always loath to quote an exact F3 because I think this misrepresents actual bass performance. Many ported designs use poor performing woofers, with a slightly exaggerated hump around the tuning frequency. This gives the initial impression of bass, but it ends up being a bit monotonous as the driver can only cleanly reproduce bass around the tuning frequency. On paper, the F3 looks good, but in reality the overall bass performance is not realistic. The ER18DXT is tuned to sag a bit, without any hump. The actual F3 is in the mid to high fourties, but the rolloff is a bit more gentle with the tuning and stuffing. There is meaningful bass output, maybe down 5-10dB to 35-40Hz.
mark
I fully understand the benefit of (very) low distorsions in the low-treble and the global smooth power response on the fr range up to very high fr.
However I wonder about this huge 2.5 - 3.5khz energy (equal from 0 to 45°).
At first thought I would not tell this "smooth power response" in such a sensitive area...Am I wrong?
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Joined 2003
I don't see any problem between 2.5 and 3.5kHz...
Btw, absolutely excellent work Mark! This is definitely a well thought out design, everything taken into account. I am curious about the transfer function though, since you have valleys in the tweeter transfer function at 1.5kHz and 6kHz, I would expect to see a tweeter roll off that is not as smooth as it is. I'm not saying you've done anything wrong, it's just interesting how that transfer function creates such a smooth frequency response.
An externally hosted image should be here but it was not working when we last tested it.
Btw, absolutely excellent work Mark! This is definitely a well thought out design, everything taken into account. I am curious about the transfer function though, since you have valleys in the tweeter transfer function at 1.5kHz and 6kHz, I would expect to see a tweeter roll off that is not as smooth as it is. I'm not saying you've done anything wrong, it's just interesting how that transfer function creates such a smooth frequency response.
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I don't believe it either, but have to admit at what was measured, maybe the phase (from both drivers) is playing a role here (would like to see matching phases to understand) what, also, is not very easy. The speakers look very good. Congratulations Mark.
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