P.O.C. #2 - Compact Tapped Horn

[Brian Steele]

I think I've come up with a smaller version of s segmented TH for the same driver:


S1 600 cm^2
S4 1300 cm^2
L12 18 cm 7.1 in
L23 204 cm 80.3 in
L34 18 cm 7.1 in

Length 240 cm 94.5 in
Segments 4
Net Volume 228000 cm^3 8.05 cu.ft.
Fc 36 Hz


I gave up a bit @40 Hz, but ended up with a bit more above 100 Hz, which might be a better fit for the intended purpose of this box. Passband SPL remains about the same (~120dB). Box size is quite smaller too (version #1 had a net volume of 12.75 cu.ft.). Net dimensions work out to 31.5" x 18.75" x 24", which seem reasonable I think.

Shoe-horning a 15" in driver into this smaller TH might be a problem though! It might not be possible to mount the driver so that the magnet structure is facing into the start of the line, as the depth at that point might be too low (6 in.!). OTOH, if I mount it so that the magnet is facing into the mouth, it might likely shade the mouth too much - some ad-hoc experiments I've done with P.O.C #1 suggest that even a small amount of shading can drastically change the response. I might settle for a version that's a bit wider to avoid these problems.

Now I just need to get my hands on one of these drivers: http://www.parts-express.com/pe/show...number=295-080....

[GM]

Why? It's a worse choice for this app than the POC. You want to be moving to smaller, high Xmax drivers to keep size reasonable in this BW or at least ones with a lower Vas, Qts.

GM

[Brian Steele]

Originally Posted by GM

Why? It's a worse choice for this app than the POC. You want to be moving to smaller, high Xmax drivers to keep size reasonable in this BW or at least ones with a lower Vas, Qts.

GM

The parameters of that driver were used for modelling P.O.C. #2

Efficiency of this model works out to around 99dB/1W/1M to 50 Hz.

Any suggestions for smaller, high Xmax drivers that can be used instead? I've put together a spreadsheet that basically chucks out the box dimensions when I put in the TH specs, it should easy to compare a few options.

[GM]

Hmm, I use compression horn theory, so for the POC I get a 21.11-209.44 Hz BW to load it from its Fs - up with a conic expansion and just like in any cab alignment, the lower the Vas and/or Qes the lower its acoustic bulk, so the Dayton is going in the wrong direction WRT bulk. Plus, it has a lower Fs, necessitating a longer axial length to a lower HF corner, further increasing its bulk as it shifts its pass-band lower, hence my remark.

Well, in the big TH threads the MCM 55-2421, its TB variants as well as the CSS TRIO8 comes to mind. For some really serious SPL (up to 135+ in <12 ft^3 in a sim) there's the B&C 15TBX100 that DSL uses in at least one of their products if you still prefer a large driver, but multiple small drivers is the future for HIFI/HT/small prosound apps whether in a single cab or spread across multiple small horns that you can carry one on each shoulder.

GM

[Brian Steele]

Originally Posted by GM

Hmm, I use compression horn theory, so for the POC I get a 21.11-209.44 Hz BW to load it from its Fs - up with a conic expansion and just like in any cab alignment, the lower the Vas and/or Qes the lower its acoustic bulk, so the Dayton is going in the wrong direction WRT bulk. Plus, it has a lower Fs, necessitating a longer axial length to a lower HF corner, further increasing its bulk as it shifts its pass-band lower, hence my remark.

Well, in the big TH threads the MCM 55-2421, its TB variants as well as the CSS TRIO8 comes to mind. For some really serious SPL (up to 135+ in <12 ft^3 in a sim) there's the B&C 15TBX100 that DSL uses in at least one of their products if you still prefer a large driver, but multiple small drivers is the future for HIFI/HT/small prosound apps whether in a single cab or spread across multiple small horns that you can carry one on each shoulder.

GM

I think that so much is different between a TH and "true" horn that it's dificult to apply the same theories to both. Look at mouth size, for example. The mouth of my POC is only 1300 cm^2, but the sims suggest response down to 40 Hz.

One thing that sticks out though from the sims I've done so far is that it's difficult to get an efficient TH with a smooth passband that goes above 100 Hz if you start with a driver that has a low Fs. All the sims I've tried so far suggesting a starting point for Fs of ~ 40 Hz. Then the Qes can't be too low either - this seems to affect the S1:S4 ratio, and if it's too small, the throat gets almost impossible to implement in an actual box (some designers are opting to place the magnet structure in the mouth to get around the problems with small S1, but this can introduce response problems because the magnet structure is now obstructing an area where velocity is at or near maximum). Finally, Vd is also important for peak output, and if you start with a small driver, it either has to have massive excursion potential or you have to use multiples of them, driving up cost.

The sim suggests a clean 120dB in the passband for a box that's a bit above 8 cu.ft. net for this 15" driver. I'm going to try out one or two of those MCMs in a sim, but I'm going to bet it's not going to approach that, at least not for the same passband. I could also swap the Dayton driver for a Kappalite 15, increase the box size a bit and get another 5dB of output for the same passband (I might even opt for that ).

[GM]

There's at least two valid ways to mathematically approach designing a TP/TH, either as a segment of a much larger horn and/or as a 6th order BP taken to its logical extreme and in either case you're always trading cab efficiency for BW and vice versa same as any other speaker alignment regardless of the driver's size.

Anyway, when I use TL/horn theory as a basis to design TPs/THs I get 'close enough' simmed matches to the DSL products that I have enough tech data on to compare them (once damped) that I feel I'm in good enough 'company' not to be swayed by your 'argument'.

Regardless, thanks for sharing your 'adventures' in (TP/TH/BP6) horn design. Looking forward to what you find out and why/what you ultimately choose for the app.

GM

[Brian Steele]

Originally Posted by GM

There's at least two valid ways to mathematically approach designing a TP/TH, either as a segment of a much larger horn and/or as a 6th order BP taken to its logical extreme and in either case you're always trading cab efficiency for BW and vice versa same as any other speaker alignment regardless of the driver's size.

Anyway, when I use TL/horn theory as a basis to design TPs/THs I get 'close enough' simmed matches to the DSL products that I have enough tech data on to compare them (once damped) that I feel I'm in good enough 'company' not to be swayed by your 'argument'.

Regardless, thanks for sharing your 'adventures' in (TP/TH/BP6) horn design. Looking forward to what you find out and why/what you ultimately choose for the app.

GM

Actually I haven't looked at the mathematics of tapped horns at any serious level yet. I'm basing my observations solely on the HornResp sims that I've done (or attempted to do) since building POC #1 for a number of drivers, including the suggested MCM driver (added that to the sim database yesterday). The results indicate path lengths that are similar to horns, but mouth sizes that are not. I did come up with a TH with four of the MCM drivers that in the HornResp sims appears to be more efficient in the passband I'm aiming for, but with S1=136cm^2 and S4=900 cm^2, it's probably going to be impossible to implement in any meaningful way. I also couldn't come up with something using the Trio8 that I was really happy with. The B&C driver looks interesting, but costs are bit out of my budget.

Thanks for your help so far - very much appreciated. I'm certainly going to review a few more options before settling on a particular driver for POC #2.

[Patrick Bateman]

Originally Posted by GM

Hmm, I use compression horn theory, so for the POC I get a 21.11-209.44 Hz BW to load it from its Fs - up with a conic expansion and just like in any cab alignment, the lower the Vas and/or Qes the lower its acoustic bulk, so the Dayton is going in the wrong direction WRT bulk. Plus, it has a lower Fs, necessitating a longer axial length to a lower HF corner, further increasing its bulk as it shifts its pass-band lower, hence my remark.

Well, in the big TH threads the MCM 55-2421, its TB variants as well as the CSS TRIO8 comes to mind. For some really serious SPL (up to 135+ in <12 ft^3 in a sim) there's the B&C 15TBX100 that DSL uses in at least one of their products if you still prefer a large driver, but multiple small drivers is the future for HIFI/HT/small prosound apps whether in a single cab or spread across multiple small horns that you can carry one on each shoulder.

GM

I've noticed that Danley begain using multiple woofers in many of his tapped horns. I think this is to smooth the response. Or at least Akabak indicates that it will.

The glue is drying on a new design, with three of the ubiquitous MCM 55-2421 drivers. I've done some preliminary measurements, and it has usable response over three octaves (40 -320hz.) There's a notch around 180hz, but it's narrow, probably inaudible.

This one should be a good all-around design, particularly compared to the two I did previously. The first one basically didn't work. And some people on here warned me it wouldn't (including you!) The second one, based on the TH-Mini works great. But some whiners complained that 4.5cf was too big for a single 12"

So the new box trades some efficiency for box size, and uses woofers with much higher xmax to increase maximum output. It's harder to build, but it's cheaper.

I'll have some pics and measurements posted soon.