Horns and Triodes...?

Greg, I have the same question as TWL, how do you front load a Lowther? I can figure backloaded horn, but front loaded? Do you have pictures of this beast, a plan or a link? Furthermore, without these schematics or some illustrations, I am curious to figure out how does it cut "mechanically" at 120hz....the subs, everything, you have described is wrecking my brain and now, I am totally obsessed. If you can give me some "fix" I appreciated.

Paul

Hi Tom, Paul- Geez, I don't have the exact measurements handy and no pics yet (I'm a bit of a wet rag -- sorry).

The whole construction is on an open baffle:
The Lowther is wearing the wooden phase-plug.
The tractrix's flare frequency was calculated for 120Hz; mouth diametre just under 1m; throat is just larger than the Lowther's cone+suspension; horn's length under ~1m; presently (and cheapo-ly) the horn is made out of cardboard & I don't remember how many-sided it is (12 probably).

We plan on using suitably treated (i.e. "good-looking") plywood strips for the final, WAF approved, version...

The back of the cone is damped -- good point Tom. In fact, we tried it open baffle (hmmm), covered the lowther with some material (admittedly better), finally made a small rectangular box, damped the spider, no box damping (best - amazingly!). The point is we were getting interfering reflections from the Lowther's back on the open baffle scheme (we *were* close to the back wall, after all -- about 1m away only)

Now to the subwoof. Quite simply, there were 15" Beyma 400's lying around. With a total q in the upper 0,2's, these were unlikely candidates... but then, they were immediately and freely available for the experiment. Xmax looked OK at ~7mm.

We attached these to the baffle (~65cm width) -- one per side. We attached two 50cm sides to the baffle -- more for structural reasons than anything else and to simulate an H woof (open baffle).

Amazingly, this Beyma/Lowther combo works! The beyma is perhaps not the most versatile transducer around but "speed"/(phase) discrepancies are not subjectively perceptible. The B's BL factor of ~22 and a reasonable mms (just over 100gr, if I remember) probably help here.

However, marrying the dba's (horn w/ the woof) was not that easy.

Putting this together:
We matched ("equalised") the Beyma/P-L combo's db level at ~60 Hz to the horn's ~130Hz (the beymas were losing ~7db @60Hz vs 120Hz) & "flattened" it with the low-pass.

As you expect, the 300B drives the lowthers x-overless. Subwoof: we are taking output from the 300B, feeding that into a passive low-pass, then into an old 400W/Ch Phase Linear (P-L; for the young, that's an old manufacturer:)) which drives the Beyma coils directly. Strangely, the passive filter sounds better than an active one we tried.

Final task: align acoustic centres (i.e., bring the woofs forward). This will have to wait until we finalise the woofer units. If finances allow, the supravox 400-EXC with the electromagnet field coil. Makes for a very versatile driver (basically, one can adjust vas & qes->qts). Also, at $7-800 apop, it's an expensive driver.
Is dreaming allowed? TWO Supravox 400 per side in a W config...

A final point-- my personal contribution to this construction is practically nill; I just insisted on trying open-baffle and supported (enthousiastically) front-loading the Lowthers. This friend calculated the exponential (tractrix) horn by himself! That's when he was oblivious to the fact that there are spreadsheets around and that the horn he was working on answers to the name "tractrix"... the fact that he is a PhD in maths & has 25yrs diy experience helped, I suppose...

Cheers!

I am crying. I think I will never be able to take a hack into your project, unless I have really idiot proof explanation and some decent diagrams, but man, does your set up sound like something out of a futuristic movie. I luv it! If you can, try to make some diagrams of it and also pics. "youngwon gidarilso iqkeso",I can wait forever until you have this!

PAUL

Paul -- thanks for yr kind words. OTOH, youngwon?? gidarilso?? iqkeso?? Ya got me there!

Greg, I understand the Tractrix front-loaded horn arrangement just fine, and there are some things like that already out there using Lowthers, like the Oris 150. However, most designs like that have a box loading of the rear of the Lowther, and I was most interested that your design did not load the cone in the normal manner.

I expected the rear radiations to play into this equation at some point, and was interested in your comments about that. I think the idea of an open baffle Lowther is interesting. It sounds to me like you ended up with some sort of "aperiodic" loading design.

To Paul: The horn contour is designed to have a "lower cutoff frequency" which is defined by the length and mouth size of the horn. At this "lower cutoff frequency" the bass sounds are no longer amplified by the horn, and the efficiency drops off correspondingly, like an acoustical filter.