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4.158.0.145
In Reply to: RE: is it ok to examine some BLH horn by converting to FLH? posted by freddyi on July 15, 2007 at 09:06:27
Hi Fred
I dunno if it answers the question or not, but it's an interesting comparison. The BLH is very rough and peaky above 200 Hz, but most of this would be rolled off by the muffler effect of the labarynth in most typical designs. While the FLH goes lower, the BLH has a nice flat plateau from just over 50 Hz to over 80 Hz, which gives some justification to using this design over a FLH where use of a reactance anulling box is possible. Contra to this is the predicted 140 Ohm peak for the BLH at just under 20 Hz (yikes!) where the FLH is nice and flat. So it looks like there's both some good news and some bad news for BLH's and us double loaders too.
Thanks Freddyi Labs!
Paul
Follow Ups:
hey Paul - here's a real BLH built by "awhite" over at the fullrange forum using Fostex 225K. IIRC throat area is around 8 sq.in.I asked the builder to put a sealed chamber over the front - he took a graph and I mucked with it a bit adding his plans to show the horn's folds
here's what he came up with FOR RESPONSE
some BLH-horn-designers insist upon smooth curved surfaces while others might take an approach ~like Olson's 1937 patent.
questions - whats important in making good and good sounding BLH? how far up must the rear wave show "gain" and how does baffle-step and what can be expected of the rear wave's contribution and summation(s)
in real world horns?imo that graph is cool - seems like method I suggested would pretty well separate the rear-wave performance and beamwidth with a few traces - whadya think?
Freddy
Fred
Some of your graph is chopped off, and there's no before and after for comparison. Anyway, this iteration rolls off steeply over 200 Hz, not really much different than Olson's BLH. The front and back wave have to integrate seemlessly (hopefully) with each other, like a good vented box. In a big BLH, the baffle step must be considered in the design, as in the rececently discussed Olson Modified Monster. Much of BLH design is still in the realm of "cowboy engineering" (try it and see), though this may certainly change some day as sims get more sophisticated.
I dunno if we're there yet.
Paul
might take another decade (if there were interest in BLH) to get similator which could handle complexities yet be "understandable" (?)
awhite's real graph goes to 1Khz but I accidently cropped off test - I think he's worried the throat at 50sq.cm is too small for 8" driver as other horn builders suggested ~0.5*Sd
is a cowboy BLH cookbook possible? - how does one manage translation of an expansion into Olson's stepped manifold? how much wer you able to jockey things and tone by changing spacing of the plates?
Freddy
Fred
It's a little hard to figure out what's going on from the truncated graph. I asssume the box was placed over the front of the normaly direct radiating 8"er? I can't read the dimensions even on a paper printout, but bass response to 75 Hz is respectable in what must be a relatively small horn mouth. The -3 dB point is at about 70 Hz, but the bass roll-off is rather gentle for a horn. Extrapolating from your previous post & graph, I would expect that the BLH (sans box) version would probably not go as deep, and that the roll-off would be steeper. As to whether the horn mouth is the correct size, a graph comparing the horn ouput with the direct radiator output would decide this.
Me and my Dad assembled my bass horns in the late 70's using glue and 8 penny CC "sinkers", so there is no way the things are coming apart to move the manifold plates around. This is why I suggested to John in Melbourn that he make the top plate removeable. While small changes in the area of the throat and horn mouth make for measureable changes in response, I have found that rather rediculous departures from what would be considered a proper horn expansion can be tolerated in other areas in such a horn. I once made some reflectors to correct for what seemed to be sudden discrepencies in the horn expansion in the lower "S" bend of mine, and was surprised to find that they made no difference in the response of the horn! Not so for even small changes in the mouth or throat though.
A BLH Coolbook is certainly possible, but the fact that these type of cabs are relatively easy to build makes the need for this a bit questionable. It's just a big box with a shelf in the middle and some plates forming the "W" manifold. Olson did provide electrical analogies for the various perameters, and it would be up to some brave person (David McBean?) to do the software. Perhaps the Call of the Table Saw & Sawzall speaks stronger to some of us.
Paul
yeah Andy's box was clamped over the front so only the rear horn would work - - - - hit "F11" and see if pic opens to fit your browser screenGraph Again
Andy's horn for FostexOne View
Another View
for this style BLH there should be certain bulk and parameters to give common cutoffs (70/60/50/40/30) without peakingRCA-Fan mentioned AJ-Horn being ~80% accurate - what happens if luck places the real thing in the other 20%?
seems to me the little fullrange BLH draw by far the most interest and coaxial's are shunned as bastards due to xover (and maybe size)
Freddy
Fred
Bulk and perameters here should follow from general horn mouth and horn length procedures, mitigated by space radiated into etc.. 70 Hz is kinda big, with WAF Z increasing exponentially as 30 Hz is approached. Below 30 Hz, the horn would be large enough to live in, following projected divorce (best dang stereo in the trailor park!). Can anyone verify (in a real world horn) that 145 Ohm peak in your sim @ 18 Hz? It seems a bit far fetched, but I've only looked at mine down to 20 Hz or so. I'll have another look with the oscillator when I'm back up and running again. Thanks for giving me something else to worry about : )
If you wind up with a 20%er with the AJ sim, that's when a sawzall comes in handy. Any driver on any horn does something, to paraphrase Tom Danley. So try a different driver, or start cutt'n some wood, or both. Push on those perams.
The full-rangers do draw some interest due mainly to their relatively small size and simplicity. One can bask in the simplicity of having no crossover, untill one realizes that there is in fact a crossover between the direct radiator and the horn (and also one between the cone and whizzer too). But things aren't all that bad as they seem, they're actually much worse as there is a 90 degree phase shift between the transfer functions of the horn and direct radiator, not to mention a continuous time lag in the direct radiator itself, which increases with frequency as Tom Danley has also pointed out. At least the whizzer should be in step with this. So full-rangers have no reason to cop an attitude over co-ax guys.
Sometimes ya have to just let the music play.
Paul
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