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Hey All,

The parallel plane universe of Q(ts) and X(max) have joined. OK, enough of Star Trek references, the world could do with a few less I suppose.

The folded Voights were sounding really good after 50+ hours of burn in. The Lyeco LY802F drivers have found their stride. I am thinking of starting a Moi decrotive garden in the backyard with discarded enclosures. Port and Voight debris abounds.

I am attributing round bass sound to X and Q. First my foe (given the writings I had first started reading some time ago), now my friends.

Next play thing is backloaded horn. Why?....because math says I can!

I can stare at a computer for so long. Then there is a critical point of "so, the computer says OK.....there is one way to find out". Ever look at a new paint color for your living room too long? You know "X"family of colors is going to work....beyond that holding little swatches next to furniture is only seeing trees and no forrest. Go, go young man....go west.

So here is. The computer illustrates the contraption, and lumber required for compiling what is supposed to be an entertaining sound. Should stands of Birch trees fall opposed to Q? Will Q deliver me from the standard boxes to Ponce De Leon and the Fountain of Youth? I remains to be discovered.

I stand (more like sitting with a few empties) before you at the launch of this endeavor. Will it work? Are your slide rules up to the task (mine has burst into flames with working the wood back and forth at a frantic pace).

Might I present enclosure (called EHT.... Enormo Horn Thing... but in retrospect maybe it should be been names Mr Q).


Hey GM,

Sorry, forgot to point out that on the second page of the .pdf there were the specs you were curious about.

45.9 inch (296cm) throat area
611.6 inch (3946cm) mouth area
49 inch (124.5) horn length
Flare Freq. 55hz
Compression chamber volume of 7.096 liters
Compression chamber fequency 220Hz
Shape factor 8 (corner horn)

I chose corner horn so that I could use full length of horn. There are so many variables in horn design, I opted for not chopping something short and adding yet another guess.

My question to you all is: At what point does the size of the throat (large) make the design of the compression chamber....well, not a compression chamber?

There is math, which I have adhered to strictly, than stuffing/damping factory........ and hopefully the fairy dust brew is ripe.
Sorry, forgot to point out that on the second page of the .pdf there were the specs you were curious about.


Well, that's what I get for not paying attention. I assume an exponential expansion (M = 1).

At what point does the size of the throat (large) make the design of the compression chamber....well, not a compression chamber?

There is no compression chamber per se in a BLH, only a low pass filter, so the question is moot.

Anyway, it appears you've designed a decent PA 'scoop' horn, though I think you should use a ~35 Hz flare frequency. Most folks probably won't find it a good sounding BLH for a FR driver though due to the excessive mids coming out of the mouth and the ~deep broadband dip from ~400 - 2 kHz that el 'Ol alludes to.

As always though, YMMV.

I thought about two german-style big sphericals with same diameter and slightly different opening. What geometry will that give between the flares? Will that be suitable for backloading?


??? Please elaborate.

It all seems to be less strange than I thought. At the Supravox homepage you can see that the 8" with doubled xmax has 3dB less sensitivity than the standard version.

??? Xmax has nothing to do with sensitivity. I didn't save the links, but a few Supravox BLH's have been posted over the years. I imagine it's high Qes keeps most folks from considering them since very few know how to manipulate the math to deal with the 'weak' motor. Anyway, above 0.707, OB or aperiodic TL, or ML-TL yields the best performance overall unless you want to use the horn to lower its effective Qts (flatten its underdamped response 'hump').


OK, so what are you saying here? A front/back horn with same mouth area, but different throat areas? Or same throat area and larger rear mouth area?

Anyway, the latter is the way to go with this, or any other rising response driver. For ~flat response drivers, a double horn will require a horn loaded tweeter to balance the system out, losing much of the fullranger's HF BW.

an illustration


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compression chamber

Hey GM/all,

But in all the reading that I have done, compression chamber on BLH designs is always part of it. Typically w/lower Qts drivers, the thoat appeture is reletively small. As the Qts (and Xmax) gets higher in number, the throat gets much larger as well.

I did not go to 35hz because that would have made the enclosure too large ........ for the moment. I want to be able to get the Lyecos dialed into reasonable size enclosure first. Than I can get go even larger in scale (mwah, ha, ha)

Having said all of that, I have seen a few designs (here and elsewhere) with not really any thought to compression chamber size.

My project aside, when is compression chamber appropriate to incorporate into design and when is it not. I have not found any writings on this.

Per mids/highs wandering their way into the horn. This is something to think about. However, with other projects it seems to be able to be dealt with by tuning.
an illustration


One of W.E.s early 'compact' one-off designs was the same in concept with truncated (50 Hz?) exponential flares and a huge phase plug to get good polar response over all of the horn's (limited) BW. Ugly as 'home made sin' IMO, but allegedly had very high intelligibility even at very high SPLs. The design didn't 'throw' the sound far enough though, so they finally settled on the sectoral format for 'compact' apps.


I can't help it if folks mis-label something for whatever reason. For compression to occur requires a means of counteracting the resistance the filter chamber applies to the driver, i.e. a chamber on the other side, hence the moniker 'compression driver' for a sealed or vented back, front loaded BP. Obviously, the air mass of the room applies some 'compression', but it's so little as to be insignificant and why all BLHs require a ~0.5 'M' factor with a large throat (not to be confused with the low pass filter terminus) to maintain a ~linear driver travel. Once you begin increasing the flare factor, ideally some means is required (front horn, phase plug, acoustic damping) to balance it out.

WRT the size of the filter chamber/terminus, it's a function of the driver's effective mass corner so the lower it is (higher Qts), the larger they must be and vice versa. Obviously, the higher the mass corner, the higher the acoustic XO, so best to add some series R to limit it to ensure that comb filtering doesn't detract from its performance. For me, it's ~250 Hz, but some designers find up to ~400 Hz acceptable, so as always, YMMV. Again, Xmax per se has absolutely no bearing on its design.

You misunderstand, I mean use a 35 Hz flare rate. You can still truncate it at whatever frequency you want.

Yes, there's probably more mis-matched driver/horn designs published on the net than correct ones, but all of course sound fantastic to their owners. ;)

Hey All,

OK, GM, I guess I am a bit confused. Nearly all the BLH designs I have seen have the driver in an sub-enclosure....... compression chamber (so as seems to be defined). There are a minority that appear to have not taken this into account.

There is a formula for that. The relation of the size of this box to the appeture (throat) has been cited as being rather important. As I have said before, in running calcualtions... higher Qts means larger opening (throat). Any other ideas on the relationship of theory to actual would be great.
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