4q

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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.
Greets!

Yeah, we're having a pretty severe 'failure to communicate'. ;)

What you refer to as a 'compression', or 'sub', enclosure is the low pass filter chamber I speak of. Visualize a BLH as a FLH with no rear cover, ergo no compression chamber. The front chamber of a FLH isn't called a compression chamber, so why should it be so when in a BLH?

I'm sure all designs at least attempt to properly design this chamber, but few DIYers apparently know how to do it correctly, or as I previously noted, they take a perfectly good design and stick in a driver other than the one it was meant for, so if you reverse engineer it you get 'spam'.

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.

There's more than one design 'system' actually, depending on the designer's performance goals, so confusion abounds in the DIY community. Indeed it is since the low pass filter and throat should have the same tuning frequency. Yes, me too, so why keep bringing it up? Theory and real world results tend to be pretty close if you have accurate driver specs, use the right formulas for the job and know how to manipulate them. ;)

Really though, not much point since anybody can be a pretty decent horn designer today, what with such powerful tools as Hornresp, etc..

GM
 
Hey...

HA! Knew we were somehow talking about different things GM! Sorry if I confused anyone with terminology.

OK, now we are on the same track.

I guess my questions are more a matter of curiousity than necessity. Yes, if tried and true formulas are adhered to.... then that alone should put me close enough...... close enough where I can tune from there without ripping wood apart! Have gotten there with other things already built.

Horns are different <insert dark sarcastic remark here>.

There is a thread here about using a driver with a Qts of 1.1. Don't remember which one... in any event, the posted proposed enclosure design has no real compression chamber (as I had been defining it thus far). I did the math.... and sure enough, a Qts that high returns a suggested throat size bigger than the damn compression chamber....so what would be the point?...does everyone else come to the same conclusion?

If this small enclosure enclosing the driver is in fact supposed to be important........... errr........ at what point (Qts) does it become important?..... or not?

Sorry to go on about this, I am curious, and have not really found anything on this topic in very much depth. Most DIY websites by people really glance over this.
 
Greets!

Since horns have so many variables, it has much more design flexibility to achieve a wide range of design goals. Obviously, if the calc'd throat is larger than the filter chamber, then you have a driver mounting hole the same size as its Sd (or whatever portion of Sd will fit) on whatever size baffleboard is required to terminate the horn's throat: http://home.att.net/~lkalin/gifs/downstairs.jpg (note late '40s Altec A5 theater horn doing center channel duty) This is for illustrative purposes only! This driver's specs didn't dictate the throat area, rather this horn represents just a small 'slice' out of a much larger, longer compression driver design and only loads the driver over a very narrow BW to extend the multi-cell's gain down into the midbass.

Qts has no bearing on the filter chamber's design beyond determining the mass corner if you're using it to set the horn's Fh. That said, when Qts = 2.0, then Fs = Fl = Fh (zero BW). IOW the ideal horn's sole function is to flatten the huge peak this creates enough for it to have a flat response to Fs. In theory a filter chamber is still required, but due to the need for a negative pressure compression chamber to accomplish this, there's no real point in providing one.

This negative pressure requirement that many BLH designs have causes the driver's excursion to be non-linear, but unless driven hard it's not audibly obvious unless it's a fairly wide BW design, and since it's most obvious down in the BW where the room dominates, even designers 'in the know' usually ignore it since they're more interested in as compact a design as practical.

GM
 
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