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-   -   BLH compression chamber. (http://www.diyaudio.com/forums/full-range/215549-blh-compression-chamber.html)

doubtingthomas 3rd July 2012 01:00 PM

BLH compression chamber.
 
What, broadly speaking is the effect of altering compression chamber volume in a back loaded horn and can this question be answered in a general sense without going into driver and horn specifics?

GM 4th July 2012 02:06 AM

It's an acoustic low pass filter same as a vented cab, not a compression chamber like a sealed cab that's used to seal up a BLH to make it a compression loaded one, so the smaller it is, the higher it's HF corner frequency, Fc and vice versa.

GM

doubtingthomas 4th July 2012 12:13 PM

Forgive my stupidity, I don't understand "corner frequency" or "Fc and vice versa." Any clarification would be gratefully accepted as would an allowance for my ignorance.

gafhenderson 4th July 2012 01:32 PM

Quote:

Originally Posted by doubtingthomas (Post 3081148)
Forgive my stupidity, I don't understand "corner frequency" or "Fc and vice versa." Any clarification would be gratefully accepted as would an allowance for my ignorance.

Cutoff frequency - Wikipedia, the free encyclopedia

i think(?) that will help.

also subscribed for knowledge. these bloody things have me completely bamboozled.

zman01 4th July 2012 02:16 PM

Quote:

Originally Posted by gafhenderson (Post 3081238)

also subscribed for knowledge. these bloody things have me completely bamboozled.

+1 and add me to the list.

bear 4th July 2012 03:39 PM

There is a difference between the volume of a chamber between the driver and the horn's throat and a "constriction" (of sorts) that takes the open face of a driver that faces into the throat of a horn and reduces that opening so the volume of air now has to run through a smaller orifice. The second causes an increase in compression ratio.

What the former does is to form a Low Pass Filter (LPF) which means that if you were using a speaker that had output up into the midrange or above, with a volume between the speaker and the throat, the higher frequencies would be made to roll off, acoustically. Not electrically.

Of course too much volume there and you lose the benefit of having a horn at all...

There are formulas for this stuff... and Hornresp (if you can figure it out) will simulate this stuff pretty well - I am told! :D

_-_-bear

David McBean 5th July 2012 07:14 AM

3 Attachment(s)
Quote:

Originally Posted by bear (Post 3081368)
There are formulas for this stuff... and Hornresp (if you can figure it out) will simulate this stuff pretty well - I am told! :D

The attached Hornresp screenprints illustrate how a horn throat chamber can be dimensioned to act as an acoustic low pass filter. Screenprint 3 compares the power response of a horn having a throat chamber volume Vtc = 1000cc (gray trace) against the same horn with throat chamber volume Vtc = 2000cc (black trace).

Kind regards,

David

bear 5th July 2012 02:34 PM

sweet!

there it is...

bear 5th July 2012 06:49 PM

Oh, somebody reading the thread mentioned to me about this:


http://www.quarter-wave.com/Horns/BL...gn_Article.pdf


Also, my presumption - perhaps wrong - is that the OP was discussing a speaker that is/was more or less like the standard "PA/SR" type folded bass horn, with the front of the driver directly radiating. One sees this with folded horns and the "J scoop" type.

The "horns" that people have been building with the rather small mouth (things less than several feet on a side for the mouth) in my mind are tapered, unstuffed, transmission lines, not true horns - no matter what anyone calls them.

If they were true horns, then the mouth size would be so small as to cause monster comb filter effects... which for all I know, they have (I have not seen them measured, maybe because I am not looking?).

Fwiw, Fyi, and all that good stuff...

Scottmoose 5th July 2012 07:36 PM

It depends if you define a horn by laying down the stricture that it must be impedance matched down to Fc. If we are to apply our definitions so narrowly, then for example a 'tapered, un-stuffed transmission lines' is an oxymoron, since a transmission line, bysimilarly narrow defining standards, would be an untapered, stuffed (or otherwise damped) plane wave tube with the flattest possible impedance as the only objective. ;)

It's really not worth getting hung up over definitions IMO. Life is far too short. As far as I'm concerned, I just call anything that expands toward the terminus a horn, since it will possess some degree of 1/2 wave behaviour.


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