Help me understand this horn loading behavior

[zx82net]

I'm trying to understand the effect of a chamber placed between a driver and a horn, per the attached diagram. Intuition tells me that the chamber will act as a low pass filter on the loading of the horn, is that correct? If it is, how would I calculate the low pass frequency? Is it dependent on the driver parameters and the horn dimensions, or only the volume of the chamber?

[billfitzmaurice]

HornResp has entries for throat chamber volume and cross sectional area. Because of the low pass property of the throat chamber it's usually beneficial to make it as small as possible, including the volume of the space between the cone and the baffle. That function can be incorporated with a phase plug.

[zx82net]

Thanks Bill, I was actually looking to isolate the effect of the chamber from the horn rather than model the horn. However, I found this reference:
http://www.bd-design.nl/contents/en-...rn_Design.html
And specifically this passage:

[billfitzmaurice]

You can still use HornResp to model it, just make the horn one centimeter long. You'll find the result is the same as a 4th order band pass with a one centimeter duct length. One thing that page doesn't mention is that the horn itself is a band pass device, so if the horn is to have maximum band width the throat chamber and for that matter the rear chamber should be kept small.

[zx82net]

You can still use HornResp to model it, just make the horn one centimeter long. You'll find the result is the same as a 4th order band pass with a one centimeter duct length. One thing that page doesn't mention is that the horn itself is a band pass device, so if the horn is to have maximum band width the throat chamber and for that matter the rear chamber should be kept small.

Bill, one question, why do you say the rear chamber should be kept small? I would have thought the only requirement for the rear chamber is it allows the driver the operate over the passband of the horn. Perhaps I am misunderstanding an interaction between the front and rear volumes.

[billfitzmaurice]

The air mass in the horn adds to the Mms of the driver, in effect lowering the driver Fs. When the effective resonance is lower than the horn Fc there's a sensitivity loss. One way of compensating for that is to use a driver that has a native Fs higher than the horn Fc, another is to make the rear chamber small enough to push the effective driver resonance back up where it needs to be. The latter is called 'reactance annulling', which is a three dollar word to describe what's actually a simple concept.