If you're building a tightly folded horn like the LAB horn it seems that Fs really dictates the feasibility of physically constructing the thing. Whenever I play with Hornresp I always get backed into a rear chamber volume that won't accomodate the driver.

In my basshorn design, I've balanced the rear-chamber and front-chamber with the driver to get better response than the LABhorn, but it does take a pretty small rear chamber to do it. Brad Litz did this too. It's actually slightly larger than what is used in the LABhorn, large enough to build a box around the driver. The frequencies are low enough that everything is pretty large, and even a small rear chamber is big enough to work with.

What's really difficult to work with are rear chamber volumes so small that the box has to contour around the magnet. I run into this with midbass horns. On midrange horns, I'm not usually worried about excursion, so I sometimes design them for open backs or large rear chambers, which act the same. I'm not usually looking for LF from a midrange horn. But midbass horns can potentially have cone excursions that make me want to use reactance annulling with a smaller rear chamber. On those, sometimes the rear chamber size required to do that is so small it's practically a sealed back driver.

The advantage / reason for using a massive driver is because for a given bandwidth, the heavier / stronger driver has a different acoustic impedance and requires a higher compression ratio.
A higher compression ratio means more acoustic power for a given cone excursion.
At low frequencies, in horn loading it was usually the case that one runs out of Xmax well before power handling, causing the famous bass horn distortion (like the famous W bin). This was blamed on “throat distortion” but was actually the driver reaching xmax and beyond.
Hence using a heavy driver made sense to me here given the available power capacity and the desire to maximize the “undistorted” output..
The BT-7 horn was aligned with a 3:1 compression ratio, the Lab-12 was only heavy / strong enough to need about a 2:1 compression ratio in a similar bandwidth and low cutoff.
Alternately, if one scales up mentally, one finds the 10:1 compression ratio in a typical compression driver, is needed to get the wide bandwidth from what is (up high) a massive driver also.
Cheers,

Tom Danley

I think that boils down to what you want to do with the Fch that determines the appropriate-ness of a driver for your desired application.

Daniel Plach reported that for a front-loaded horn (sealed Vb), the use of a linear motor with a low Fs is desirable, the bandpass being somewhat more limited and subordinate to the efficiency and lower distortion the loading is capable of. He said that the Vb could be practically reactance-annulled (for a Hyperbolic/Exponential flare), the course of which naturally raises the Fs of the driver, and therefore, the driver should start out with an Fs below that of the horns Fc.

He also said that for back-loaded horns, the use of a driver with a rising response curve was in order with Fs above the horns Fc, because it could not be reactance-annulled due to tuning requirements (Fr) of the back chamber usually being much lower than the horn's Fc.

DM

There was some debate about this earlier with Danley championing Leach math and woofers with relatively heavy cones and Edgar prefering Keele math and woofers with lighter cones and stiffer suspensions. I tended to see their arguments as flip sides of the same coin but the debate was on, nonetheless. Seems to have been clarified to everyone's satisfaction now.

I would assume that a 3-octave limited horn would/could "enjoy" a large-mass driver, i.e., it won't matter whether it has a light cone, because it technically doesn't go high enough in frequency band pass to actually make use of it.

I would also mention that large-mass drivers tend to heat up the VC (comparitively) more than a lighter assembly MIGHT, simply because more energy is required to move the mass, overcoming inertia and all that. In the case of sealed back chambers, this could be a potential design consideration brought about specifically by the choice of driver mass and power handling capability, depending on the application, of course. Large SPL requirements would definitely put this consideration on the designer's plate, I would think.

Personally, I've always regarded large-mass drivers (i.e., heavy cones) to be more appropriate for direct radiator use rather than used as horn drivers. Sort of a bellweather for me in selecting drivers.

In a resistive, pressurized environment such as that presented by a horn throat or more likely, a throat cavity opening, will naturally decrease the cone excursion to a great degree, and hence the potential for cone deformation is severely reduced. I tend to completely discount cone deformation as an important consideration in horns for this reason.

I think the "old guys" had it right the first time. I look for horn drivers with relatively light cones and large BL's and the lower Qts, the better, but then I'm going for 4 octaves or more, too.

Dana