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40Hz horn w/ reflex port QUESTION


D-MAN
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Guys, I got a question that's beyond me...

I have a design for another 40Hz folded horn with enough void space to allow for a 4th order reflex port. There isn't enough room to use ducted ports, so a reflex "slot" type will suffice, I think.

The design has 7711 cu. in. total void space (raw) available, which the new Bob Crites driver is perfect for, with a little insulation (4.97 cu.ft Vb).

The F3 for the horn is 33 Hz, and the Fb for the port is 31.6 Hz. I'm figuring on 257 cu. in of port size.

The horn-loading is certainly not going to be annulled. I can't find anything on porting a 40Hz horn. The Keele T/S criteria for the horn indicates a much smaller Vb for a sealed back chamber, of course.

The question is: the horn performance will expectedly be degraded by too much Vb in order to capacitate the port?

I guess that I have another question, too: Is it worth doing at all? I haven't seen any 40Hz horns that are ported - perhaps there is a reason for this...

DM

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Believe it or not, the A7/828 is an 80Hz horn (it's 14" long) - reflex ported with a Fv of 35Hz. Real darn close to the DJK-modded LS...the LS being a little smaller and has a slightly lower Fc (70Hz). But the relationship to horn size, cabinet size and porting is there...

A true 50Hz Fc horn would need an overall mouth cross-section approaching 4 sq. feet. for operation as a free-standing unit, and the A7 ain't got that (but the University Classic does and the Classic and the A7 cabinets are almost exactly the same size!).

This is my dilema - there are no front horns with a 40Hz Fc that are reflex ported that I know of.

DM

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Been thinking some more about this...

The Bob Crites 15" driver is just about perfect for this application:

Ported Vb = 4.97 cu. ft. (8588 cu in)

Fv = 31.6 Hz

40Hz Fc horn - Vb = 5400 optimum (3 cu.ft approx).

F3=33 Hz

St = 78 sq. in

Mouth = 4 sq. ft. (approx).

There is one major difference between the horn pic posted by djk above and the one I am contemplating - that is - mine, being a corner horn (what else!?) will have 5 feet of overall horn length (slightly over 25% of 1/2 a wavelength of 16ft (40Hz =32ft) and an appropriate mouth size for corner placement. So that much is taken care of according to horn theory.

The tricky part is the helmholtz resonator portion pertaining to the reflex port, which for a 4th order tuning would be around 31.6Hz.

Sizing the resonator is fine, but that is going to make for a rather loose back-end for the horn driver. I was looking for information on how to calculate the compromise there, but evidently, there isn't any information that I can find on the subject, anyway.

The resonator portion has the more extreme Vb requirement compared to the horn, and so I figure the horn driver is going to get a bit "sloppy" on the back side in order to get it to resonate at the appropriate frequency. Being that I've got a relatively long horn on the front-end, there is definitely going to be a mismatch (i.e., higher IM distortion caused by unequal cone excursion).

The available back chamber volume (Vb) is 7711 cubic inches of space.

Subtracting 400 cu. in. for the insertion of the motor board/driver combination and that leaves 7300 cu. in. thereabouts. The target volume to lower the resonant frequency of the chamber is 4.97 cubic feet, or 8588 cu. inches.

The variance can be "made up" with sound absorpting material (i.e., fiberglass, etc.), with the accepted value of upto 25% of the original volume can be added using this method. That remaining variance is 1288 cu in, which falls easily within the 25% value, totalling less than a cubic foot of space needing to be "made up" with stuffing.

The proposed volume of the reflex port is 3" high x 20" wide x 4.5" deep, giving a total of 257 cu. inches of volume, which should be very close to the 31.6 (or 32) Hz target. This is very close to the DJK-mod ported La Scala.

The 4th order alignment is chosen for a couple of reasons, first being that its possible because of the cabinet size and driver chosen, and second because it is a somewhat linear response just below the -3db point of falloff for the horn (33Hz).

The approach I have chosen is not going to be "tunable" or adjustable, unfortunately, being that the location of the port and the nature of the cabinet structure doesn't support that other than using removable panels to actuate the port or not. In either case, the cabinet will still have a port slot right in the front whether used or not. To taunt me, most likely.

DM

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  • 2 weeks later...

Been worried about the possible uncoupling of the horn driver requirements imposed by the front-loaded 40Hz 1/4+ wavelength horn and the Helmholtz resonator for the back chamber.

The K33E/Crites 15" driver theoretically anulls at approx. 5 cu. feet of back chamber space. This is pretty dang close to the Vb required by the Helmholtz resonant frequency (31.6 Hz) of the back chamber.

All this worrying over nothing! Who knows - none of us have actually heard an anulled K33E or the Crites version in a horn - yet!

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