MJK's Fostex FF125wk BLH project

[hajj]

I have been considering the idea of adding a helmholtz resonator to a BLH design I've been working on lately and was wondering if the resonator would add anything to the horn's impedance outside its "working" freq range.
The reason I ask is because designing a helmholtz resonator for a certain frequency and BW is rather easy with a lot of online calculators available, but a potential pitfall would be the resonator's effect on the rest of the horn's BW.
Furthermore, would there be an ideal placement for the resonator port along the horn length (pressure maximum, or velocity maximum of the resonator's design freq)?

[Scottmoose]

Since a back-horn is by definition a relatively narrow-BW gain device, I wouldn't be too concerned over the former. It's unlikely to have an negative impact. For the rest, to an extent it depends what it is you're trying to do with it, but again, it's not likely to so severely affect its operation as to negate its use, so I don't think you need to be overly concerned about it.

[hajj]

Originally Posted by Scottmoose

For the rest, to an extent it depends what it is you're trying to do with it, but again, it's not likely to so severely affect its operation as to negate its use, so I don't think you need to be overly concerned about it.

Thanks for weighing in.
I'll be using the resonator to get higher low pass slope to the horn at its higher cutoff frequency, and hoping to tame any higher frequency resonances using stuffing.
The design (a very compromised horn profile) calls for a horn working between 40 and 150hz, and the first resonance happens at around 500hz (this will need to be ironed out using stuffing

[Scottmoose]

Possibly not, if you get your acoustic low-pass right in the first place. Leaving the drive unit aside, as you've obviously considered this, you need a reasonable low-pass slope in place anyway at 150Hz, preferably 2nd order or higher. That should put the horn output ~20dB down at 500Hz in itself. You can then add a Helmholtz at or slightly above your chosen Fh depending on phasing etc. which should increase the slope and place the resonance[s] you are concerned about at a sufficiently low level that you may not need any damping, beyond the usual. YMMV of course & it will depend on the low-pass slope you start out with.

FWIW, I haven't tried measuring one myself, but AFAIK the physics of the situtation are thus: The internal Helmholtz resonanater is effectively an acoustic notch-filter. If you measure the output of a horn containing one, you will likely discover that it resembles a Cauer filter, with the internal resonantor helping provide a steep initial rolloff, but above this, a secondary post-notch peak since it (the internal absorber) is a narrow band device. Therefore what you need to do is ensure that the rolloff is steep enough to shunt this peak sufficiently low in level to be unobtrusive. Not too hard in itself, but it's not a panacea, and you need to select the XO frequency carefully because you don't want this secondary peak to coincide with an existing peak in the output, since when combined, the levels may be increased sufficiently to be audible.

[larryldspkr]

Very interesting article and design, it's hard to tell if the opening in the resonator chamber is a full width slot or a circular hole! Does anyone know which it is?

Larry

[chazdrumzalot]

Originally Posted by larryldspkr

Very interesting article and design, it's hard to tell if the opening in the resonator chamber is a full width slot or a circular hole! Does anyone know which it is?

Larry

Good question. Correct me if I'm wrong, but I suppose the shape of the hole doesn't matter provided it tunes the chamber to the correct frequency.

[larryldspkr]

Shape does not matter but SIZE does, so I'm told! lol I guess you could sim them both and determine the Helmholtz frequency.

Larry

[xrk971]

The way the plans are drawn, I would say it is a rectangular slot 1.375 wide x width of cabinet. If it was a round hole, that would have been specified.

[larryldspkr]

The hole works out to 135hz and the slot tuning is 283hz. I would think the higher tuning is correct.

Larry

[chazdrumzalot]

I assumed the slot also.