Vog

[mashaffer]

Voice of the Garage...

Musing on some salvaged 12" MI speakers got me to thinking of a possible project. A set of speakers patterned after the VOT concept. Usage would be garage audio and occasional light duty PA. I tried modelling short horns in hornresponse and I don't quite see how Altec got them to run up to 1KHz unless the horn response modeling is not showing a major factor like direct radiation from the cone.

What would be the best way to approach designing of mini A7? Assume that usable output down to 40 or 50 Hz is quite adequate. For the sake of similarity to the original I would prefer two way but if we need to use a mid horn and a HF horn that would be OK. Since utmost fidelity is not critical my thoughts turn toward Piezo horns on top or modest 1" compression drivers.

Thoughts?

mike

[GM]

Greets!

You surmised correctly, no program can model a driver's TL and break-up modes, only its pistonic action (the BW > in area than its Sd). The Altec horns assumes that the driver will have a big 'bump' in its HF response to extend the horn's on-axis HF response, e.g. horn, bass, or lead guitar drivers: http://www.lansingheritage.org/image...15b/page02.jpg

Anyway, you'll have to know your driver's T/S specs and its FR for best results. FWIW, back in the pre T/S days I use to make simple one fold pipe (aka Voigt) horns where its length was a function of the desired Fp and the vent area ~ = Sd based on an old Altec chart, so for a 12"/40 Hz Fp:

L = 110.375"
St = ~0" (terminates to a point)
Sm = 628"^2 (25.06" square base)
vent = 78"^2 (10" dia)
driver down ~42.187" from top for max gain, but some major 'ripple' in its HF BW or ~68.25" down if smoothest response is more important than acoustic gain.

Obviously, different spec drivers can perform quite differently in a given alignment, but can be tweaked into working acceptably by experimenting with stuffing and/or damping the vent since net Vb will always be more than T/S will dictate for any driver suitable for a vented alignment.

Typically, if max acoustic gain wasn't a performance goal, then all I did was stuff a handful of R-19 fiberglass insulation up into the cab's point and line one side wall and back with 1" acoustic fiberglass insulation around the driver to quell any HF reflections back through the cone and if it sounded underdamped ('boomy'), then I damped the vent by stretching as many layers of whatever open weave cloth I had laying around as required over it.

WRT HF duty, you could do a lot worse than inexpensive used 511 or 811 horns and either Altec or B&C ceramic motor 1" compression drivers.

GM

[mashaffer]

Thanks for the reply. The drivers are currently slapped in some old JBL boxes (not properly tuned for them either) to provide at least some bass in my minivan. I will need to pull them and measure sometime. Based on how they sound in the current cabs I expect an Fs around 50 and rather largish Vas. I fear that the Q will be a bit higher than optimum as well but I won't know until I give them a spin.

Would your approach be to use horn response to design the "horn" low end roll off as well as maximizing smoothness and then build a mockup to see what the high end of the woofer range does? Or is there a more calculated approach to figuring out the HF output of the woofer? Would it be safe to assume that both the volume of the back chamber and the port tuning will have an effect on the response of the front horn?

mike

[GM]

Greets!

You're welcome!

I don't use Hornresp, but it can't sim A7 style cabs, only the basic horn response. Normally you measure the driver's on axis FR for designing an A7 style truncated horn, which is a half size 55 Hz expo, or ~110 Hz Fc, truncated to a ~338 Hz Fh and ~242 Hz axial length. As you can see, getting a good blend between driver/horn isn't a 'plug numbers in and build' unless you have all the right numbers, but even then with this style cab it can take quite a bit of fine tuning/compromises to get them to sound really good and why folks normally just drop whatever driver they have/want into an A7 or JBL 4560 cab or clone and fiddle with vent tuning and XO point.

You assume correctly.

GM

[mashaffer]

Do you think there might be any advantage to going with a rear horn like a scoop bin? Not as VOT like but might it be easier to get smooth response out of it?

mike

[GM]

Greets!

'Six of one, half a dozen of another'........... That said, for VOG or relatively small room PA apps, the scoop bin makes more sense due to its wider dispersion. Regardless of the A7's one-time popularity in the home, you really need to be at least 25 ft away to get into its 'sweet spot'. Anyway, for smoothest response with some gain over a BR, my folded pipe alignments will outperform a scoop bin, but they're larger.

GM

[mashaffer]

Your pipe is essentially a transmission line (i.e. 1/4 wave) system right? I need to do some studying on those. I really appreciate all of your input.

mike

[GM]

Greets!

You're welcome!

All pipes and horns are 1/4 WL TLs over some/all portion of their BW, with horns morphing into 1/2 WL resonators due to their expansion. Mine are technically mass loaded BLHs like the A7 cab.

GM

[mashaffer]

Not sure I quite understand yout TQWP design? Is the bottom of the pipe a 25"x25" square plate with a 10" diameter hole in it? I thought that the normal process was for the entire end of the pipe to be open? Can you post a picture/diagram (or link)?

mike

Oh, Never mind. I was still thinking of the classic TL/pipe.