Theoretical multicell Q

[bfish]

Just wondering if you scaled down a multicell for a cutoff suitable to pick up where a large format driver begins to fall, how high could you go before cell beaming...

[Panomaniac]

Do you mean a tweeter multicell, or something else?

[bfish]

Exactly. A tweeter multicell for use above the range where a large format driver rolls off.

Having access to a nice sheet metal shop with a plasma cutting table with nesting software makes a guy wonder about such things.

[Panomaniac]

IIRC, the early Altec coax drivers used a true multicell tweeter. I was said to sound much better than the later faux multicell and other horns.

Someone here will know for sure.

It's going to be small and tricky to build, right?

[martyh45]

I made a cast of one of the larger 604 horn cells. Flattened out, the length of a side is 3-5/128ths of an inch, small end is 9/16? and the large side is two inches. The flair sides are a 1?1? radius connecting the two points. A guy could probably do a better job of transitioning from the throat if you didn?t have to make it fit inside of another speaker. A 10 or 15 cell horn based on these little sections sure would look cool but I can?t say where you would start to experience comb filtering. Perhaps someone can use the dimensions I provided to calculate that for you.


If anyone has a beat up 808 they can take a casting from I would love to have one. I always wanted to build a 1508 type multicell for tangerine driver use.

[bfish]

Well, the old 604 MCs were kinda what I had in mind, but while they're closer to a true MC, there's still a lot of compromising going on to make it work in a duplex.

I was thinking more along Wente's patent design. For ~5k-20k, or a 2 octave BW, my weak brain thinks it should be possible, I just can't do the math.

http://www.google.com/patents?id=KOB...J&dq=1,992,268

Then there's the requisite for a plane-wave at the throat. Is that even possible with today's available drivers, or was that already compromised out by the time Altec built MCs for 288s?

Thanks for the dims Marty, that's exactly the kind of info I'd need to work from (cut dims instead of flare rates).

[Audio_by_Goodwill]

I've got a pair of little Pioneer super tweeters that look like little Altec multicells...... I have no idea if they actually function by the same principles, Though. I got them from a pair of trashed CS-88A's

[martyh45]

I was thinking more along Wente's patent design. For ~5k-20k, or a 2 octave BW, my weak brain thinks it should be possible, I just can't do the math.

ACOUSTIC DEVICE - Google Patents

Then there's the requisite for a plane-wave at the throat. Is that even possible with today's available drivers, or was that already compromised out by the time Altec built MCs for 288s?

I don?t know that that would be superior to allowing the inlets to form a roughly spherical plane. I believe the early western electric multicells had bent cells at the exterior but they switched to a more spherical layout on the inlet and outlet sides. IIIRC, the drivers already include a small section of 800Hz conical expansion as part the body so I?m thinking that would preclude a perfectly planar wave at the inlet of the cells anyway. But I?m speculating rather than calculating so take that last with a grain of salt.

[bfish]

That's how I see it too, though I can see the advantage of a plane wavefront inlet when the WL becomes short enough in relation to a ~spherical inlet to present ~half-wave phase anomolies across the BW. On Wente's design, ALL the cells were bent, the interior cells just to a lesser degree. This took a plane wave input to a spherical output while maintaining uniform length/phase. I think the crux of this is esplained in the patent, but the math still eludes me;

"...It has been found experimentally that for frequencies below a value of about

(10 to the seventh power over d x fo)

the wave front at the mouth of a straight exponential horn will be spherical with its center of curvature lying on the axis of the horn, where d is the diameter in centimeters of the mouth opening and fo is the cut off frequency. For frequencies above this value the wave at the mouth opening will be approximately planar. The sound density will be approximately uniform over the mouth opening in either case until a frequency four or five times the above value is reached.

The sound wave front at the mouth of a horn built with a plurality of sound channels as above described is thus made up of contiguous areas of spherical waves at the lower frequencies and of planar waves at the higher frequencies. This wave front will produce the same distribution of sound in space as a spherical wave front coincident with the mouth opening provided the departure between the two is nowhere greater than approximately a half of a wave length of any frequency within the transmitted range..."

Still, I guess it's all moot unless you can present a plane wave front to the MC inlet. As I think I understand modern domed-diaphragm drivers, the only place you have a plane wave front is at the point where convergent waves become divergent, i.e. at the phase plug exit/integral throat interface.

Then again, my "take" could be way out of phase.