A7 Cabinet Resonances?

[Tom Brennan]

Dave---Yes.

[just me now]

Mr Bozak's outstanding designs wern't limited to speakers either....

he designed one of, if not the, most coveted DJ mixer to hit planet Earth

to this day, ppl seek out (and pay big dollar) for his CMA 10 2D and it's variants


ppl regularly pay a grand or more for units that still need complete restoration!

[dread64]

Alright guys---be nice. And I hate the word "disingenuous". It's beaten to death
by media morons all day long, and I'm tired of it. It's one of the many reasons why I no longer listen to talk radio, or watch the news.
Now you've got me getting nasty. I need to stop and so do you. This bickering is not what this message board is about. Let's get back on topic, please.
Okay? Can we stick to just talking about our hobby? Please?

Dave

[Steve Mac]

Tom...I apologize if I've offended you.
Dave...sorry for the heated post.

[Steve Mac]

Originally posted by Tom Brennan:
By the way, 605As have fabric surrounds.

<font size="2" face="Verdana, Helvetica, sans-serif">Hey Tom...is this true?
I spoke with Bill at Great Plains Audio and he said the first 605's had a stiff paper surround.

Then I ran into a post on the lansing heritage site and found a guy named speakerdave who said he poked around for information on the differences between the 604 and 605 and he mentioned the same thing...that the first 605s had a paper surround.

[GM]

Greets!

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>I'm sorry, and I apologize but foam does nothing...absolutely nothing to diminish resonance. Weight deters resonance and sand is the most effective way to do this. Resonance is distortion, plain and simple. I've built 6 or 8 huge sub woofers and used concrete, sand, double skinning, 2x4 braces internally and the only way to stop resonance is with weight.
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Hmm, mass just lowers the resonance, ergo its amplitude, it's the impedance mismatch of diverse material properties in constrained layer construction that rapidly shunts a resonance. WRT whether foam has no effect in the midbass horn's BW I don't have any experience, but frankly, I find it hard to believe based on what I do know since the impedance mismatch to the thin horn panels would be considerable. Down in the LF BW though, the impedance mismatch wouldn't be enough, so damping the rest of the cab requires copious bracing (as you note) and/or constrained layer damping for best performance.

WRT sub construction, you're 'chasing your tail' by using mass loading to damp it since you have to lower its Fs to below the sub's passband. Better to build a rigid cab that raises it above the passband where it can't be energized, so beginnng with a rigid (high MOE) material and proper bracing is the 'hot ticket'. It will still be fairly heavy, but no where near a truly 'dead' cab. My 20 ft^3 braced 3/4" marine grade plywood cabs ring like they're made from metal when rapped with a hammer, but when BW limited by disconnecting the 500 Hz horns there's no perceptable resonance. I can balance a quarter on edge and play movie special effects at loud enough SPLs for me. They weigh a calc'd ~245 lbs, but my mind reels at what they would have to weigh to push their Fs below their usable ~11 Hz lower cutoff.

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>In my opinion the a7 cabinet reconances are somehow inherent to this system. The big box allows the air to resonate in various fequencies, to make the box out of stone would not help this. The compromise to let the A7 act as half bass reflex and half backloaded horn is the trick to make the A7 as eficient as it is down to about 40Hz. The drawback are these bothering resonances.

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It's more than just your opinion, the cab is tall enough to develop 1/4 WL pipe resonances in its passband and combined with the expansion from the small upper chamber to the larger lower one, it acts as a crude midbass horn that's mass loaded by the vent to a lower cutoff than its length implies. Until MJK released his various MathCad TL worksheets though, there was no way to sim it AFAIK. Anyone who's simmed it in a typical reflex box program and compared it to a measured response knows it's anything but a simple reflex.

Right, making it out of a super rigid material such as stone would increase the amplitude of these TL modes, a good thing actually IMO since it would increase its acoustic and electro-mechanical efficiency.

Its various distracting distortion resonances are due to shoddy construction, so-so materials, and a lack of proper bracing, not poor design per se. For the cinema/PA apps it was designed for, it didn't matter, but for HI-FI apps it's better and less work to just scratch build them with 19 mm BB or marine grade no-void ply and brace it sufficiently.

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>If I were to use kitty litter, should I use the standard kind or the clumping kind?

Other than maybe a little less weight, what attributes may kitty litter have over sand?

Would pea gravel be worth considering since, if rinsed well, it would have less very small particles and be less likely to get into woofer voice coil gaps if the seal is broken (or would this be making a giant castanet!)?

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I never noticed, it was whatever was cheapest, so I assume non-clumping.

Don't know, I'm not a materials expert, but the weight difference is considerable according to my weak back.

Don't know for sure, but seems to me that it would create too many voids and the potential for shifting over time and/or movement, so I wouldn't use it without some long term testing if I couldn't find some reliable prior testing results.

How could it get into the VC gap? We're talking about walling off the horn cavities with wood and calking any seams, so there's no way for any dust or particles to reach it unless you don't either remove the driver or at least cover it with Saran Wrap or similar.

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>Kitty litter...why?
If it's lighter than sand it is worse than sand.
Pea gravel might work, but if some get loose somewhere you might hear a maraca. But again, sand is king.

====

Why not?

Sand isn't 'king', Portland cement weighs more and is much finer, so more can be packed in, and then there's the custom engineered ($$) viscuous damping bearing fluids, not to mention lead...... Anyway, there's a point of diminishing returns once the panel's Fs is pushed below its passband and kitty litter suffices IMO, but as always YMMV.

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>Maybe not today but I think speaker design was different in the 1940s, things were trial and error and if a good result was obtained they were happy.

A friend analyzed Bozak CGs many years ago and came to the conclusion that the box was deliberately designed to vibrate and help give that lush Bozak sound. Now ole Rudy Bozak knew what he was about, if the box vibrated he knew.

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The early speaker designers were electrical and mechanical engineers, and while they didn't know as much as we do today thanks to computers and vastly superior measurement systems, they definitely had a clue what was going on WRT pipe modes and Helmholtz resonances, just look at the WE horns and Thuras's 1932 reflex patent which shows the original ducted port 'Onken' layout and accurate FR plot, but since they had no way to sim it beyond calculating it theoretically, measurements/tweaking was the order of the day.

WRT the original A800's design that became the A7 series, obviously I don't know if its basic concept was by design or a consequence of packaging limitations Vs performance tradeoffs, but IMO it was more by intelligent design. I asked when I toured the plant in the '60s since I was trying to learn speaker/horn design by studying patents and reverse engineering proven designs, but all I got was a knowing look and a short marketing presentation on Bell Labs/W.E./Altec's elaborate and extensive testing history.

I agree, Rudy was an innovative designer and I have no doubt any resonances were either designed in (a common practice back then thanks to P. A. Voigt) or the consequence of what he considered an acceptable trade-off WRT weight/cost Vs performance, etc.. Unfortunately, he didn't use good business judgement in some cases after stereo became the norm and it cost him. His Concert Grands were fabulous in mono and a sonic nightmare in stereo, but between his too little too late 'band-aid' fix and its size, it was guaranteed to be discontinued. Maybe if he'd converted it to a vertical WMTMW tower............... More's the pity his driver designs didn't survive, though. As much as I like Altec, his drivers were my fave for all cone designs.

GM

[GM]

Greets!

Hmm, fill a hollow metal tube equipment stand with sand, another with Portland cement, another with one of the newer engineered micro-ball mass loading systems and sand will come out a distant third. not my idea of a 'king', but as always, YMMV.

GM

[Steve Mac]

Originally posted by GM:

Sand isn't 'king', Portland cement weighs more and is much finer, so more can be packed in, and then there's the custom engineered ($$) viscuous damping bearing fluids, not to mention lead...... Anyway, there's a point of diminishing returns once the panel's Fs is pushed below its passband and kitty litter suffices IMO, but as always YMMV.

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<font size="2" face="Verdana, Helvetica, sans-serif">Sand IS King...but cement is better.
I can't think of an instance where sand didn't have the mass to do the job. But I've always wanted to pour a cement sub-woofer. I've just never had the ba$$s to do it. I think when all is said is done, cement has the lowest W.A.F. of any audio material.