Large format driver performance question

[dhook17322]

One recurring question that crops up on large format drivers is lack of high end response. Their response curve (288x, 299x) seems to bear this out as it drops off very steeply around 15,000 Hz. Has any experimentation been done to extend this to 20K, maybe a different diaphragm material or design? Seems odd that these cut off so sharply when the small formats go to 20k (according to the specs).

[Todd W. White]

It's a matter of physics - the larger the mass of the dome, the harder it is to get it to vibrate at the extreme upper end above 16 kHz (where no one can hear, anyway, by the way) without making them so delicate that they can't handle frequencies that are too low, else they self destruct.

It's also about output - if you need the higher output levels, the odds are that you're using these in large venues such as auditoriums and such where the extended HF response wouldn't be useful - it's just too difficult to transmit them to such a large area, due to problems with HF attenuation in air.

Therefore, Altec (i.e., Dr. John K. Hilliard) opted to go the route in large systems with large-format HF drivers that had excellent response as far as possible (Altec drivers are the ONLY big drivers that go out to 16 kHz on their own without some sort of "help"), combined with excellent acoustic output with low distortion.

If you're looking for units in smaller venues, then the smaller drivers work better there.

Hope this helps.

[dhook17322]

Thanks much for the explanation. Expecting a driver to perform from ~300Hz on up is asking a lot from a diaphragm. Just wondered if there was any experiments with newer materials that would extend the range.
You mentioned that very high frequencies don't project well, or are absorbed at a higher rate. I have noticed at orchestra performances that very high frequencies are heavily included in the high brass and strings overtones and are critical to the definition and quality you expect in very high degrees of fidelity. You notice this in very high quality recordings (like DG). Not sure how much of this (if any)would be lost if you are limited to 16kHz?

[Old Guy]

The problem is not so much material as mass. A larger diaphragm with more mass will go lower, a smaller diaphragm with less mass will go higher.

If you lower the mass of a large diaphragm by using different material it will raise the resonance, affecting how low the driver can go.

It's a perpetual trade. The most successful engineers are the ones who make the best trades.

There are Beryllium diaphragms. I won't own them because beryllium is toxic, and frankly I think they are over rated.

Titanium isn't so bad as a diaphragm but makes a terrible surround. The best sounding titanium's I have heard copy Altecs Symbiotik idea of the Mylar surround.

There is the approach taken by BMS in that they build a large format and small format driver together like a coax speaker. These go from 300 to close to 20k no sweat. And sound good. IMHO they make Just Big and Loud drivers sound like a joke.

http://www.bmspro.info/index.php?sho...279&id=5059986


That's the 1.4" version there are several 2" versions also.


You still have a crossover. But if you go with a good electronic crossover you eliminate 99.9% of the traditional problems with crossovers.

[Russ Farrell]

Altec did experiment with various diaphragm materials during the late Anaheim years. Bill Hayes, who was Chief Engineer - Acoustics, had some interesting small format diaphragms in his desk. He had some carbon based diaphragms as well as the types mentioned by Old Guy. They all had their drawbacks.

Just getting a 1" driver to 20 kHz was a major problem. The 950 driver in the "Japanese market only" 9862 used a Tangerine phasing plug with a larger number of slots than the 11-slot 802 plugs. Had to be a prime number, so probably 17 or 19 slots were used. Getting the output near to what I believe was a claimed 22 kHz required each driver to be tweaked in the acoustics lab. It was a slow, and very expensive, process. Thats why the MR902-16HF became Altec's "supertweeter" instead of the 950.

9862 Monitor for Japanese Market

Maybe someday large format drivers will get to 20 kHz. I heard that there was some unusual very light metal found in a farmer's field in Roswell, New Mexico once. Hmm, I wonder.....

[Russ Farrell]

Latest rumor is the metal is stored in a warehouse on some small Air Force base in Ohio. So if you really want to try it.....

Now as for the voice coil, when the last kinks are worked out of room-temperature superconductivity in high magnetic fields, we can have a very efficient coil (flat wound, of course).

www.fas.org/sgp/othergov/doe
/lanl/pubs/00285656.pdf+superconductivity+high+temperature&hl =en&gl=us&ct=clnk&cd=2&ie=UTF-8]High temperature superconductivity in electrical wire[/URL]

Trouble is, the amplifier output devices now see a dead short. I see two possible answers: design a superconductive semiconductor, or use two 6L6's in a push-pull class A amplifier for about 18 watts out. Oops, I bet it will only work with a Peerless output transformer.

[Old Guy]

Hmm...antigravity metal- we could have a large diaphragm with less than zero mass...

Flat response from 1 hz to over 1 megahertz...with tremendous transient response.

The only problem so far is the prototypes keep floating away...