What the!?!... Weird tweeter

mhardy6647 said:

The flaw in most piezo implementations is not to use an appropriate crossover. They really only get spitty when allowed to 'run free' and too low.
Lots of good advice on handling piezos at James Melhuish's Full Range Driver site. Good forum there, too. www.fullrangedriver.com

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CORRECT! While I use the 1800 hz units (KSN-1165a, KSN-1141a, KSN-1142a, and the older KSN-1025a) right at 1800 hz and they do perform well there, the large KSN-1188a, which on paper goes down to 650 hz actuall sounds better if I cross it over between 800 hz - 900 hz. It has a very smooth midrange but gets the top end well, too.

M Jarve said:

I took apart a Motorola Console today. Three-way system with a 12-inch Golden Sound woofer, 5-1/4" GS mid, and what I though to be an ordinary horn tweeter. I got home and noticed the resistor in the x-over was not 1.20-ohms like I thought, but was 120-ohms. I then removed the cover on the back of the driver, and the attached pictures are what I saw. A transformer, a needle, a metal diaphram, and a thin wire. No magnet or anything. It works well, just never seen anything like it before.

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I visited the Moto factory (Quincy, IL) where these were being tested (if not built) in the early 70's. They had a test sequence of four beep tones rising in pitch that would have driven me crazy if I had to listen to them all day. I always assumed that the "needle" IS the piezo element - the length is needed to get the required gain (lengthening/shortening some percentage of total depending on drive voltage, so actual stroke is proportional to the length of the element).

A later design used a piezo disc like this:

http://www.pulsardevelopments.com/products/detail/piezoan.html

I did some research - the tweeters you have were the first piezos introduced by Motorola in 1965, and the "needle" is indeed the piezo element. Previous attempts at using piezo motors involved a cantilever with one end mounted to the frame and the other end to the diaphragm, or a square element mounted on three sides to the frame and one corner to the diaphragm. Motorola published a paper on the disc bimorph motor in 1975 and some improvements on it in 1980.

By the way, one thing I didn't realize was that the later disc drivers were supported by the diaphragm at the center and free at the edges, so they worked against their own inertia. An April 1980 paper, "A New Piezoelectric Driver Enhances Horn Performance," describes some partial edge mounting that suppresses unwanted subharmonic resonance of the disc. (From JAES April 1980)

There were other articles published about horn/throat design.

Very interesting thread.