On Wed, 22 Dec 2004 22:04:32 -0800, Harvey Gerst
wrote:

Not from what I remember. We use to sell them to the Army (for God knows what
purpose), and to the Highway department (to use as vehicle counters at 25kHz).
There WAS NO off-axis response; at JBL, we called it affectionately "The
Acoustic Laser". snip

Beaming on the "Burton Bullet" was pretty bad just below 10 KHz and
incredible at 15 KHz, per JBL's own polar plots. JBL's own literature
showed response down 3 dB just over 15 KHz. Sure, they went out to
the ultrasonic range, but certainly not "flat" when compared to its
audio passband.

The "slot" was 077 = consumer version, 8 ohms 2405 = pro version 16
ohms -3 dB down at 25 KHz on axis.

That was Ed May's slot-loading modification to Bill Burton's original 075
design. The only difference between the two was the dispersion, IIRC - same
diaphragm and magnet assembly. snip

Yes, I made many 077/2405s out of 075/2402s (it was 2402...I retrieved
the original documentation from file) by simply changing the face
piece and the center "nose cone" with parts picked up from Balboa
Avenue. In fact, the pro and home versions shared the same diaphragm
too, with the home rated at 8 ohm and the pro at 16, when actually
testing in the passband showed it to split the difference at 12! This
was taken into consideration in the crossover. May's ingenious
variation got rid of the horrid beaming of the 077/2405 and exhibited
100° horizontal dispersion at 10 KHz, where the 075/2402 was narrowed
down below 60°, conical pattern, and got a lot worse as you went up.
The 2402 was still the transducer of choice for longer throw
installations, however, and still is. The only serious competition
back in those days was the Gauss "Big Tweet," which handled a lot more
power and used an odd multicell concentric horn section to try to
improve dispersion.

40 KHz? It might be there, but way, WAY down in the mud.

I don't think so. snip

OK, well, I DO know this from experience. One day while changing
diaphragms and parts on some of these, I had connected a 2405 through
an HP matching transformer to the 600 ohm output of an H-P 200CD,
which I used to sweep for obvious resonances. Just for laughs, I went
up past 30 KHz. I know one thing...it REALLY got my dog's attention
in that range! The only other transducer that was able to rate his
attention was a $2 piezo, and we all know what THEY sounded like.
Back in those days, I could easily sense 17 KHz, and it was definitely
there at more than 45° off axis on the 2405, as mic tests proved also.
I have no doubt at all that the 2402/2405 could easily put out 40 KHz,
but I do doubt that the output was within 3 dB of their acoustic
output at crossover, which was 7 KHz with the May modification and 5
KHz with the original "Burton Bullet." Why was the May version
flatter at the top? Easy...the bullet had bad phase cancellation
problems in that area at anything other than precisely on axis; thus,
the moniker "acoustic laser." May's "crossfire" arrangement neatly
took care of that by keeping the wave fronts from either side of the
diaphragm phase coherentin the horizontal plane. This was also the
reason why the 2405 was rated 2 dB higher in efficiency than the 2402
while using the same drive components.

dB