Big Voice at Cal Poly
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Compiled by Pete Woodworth, CTS Typically, very few people ever notice that installed on top of the Administration Building at Cal Poly is a row of 4 bizarre enclosures, each with 9 horns. The system is
affectionately referred to as
From
the manual:
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| The system was designed
and built by Western Electric for the Navy (well
prior to becoming part of Altec
Corporation). Each of the 4 cabinets are rated
at "250 Watts", and contains 9 individual high-power
speaker drivers, with a rated output capability
of 116 dB at 30 feet! Each of the
individual drivers are equipped with 'blast valves' to protect them from the impact of 'gun blasts'.
The diaphragms are phenolic, and replacements can
still be purchased from Altec. According to the
manual, the original amplifier was a 250 Watt, forced
air tube job. I wish I could have seen it.
So, what's it doing on the roof of the Admin building? Well, it seems that it was originally installed back in the '60's. War surplus even then, it was installed as an emergency announcement system so that the campus President, the Department of Public Safety, and a handful of others could make announcements campus-wide. Now why would anyone have wanted to be able to make emergency announcements in the 1960's? Nobody remembers, but this is just my opinion:
So, somebody dug up this surplus hardware, bolted it to the roof of Admin, built an enormous custom amplifier to power it, and even added a reel-to-reel tape player...to play music, of course! Everybody knows that if you play classical music during a RIOT, it will pacify the participants! (I guess somebody forgot to throw the switch after Poly Royal a few years back.) A while back, our department was required to move a bunch of equipment around on the fifth floor of the Administration building. Among the equipment was the amplifiers and switching systems for Big Voice. Since the system had really never been used, it was determined that we needed to perform a complete evaluation of the system, and a full-power test. Tom D. got the heavy work of moving the amps and rerouting the speaker cables from one end of the roof to the other. I got the job of system evaluation and testing. So I hauled my beloved Crown amp, a pink noise generator, a UREI EQ, good ol' Radio Shack SPL meter, tools and a mover's blanket up on the roof. |
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This is a view of Big Voice, facing roughly due west. The back panels of the cabinets have been removed for access and inspection. The big coil of cables in the foreground are the speaker wires due to be rerouted. |
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This view is west-by-northwest. The speakers under test have the mover's blanket over the top of them, so as not to bother the folks below. Even with the blanket over the 9 drivers under test, this sucker is LOUD. The amp, noise generator, and eq are in the lower right. There's only about six feet between the front of the speakers and a six story fall, so SPL measurement is interesting. |
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This view is facing north. The individual speakers are wired in groups of 3 (vertically); that's also how I tested them, initially. Basically, I just let them cook on band-limited pink noise at an output of about 120 dB for about 5 minutes, measured at the mouth of the horns. Then, with ear plugs well in place, I rolled back the blanket, and cranked up the amp until I sustained a level of about 120 dB at 4 feet. OMG! |
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A view of campus, with Big Voice taking direct aim at "Fort Fisher." We did need to have some replacement rear panels fabricated, as the old ones were rusted through. You can see there's only two screws holding the field-replacable magnet and diaphragm assembly. You can also see the soldered barrier strips for the complex series/parallel wiring originally used in the cabinets. |
After all of that, I found NO defective drivers! God bless American technology. I permanently installed a replacement mixer/limiter, equalizer, the Crown ComTech amp, and some other surplus interface gear in the equipment room on the floor below. Setting normal operating levels was going to be pretty subjective, since I wasn't about to sit in front of Big Voice and listen for clipping. When I first keyed the push-to-talk mic, the system fed back through the roof and wouldn't stop!. It scared the h*ll out of me! Eventually, I set the level by having a second person listen for adequate speech articulation from about 1/4 mile away. I recommended installing a digital message repeater to eliminate any future feedback concerns. Here's another Beachmaster link. (off site) |
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All questions welcome! pwoodwor [at] calpoly [dot] edu |
Images and content © Pete Woodworth |