The Mother Of All Speakers (MOAS)

Written by Bill Leebens

I had begun writing a piece about the history of horn loudspeakers (still to come, don’t panic) when MOAB—the Mother Of All Bombs—appeared in the news. That triggered a memory for me of the biggest, baddest horn loudspeaker of them all, one of the weirder annals in the history of American acoustics: MOAS, the Mother Of All Speakers.

At the outset of the first Gulf War, the US Army saw a need for a portable, high-output sound-source that could be used on the battlefield to call for the surrender of Iraqi combatants. The late Dr. Hank Bass at the University of Mississippi’s National Center for Physical Acoustics was charged with the project. Dr. Jim Sabatier, now recently retired from NCPA,  helped design what Bass’ wife Cathy dubbed “the Mother Of All Speakers”,  after Saddam Hussein’s promise of “the mother of all battles”.  Internally, the project was quickly known as MOAS; the Army retained the acronym but cleaned it up to stand for MObile Acoustic Source, or Mobile Outdoor Acoustic Source.

Additional requirements called for reproduction of a 10Hz signal at 140dB @ 1 meter, so as to allow for realistic reproduction of the 11Hz rotor noise produced by helicopters…and maybe the rumble of approaching tanks, as well. And of course, you should be able to hear it miles away. Simple, right?

The frequency and volume requirements dictated the basic parameters of the horn, and almost made a mockery of the term “mobile”: a round-section exponential horn, 56′ long, with a mouth diameter of 10′. Regular old drivers couldn’t meet the required specs, and so sound was to be produced by an airstream modulator, basically a valve controlled by a voice coil.

The pressurized air to be modulated would come from a large air compressor producing 3000 CFM at 12-15 PSI. And how does one drive such a compressor in the middle of a desert battlefield? With a 150 HP diesel engine, of course!

The modulator was the Wyle Laboratories WAS-3000, still used today in simulations of rocket launches and airport noise-cancellation projects. On the MOAS project it was quickly discovered—and anyone familiar with supercharged engines could’ve told them this!—that the compressed air being fed to the modulator would be very, very warm. Warm enough, in fact, to melt the voice coil driving the valve.

Just as is often done in supercharged engines, air-to-air intercoolers were installed to cool the incoming compressed charge. An occasional spritz of compressed nitrogen or carbon dioxide also helped. As I said: simple, right?

The Army also authorized the construction of a second horn which would point up in the air, in an attempt to extend its range and to better simulate the sounds of copters and tanks.

Neither version ever made it to the battlefields of Iraq: blessedly, the war ended too soon. The horizontal MOAS did make it to White Sands Missile Range in New Mexico for testing, where, lo and behold, it acquitted itself admirably. In the years since it has been used in noise-abatement testing at airports and other sites, where it was utilized in a complex noise-cancellation set-up.

These days, MOAS is mothballed at the Army’s Aberdeen Proving Ground in Maryland. Who knows? Maybe one of these days it’ll be trotted out to do sound-reinforcement at Coachella or Burning Man….

PS: I spoke too soon. B. Jan Montana sent this pic of a horn speaker used at Burning Man in 1999. Alpenhorn, anyone?

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