Stuart Dempster has recorded his trombone in a cistern at Fort Worden, mimicked Gen. Douglas MacArthur's "Farewell Speech" through his instrument and "massaged" people with air blown through a shower hose.

If you think "there oughta be a law" against such musical shenanigans, you're not alone.

Once, back in the '70s at the University of Washington, Dempster and clarinetist Bill Smith were performing Smith's piece "Encounters," which called for them to wear animal costumes and wooden clogs. When they walked out into the foyer, two UW campus cops "pulled them over."

"I think that was our only tangle with the law," recalls Smith. "They almost arrested us."

Many folks in the world of music would have rushed to bail them out. Proof positive is the lineup at the gala 70th birthday tribute, "Dempster Diving: A Sonic Extravaganza," at 2 p.m. today at Town Hall, part of the Earshot Jazz Festival.

The show offers a rogue's gallery of experimentalists from Seattle and beyond, including visionary composer Pauline Oliveros, with whom Dempster plays in the Deep Listening Band; sound sculptor Trimpin; composer/pianist David Mahler; the Seattle Harmonic Voices; the DidgeriDudes; trombone-playing unicyclist Nathaniel Oxford; the Degenerate Art Ensemble; flute virtuosi Paul Taub and Jeff Cohan; improvisational dancer Sheri Cohen; the Tibetan Buddhist Long Horns with Phursang Kelak Lama; jazz saxophonist Michael Monhart and Sunship (with Dempster); cellist Loren Dempster (Stuart's son); sound artist Suzie Kozawa; a trombone choir; and performance artist/juggler Thomas Arthur.

All come to celebrate a musical agent provocateur who, for nearly four decades, has quietly enlivened — and enlightened — Seattle music.

Dempster, whose actual birthday was July 7, has been feted all year, recently in Berkeley, Calif., where he grew up.

"Terry Riley [father of minimalism] was there," says Dempster. "It was very sweet. This whole business is very gratifying and very humbling. And it's only getting worse."

That last remark is a glimpse of Dempster's irrepressible wordplay, which has produced some hilarious titles over the years, such as "The Saucer's Apprentice," "Not Very Deep Hockets" and "Hi Bali, Hi."

But don't let the fun mislead you. Dempster is a highly respected fellow. A music professor at UW for 30 years (he retired in 1998), Dempster once played principal trombone in the Oakland Symphony and wrote the 'bone player's bible, "The Modern Trombone: A Definition of Its Idioms." He has commissioned works by the likes of Luciano Berio, Andre Imbrie and Ernst Krenek; been awarded a Guggenheim fellowship; and until last year, worked regularly with dance giant Merce Cunningham.

Dempster also was the main popularizer in America of the Australian wind instrument, the didgeridu, that long pipe of eucalyptus wood you hear droning everywhere these days.

Dempster's passion for experimental music began in the 1950s as an undergraduate at San Francisco State College and intensified when he returned in 1960, after a stint in the Army, to get a masters degree in composition and performance.

"The culmination of my pollution was when Pauline organized a festival, in 1964," he recalls. "That was really an interesting time. The Vortex Series was going on at the [San Francisco] Planetarium, with 20 speakers and light shows on the ceiling."

Touring with Oliveros, Dempster stopped in Buffalo and was invited to stay. He played there in the premiere of Riley's "In C," the piece that launched the minimalist movement, which included Steve Reich, Philip Glass and John Adams.

After Buffalo, Dempster came to the UW, planning to stay for only a year. That was 1968. Since then, he has taught hundreds of students, performed constantly and raised two children — Brian, an English lit teacher in San Francisco, and Loren, who has followed in his father's footsteps, playing experimental cello in New York.

Dempster's combination of seriousness and fun — and his fascination with transcendent states — reflects the broad influence of John Cage. Inspired by Zen Buddhism, Cage proposed that the world is already quite musical in and of itself — if people would only listen. The role of the composer is not to impose some grand, authorial opus, but to appreciate and collaborate with it.

The composition that made Dempster famous expresses just this view. It is the cult classic "In the Great Abbey of Clement VI," (1987), in which he plays solo trombone and didgeridu in an Avignon church where the "hang time" for reverberating notes is a full 14 seconds.

"It was the most amazing sound sensation I have ever experienced," he wrote in the liner notes.

Dempster studied the didgeridu in Australia in the field, with native players, on a 1973 Fulbright scholarship. A supreme virtuoso, he taught himself how to play the instrument from a 45-second recording lent to him by a friend.

Brian Pertl, who learned didgeridu from Dempster, compares his former teacher to a Zen master.

"Before I came to the UW, I was studying in a Zen Buddhist monastery," he says. "People always think that would be real quiet and somber, but actually, the monks laugh a lot. At Stuart's lessons, there was always a lot of laughing."

Dempster is still a devoted fan of the musical humor of the irreverent musical comic Spike Jones.

Composer David Mahler, who wrote a baseball performance piece for Dempster, adds that, unlike many experimentalists who have the reputation for being abrasive, Dempster "caved in to beauty ... [and] overt sensuousness."

Without sounding too New Age, the idea that music should be beautiful — and make you feel good — is central to everything Dempster does.

"Stuart is just such a totally nice guy," says Taub, who performs Dempster's "Alternative Realities" today, a score of written instructions in boxes connected by lines. "Stuart likes to say he doesn't own any staff paper, though he's a composer. He's a great role model, in terms of his devotion to what he's doing, his integrity, his wonderful trombone playing and his sense of humor and personal warmth."

Who could ask for a better birthday card than that?

Paul de Barros: 206-464-3247 or pdebarros@seattletimes.com