SOUTH KINGSTOWN, R.I.

Between each act at the Newport Jazz Festival, as the audience cheers and crews clear the equipment, Bill Calhoun darts onstage with a fistful of tools and parks at the piano. He cocks his head, lowers his ear to the piano, taps ding-ding-ding on the keys, tinkers with a tuning pin here and there. When he's done, he scurries off stage. It's a routine Calhoun has perfected. He marks his 25th anniversary this weekend as piano tuner for the celebrated jazz festival, which since 1954 has hosted such luminaries as John Coltrane, Miles Davis, Dizzy Gillespie and Ray Charles.

Calhoun generally has minimal interactions with the musicians, but his behind-the-scenes role is crucial to the festival's success. When Dave Brubeck hits a note that rings just so, it's of course a credit to his talent but also a testament to Calhoun's craftsmanship. "The musician playing the piano has never played this piano before. They're going to walk on stage, introduce themselves to the audience and then sit down at a piano that they have never played," Calhoun said. "In a sense my job is to make it so that they have total trust in what the piano can do for them and how the piano sounds."

Performers who play Newport take turns on the festival's rented pianos rather than bring their own, creating the need for an onsite tuner sensitive to the instrument's notoriously fickle nature: Humid weather, common during the annual August festival, can knock the pitch out of whack. So can a pianist who pounds the keys especially hard. "I'm insurance that the pianos will be in tune enough and in good enough repair," explained 55-year-old Calhoun.

Normally both he and the performers are too busy to greet each other, though sometimes they do. One time Chick Corea asked to meet Calhoun to feel him out and get a sense of the piano he'd be playing. He's also met Dr. John  a pianist of a strikingly "gentle" style, Calhoun says, despite his "funny little meaty hands." And one summer, he found himself huddled beneath a piano with Herbie Hancock and his manager, investigating the source of a terrific bam that occurred after a structural piece at the instrument's base fell during the Grammy winner's performance. "His manager looks at me and goes, 'Are you the piano technician?' I said 'yes.' He goes 'good' and then he looks at Herbie and says, 'Herbie, get out of here!'" Calhoun recalled with a laugh.

As a boy, Calhoun was always more interested in trying to take apart his parents' piano than in playing it, though he is himself a pianist and fond of bluesy jazz.

The Dartmouth College graduate taught high school science before deciding to mesh his interest in music and physics. He enrolled in the piano technology program at the New England Conservatory in Boston, then was hired at the jazz festival in 1986 after making what he calls a "ridiculously low" offer for his services. He's been there ever since, also working the Newport Folk Festival when a piano is needed for a performance.

Calhoun arrives at 7 a.m. and hangs with the sound engineers near the stage. He does full tunings and checks octaves at the start of the day, but between acts is when he really hustles  moving between performances on the festival's three stages with a tuning wrench and pair of rubber mutes. He typically has a narrow window to test the strings that correspond to each note, check the octaves and finger the keys to make sure the sound is pitch-perfect.

"He's very good and very fast, which you have to be. Sometimes there's only 15 minutes between two piano players. He's got to pipe right up on stage and make sure the piano's in tune," said Bob Jones, a senior producer with New Festival Networks, the festival's production company.

Calhoun is one of the festival's many unsung contributors who return each year and are vital to behind-the-scene operations, said Tim Tobin, the festival's operation's manager. "They do feel as if it is a privilege to work for this festival because it is in fact the granddaddy of all jazz festivals," Tobin said. "When you're working with your family, they tend to stick around."

Calhoun says he's tuned the piano of virtually every Newport performer in the last quarter-century, though one notable exception sticks out in his mind. One summer Bruce Hornsby swung by Newport while on tour but enlisted his own keyboard player to tune his 9-foot Baldwin piano. It was, Calhoun politely suggests, perhaps not the best decision. "Let's just say had I tuned the piano it would have sounded better, but you know, Bruce Hornsby didn't seem to care one bit what it sounded like."

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