Sandi Geary doesn't skip a beat as she describes how to imitate the sound of breaking bones.

Grab a crisp stalk of celery, she says. Then, snap it cleanly in half."It gives you that wet, cracked and snapped sound," she says.

Geary knows all about sounds that give people the creeps.

She is director of sound and music at SingleTrac Studios. The Salt Lake City software company makes games for the Sony PlayStation.

Its creations include some of the top-selling games for the console, including War Hawk, Jet Moto and Twisted Metal.

Geary designed the tracks that give those games just the right mix of sounds to make the simulated worlds seem real.

It's her job to make sure weapons ka-boom when fired, motors rev when gunned, and there's a POW! when a punch hits its target.

But face it: She also wants to make a player's heart race and skin crawl.

To do that, Geary takes advantage of universal and deep-seated fears. Wild animal sounds. Human chanting. Beating hearts.

"Anything that is wet and slimy," she says.

Sure, the sounds fit the settings and action on the screen. But the fun comes in weaving frightening aural motifs into musical patterns, where they work subconsciously to evoke mood.

"A lot of what I try to do with sound is drive emotion," Geary said.

Geary didn't set out to be a sound engineer for a game company - it wasn't even an option back when she was in college.

She pursued a more traditional track after receiving an engineering degree in 1986 from the University of Utah. Geary worked at Unisys and then Evans & Sutherland, where one project she oversaw was development of a military flight simulator used to train pilots for Desert Storm.

She also is an entertainer. Geary plays the piano, guitar, saxophone and also sings. She's performed at local parties and was onstage regularly at the former Mark Twain restaurant in Salt Lake City.

Her musical and technical careers seemed to be running on separate tracks until 1994 when SingleTrac put out word it needed a sound engineer.

"This is the kind of job I dreamed of but never thought I would get to do," she said.

Geary worked solo on the first titles released by SingleTrac; she now has a staff of three and is trying to hire two more employees.

Her work begins when a game is at the design phase, long before any actual building begins. Sometimes there are sketches of the new game, but often it consists of nothing more than a vision in someone's head.

Geary fleshes out as many specifics as she can: What's the game about? What will the biggest weapon look like? Where will the scariest places be? What sort of monsters or opponents will players confront? Geary pulls sounds from a variety of sources. She uses stock sound CDs, music synthesizers and samplers.

She also books local musicians - and even on occasion members of the Utah Symphony - to create sound tracks for games.

She wrote, with the help of local composer Chuck Meyers, an original tune for one SingleTrac game, the underwater submarine combat "Critical Depth."

And she and her team always have their ears open for live sounds that might make a perfect game effect.

That's how the team ended up once outside the SingleTrac offices, banging on garbage can lids. They needed a sound to fit with the crash of the hover cycle in the Jet Moto racing game.

"It didn't work," she said.

She's hoping to find a way to use a sound she discovered the other day in a future game.

Geary had surgery to correct extreme nearsightedness. And the laser the surgeon used sounded really cool.

"It would make a really good weapon," she said. "It charges up with a big synthetic release, an electrical zap. I could start with that little sound and get in there and beef it up."

Games, after all, are a lot like the movies.

"Everything is bigger than life." If a laser needs tweaking to sound like a weapon a robo-warrier would cart around, so be it.

And that's why much of what you hear on a game sound track doesn't sound like anything you've ever heard before.

Take the clown laugh that became the signature of both versions of Twisted Metal, a car combat game. The sound is activated whenever the ice cream truck character fires flaming ice cream cones at opponents.

Geary took a laugh from a stock CD and "made it strange." She added echo, a reverberation, shifted the pitch, broke it apart and then put it all back together.

The result? A hysterical, demented laugh befitting a twisted clown.

Geary is at work on more digital sound magic for a new game that SingleTrac will release this spring.

It's called "OutWars" and features a military character who's trying to save the planet from alien invaders.

She concocted all sorts of "voices" for the aliens using animal noises. Geary mixed badgers, raccoons and fighting dogs for the first-line aliens. The more advanced "hoppers" use pig sounds, though the oinking and squealing is like nothing you've ever heard before.

"It's like a choir singing together but they're really bad and all singing off-key," Geary said of the pitch shifting and other special effects she uses to alter the sounds.

Then there's the alien queen: Her sound is a jumble of lions and horses, with a very special touch. Geary added recordings of Ken Teutsch, a video editor at SingleTrac, roaring and snarling.

It makes the queen sound "more intelligent," she said.

Putting it all together is a complicated process. When the tracks are added to a game, programmers have to make sure the right sounds are triggered by appropriate actions. You don't want the hero snorting and shrieking like an alien.

They also have to make sure sounds don't conflict and that players can hear the sounds they need to. Geary once had to re-record an electric guitar riff because it was the exact frequency of an engine.

As many as 20 sounds can be taking place at once, but humans are only capable of making sense of about seven sounds at a time. Geary works with programmers to decide which sounds will predominate.

It's tricky, too, since games are nonlinear, which means players can jump in and out of the play at any point. The music and sound effects have to change accordingly in a flash.

Geary's work has received raves from the industry and game players.

Twisted Metal received the Sound Effects of the Year award from Electronic Gaming Monthly in 1995.

"We were pretty happy because EGM is one of the largest and oldest publications in the industry," Geary said.

She even gets requests from players and garage bands who want to buy the sound track or sheet music to their favorite games.

She has to turn them down. SingleTrac doesn't release audio-only versions of the music and "it never gets written down," she said.

But, if you're really hankering to hear Twisted Metal or Jet Moto or sounds from another game, you can pop the disc in a regular CD player. It will work just like a regular audio CD, which may be music to your ears.