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In movies, when someone lands a punch there's this nifty slappy sound that real punches just don't make. What is that sound?

--Adam S., New Haven, Connecticut

Welcome to the world of "Foley artists," the unsung geniuses who create the larger-than-life sound effects that make a flick come alive. For a good face punch, a Foley artist might hit a piece of raw meat with his fist, maybe wearing a tight leather glove for enhanced smackiness. I'm told rib cuts are particularly good because they have bones to give a crunchy effect. Then again, maybe the Foley artist will just punch himself, hard. The beauty of Foley--named after Jack Foley, chief sound effects guru at Universal for many years--is that nobody's telling you exactly what you have to do. All that counts is that it work on-screen.

Foley art is made necessary by the fact that (1) you need ambient sound (i.e., more than just the actors talking) to make a movie scene seem real, and (2) miking the entire stage or location during shooting just isn't practical. Even if it were, real-life sounds often don't have the oomph the big screen demands. In addition, dubs for foreign markets often require that a sound track be created completely from scratch. So Foley artists add sound in postproduction. The most basic type of Foley consists of one or more people walking around in a well-miked "Foley pit" filled with gravel, sand, loose audio tape (to mimic the sound of crunching leaves), etc to re-create the sounds of the actors in motion. They do this while watching the movie with the sound off, synchronizing their movements with the action on the screen. This requires a good sense of timing and rhythm, and maybe for that reason a lot of Foley artists are also musicians.

Some Foley effects have been around since the dawn of the talkies--for example, walking on cornstarch in a burlap bag to create the sound of crunching snow. Another time-tested technique is drawing a paddle full of nails across a piece of glass to create the sound of branches scratching on a windowpane.

Other sound effects are of more recent vintage. Foley artist Greg Mauer told us he was recently working on a vampire flick that had a scene in which a character's guts get pulled out. Greg used raw chicken, which he likes because you get a nice moist sound he describes as "slimy." For a simple broken bone there's nothing like the crisp sound of snapping a stalk of celery or a chicken bone.

Not all Foley is fake. If a scene calls for somebody falling, a lot of Foley artists figure there's no substitute for actually falling. Same with walking on sand. But if you need the sound of 150 people running around, no way you're actually going to cram 150 people into the Foley pit. Instead you have maybe three Foley artists laying down a half dozen tracks. Mix 'em together and voila--crowd noise.

As you might surmise in this age of high-tech special effects, cinema sound can involve lots of fancy enhancements you'd need a master's in computer science to understand. But it's good to know that, at least for some stuff, they still rely on guts and red meat.

Is there such a thing as spontaneous combustion? I'd heard it was a myth.

--P.J., New York

Spontaneous human combustion is a myth. (It's all masterfully explained in my books The Straight Dope and More of the Straight Dope.) Spontaneous combustion of other things is all too real. Often it involves vegetable oils such as linseed oil or tung oil, but sometimes just damp hay is enough. In the typical case, somebody refinishing furniture or a wood floor piles a bunch of oil-soaked rags in a bucket, corner, or other confined place overnight. The oil oxidizes, a process that generates heat. (In the case of damp hay, bacteria commence digesting it, which also generates heat.) If the heat can't dissipate, it builds up, dramatically speeding up the oxidation reaction and generating still more heat. Eventually the oil, rags, or whatever other potential fuel is on hand reaches "auto-ignition" temperature--that is, the point at which the stuff bursts into flame without the need of an external ignition source. This can occur at surprisingly low temperatures, only a few hundred degrees above room temperature. So spread your rags out to dry in a well-ventilated place, lest there be a hot time in the old house tonight.

Art accompanying story in printed newspaper (not available in this archive): illustration by Slug Signorino.

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