SPL levels in competition can reach unbelievable (not to mention preposterous) levels.

The stereo shop that does my work (unsolicited plug: Audio Advisors, in Fairbanks, Alaska) has a big Chevrolet Dualie that competes in SPL, he can put a brief burst of 18,000 watts to his speakers (12 fifteen-inch subwoofers) and has hit an official 164 decibels in sanctioned competition, and unoficially has approached 170 db. This is with the measurement taken at the base of the windshield, not with the microphone stuffed down a speaker port or anything.

As a frame of reference, I have been told that a Boeing 747 jetliner during its takeoff roll will generate 115 decibels, but I don't remember how far behind the airplane the measurement was taken. 105 decibels is enough to cause permanent hearing loss. I have also been told that if you were actually inside the vehicle when it was making 160+ decibels, you would not survive, the sound pressure would kill you. I don't know if this is actually true or not - it may be another of those urban legends.

SPL cars are so extreme, you'll find them with the glass replaced with half-inch-thick lexan, screwed right into the window frame because the pressure will blow the windshield right out of the car. I have seen cars with chains and turnbuckles between the roof and the floor of the car to keep the roof from flexing to the point where the doors pop open and the windshield pops out. I have seen more than one competitor find out that a CD player can only generate 150 db (give or take a couple) because at that level the CD starts to skip. Some cars have been built with solenoid powered wedges to hold the doors shut.

The SPL people are, in a word, truly weird. Hmmm... I wonder how many decibels an empeg could endure before the hard drives started malfunctioning? In any case, I would venture to say that a built-to-the-max SPL vehicle would generate more sound pressure on the microphone than you would get by tapping it with your finger!

Hmmm.... I wonder too how many SPL people I offended with this posting? Worst sound quality I ever heard was Audio Advisors SPL truck. We played the 1812 Overture and couldn't even recognize the music enough to find the cannon shots. (His entire system is the dozen 15" subwoofers. Not a lot of mid bass or high end response there.... ;-)

Anyway, Mark, welcome to the wacky world of SPL competition. I take some pride that I have been told by the Vice President of IASCA (the body that sanctions the sound-off competitions) that I have, to his knowledge, the quietest car that has ever seriously competed. I can get a mighty 112 decibels maximum out of my system using the official SPL tracks on the competition disk -- I got 115 with the 1812 overture. Typical car in my class gets 125--135 decibels, and when you figure that each three decibel increase is a doubling of the sound pressure, either their cars are really loud, or mine is really quiet... but it sounds really, really good. ;-)

tanstaafl.






"There Ain't No Such Thing As A Free Lunch"