Easy Decibel Check-August’00

Eventually, your loud ATV engine will cause you problems?and not only in the form of permanent hearing damage. Many off-road riding areas are starting to crack down on excessive noise. If you ride a stock ATV, you’re almost certain to fall within the acceptable limits. But if you’ve done any intake or exhaust mods?including something as simple as removing the airbox lid?or your silencer needs repacking, you might be over the line.

Sound regulations vary from one off-road area to another. Because of the close proximity of private residences and the unique acoustics of the area, the Oregon Dunes have been forced to get really tough on offenders. If they don’t, OHV opponents will move in and we risk losing the area to off-road traffic. The current limit there is 93 decibels (dB).

How do you know if your unique set of mods are over the line? The manufacturer of your aftermarket pipe might be able to tell you, but can you trust his numbers? Even if his numbers are correct, a poorly packed or worn silencer can change the sound output dramatically.

ACCURACY FOR $50

To avoid the extremely depressing scenario of being turned away from an off-road area, the safest bet is to check the decibel output yourself at home. The test takes just a minute and the two hand-held devices you’ll need will set you back around $50. You could even split that cost with a fellow rider or your riding club.
Pick up an analog sound level meter from Radio Shack for $35. It’s accurate, but if you want a slightly more durable unit, get the digital version for $60.

You’ll need to take your sound level readings at a specific rpm, so you’ll need some sort of tachometer. You can pick up a vibrating reed tach for around $20. Ours is a German-made Treysit. The unit needs no power source and the reed is actually a wire that protrudes from the unit and vibrates most vigorously at the rpm you select. At lower rpm settings, the wire protrudes farther from the unit. Most lawnmower or chainsaw dealers can order this unit for you. The Briggs and Stratton Vibra Tach part number is 19200.

What rpm do you select for your particular machine? If your four-stroke ATV has the stock bore and stroke, divide 250,000 by the motor’s stroke (in mm) to determine the test rpm. For two-strokes use the number 200,000. For many ATVs it will be in the 3000 to 4000 rpm range. If your ATV does not have the stock bore and stroke, divide the redline by two.

THE TEST ITSELF

Even if you can set the idle high enough, you’ll still need someone else to help you do the test. Have a friend operate the throttle and read the tach. We found that placing the tach either against the handlebars or the grab bar gives the same reading. Our test ATV was equipped with a $200 onboard tach and the little $20 Treysit matched it exactly.

While your friend keeps the proper rpm, you hold a tape measure, the sound level meter and take the reading. To get the most accurate reading, you should be in a open area where the sound can’t bounce around. Set the meter to the “A” weighting, “slow” response time and dial in the decibel range. Hold the meter exactly 20 inches from the exhaust and at a 45 degree angle to it.

Now, you can go to your favorite riding area confident that you won’t be turned away. Plus you won’t annoy others and contribute to the closure of the area. As the saying goes, “Less sound equals more ground!”