Decibel Automotive Terms - Performance Dictionary

Decibel (DES-e-bel) noun [origin: Latin] A unit of measure used to quantify sound pressure. Also known as db, dB, dB(A), and Tenth-bels.

Huh? What Was That, Again?How many times have you had a conversation with an old-time drag racer and had to repeat yourself two or three times? Or how about those guys with the gigawatt car audio systems?

I sat at a stoplight the other day near a Honda Civic that was visibly pulsating up and down-and it wasn't because of some tricked-out VTEC engine swap, either. The vehicle had so much alternator horsepower flowing into the mega-buck stereo that it was hard to believe there were any crankshaft ponies left over to actually accelerate the car. Two lanes away, my own 1-1/2 ton ride was physically vibrating in response to the sound waves the Honda was pumping into the atmosphere. I wanted to tell the driver he should stock up on hearing-aid batteries. It wouldn't be long before he was speaking like all those old-time drag racers. Huh? What was that, again?

To many of us gearheads, there's nothing quite like the sound of uncorked exhaust headers and wide-open throttles. But what today is music to our collective ears may in fact be just muffled noise in the future. Hearing damage is a cumulative process. The longer you listen to loud and damaging sounds, the more and more hearing ability you're going to lose. And medical science hasn't yet figured out a way to reliably bring it back, either. Once your hearing is gone, friends, it's Miracle-Ear(r) time.

But personal health concerns aren't the only reason to think about automotive decibels and noise reduction. Excessive sound levels have recently become a hot issue at most race tracks and speedways. A few years ago, the National Hot Rod Association (NHRA) decided to take a pro-active stance on noise reduction. The NHRA was not the first to do this, either. The Sports Car Club of America (SCCA) and others now have maximum allowable noise values as part of their rules, as do more and more drag strips and sanctioning bodies. Like it or not, it's time to start thinking about the noise that our cars are pumping into the air.

What Is Sound?In simple terms, audible sound is just quickly changing pressure waves in the air. To picture this, imagine a large speaker cone vibrating to the bass beat of a low-frequency rock and roll song. You can observe the movement of the exposed speaker cone if the frequency is low enough in the audio spectrum.

As the cone moves outward, the air resting immediately in front of the speaker is squeezed, or compressed slightly, which causes an increase in air pressure. As the speaker cone relaxes and travels back to its rest position, the air in front of the speaker experiences a pressure drop. By rapidly alternating back and forth, intermittent waves of high and low pressure are radiated outward from the speaker through the air. Almost all objects vibrating in the air create sound waves in this manner, including speakers, engines, and exhaust systems.

Our ears sense these sounds by channeling the pressure waves down the outer ear canals to the eardrums. The eardrum vibrates in response to the pressure variations and, in turn, transfers the energy inward by way of three tiny bones in the middle ear. From there, all this vibrational energy is changed into a type of compressive wave in the inner ear, or cochlea. This fluid-filled spiral coil (think of a nautilus shell or turbocharger compressor shape) is filled with over 10,000 tiny hair cells. These cells sense the minuscule compressive waves and change them into nerve impulses, where they travel the auditory nerve highway to the hearing center of the brain. Voil, we have audible sound.

How Sound Is MeasuredScientists like to express pure sound levels with something called a bel. In basic terms, one bel is the difference between two sounds, where one sound is 10 times greater in intensity than the other. One tenth of a bel is, of course, a decibel, and it is the more common term that is used by scientists and engineers to express sound levels.

It is also probably one of the most misunderstood units of measurement since the introduction of the furlong and the cubit.

The decibel, or dB, is actually a logarithmic unit used in many scientific fields to quantify things that are relative to each other. Unlike an inch or a gallon, a dB is not a pure physical measurement. You can't go down to the National Bureau of Standards and physically look at a dB to see how "big" it is. Instead, a dB is just a comparison between one value to another known reference value. Normally, the reference value that is used is the smallest known measurement of that quantity. The Richter scale for instance, which is used to measure earthquakes, is a dB-like logarithmic scale. An earthquake that measures 6.0 on the Richter scale is 10 times more powerful than one that measures 5.0, and it's 100 times more powerful than one that measures 4.0.

For audio engineers, the smallest audible sound (that is used as the basic reference value) is termed "near total silence." This is classified as 0 dB. Very faint whispering is about 10 times louder than "near total silence" and is therefore classified as 10 dB. The rustling of leaves can be thought of as about 100 times louder than 0 dB, so it gets classified as 20 dB. Working up to "normal conversation" gets us to about 60 dB, and "loud shouting" at 3 feet distance comes in around 80 dB. A diesel truck from about 5 feet away is typically about 100 dB, while a chainsaw at 3 feet is something like 115 dB. Above this level, sound is uncomfortable for most of us, and actual pain starts to set in above 130 to 140 dB.

How Much Sound Is Too MuchRacers and aftermarket engineers have been paying more attention to sound, and have started investing time and money into maximizing performance while holding sound levels down. This research into pressure levels and exhaust flow patterns has lead to bothquieter and more powerful racecars with optimized exhaust designs. Better still, much of this racecar engineering eventually finds its way to OEM and aftermarket street accessories. That's the good news. The bad news is that too many racers live by the adage that "louder is faster." Open headers and uncorked exhaust systems at the track can emit sound levels that are above 110 dB, which is well into the danger zone.

Hearing experts say that it's best to avoid any unprotected exposure to sounds above 100 dB. They also say that hearing protection should probably be used whenever you're exposed to levels above 85 dB, especially if the exposure is prolonged. A rule of thumb says that if you have to raise your voice to be heard, the sound is at or above 85 dB.Racetracks are notorious places for big dBs and it's a good idea to bring along some kind of hearing protection whenever you head to the races. When in doubt, use sound protection. Hearing damage is both cumulative and irreversible, and it's not something to fool around with. You are listening to this advice, right?

Huh? What was that, again?