Sound Control


Sound control is described in section 15 of the GCR and its purpose is a self policing effort to keep racing alive by keeping race tracks open. By keeping our cars quiet, we will be nice to the neighbors of the race track and hopefully won't then fall under the mandate of the government or town control. Regardless of "who was there first", development and encroachment of homes (read local voters) near race tracks has caused SCCA to address excess noise before racing is banned by local communities.

The rules for a measurement location include: full throttle, a microphone located 50 feet from the track, 50 feet of clear area around the microphone (no reflections), 50 feet clear area on the opposite side of the car/track, not within 200 feet of tunnels or bridged, not near billboards or walls, the ground between the car and microphone should be hard and clear of brush, the microphone must be 3-6 feet above the ground, and finally, SAFE!

At Summit Point, the sound control microphone is on the main straight on the left side of the track just past the second cross over road. The microphone cable is run under the track to the meter on the right side so that the Sound personnel can sit in the spectator area and have easy access to the tower and post readings on the results board located at OG Raging in the paddock.

The rules for taking measurements on single cars is that there must be at least a 10dB rise and a 6dB fall in level between cars for the reading to be considered valid. Cars running in packs therefore cannot be effectively measured. All readings are rounded down to the lower whole number, thus 105.9 dB is recorded as 105dB.

The present legal limit for cars (nationally, some divisions and tracks are lower) is 103dB. To be considered in violation of the limit, you have to be recorded at least two times to prove consistency and accuracy in the readings.

If a car is over the limit, the procedure is for the sound judge to notify the operating steward who will initiate action. The steward may BLACK FLAG the violating car or wait until the session is over to assess a penalty. The penalties for being over the limit are similar to other technical inspection violations. In qualifying the time will be disallowed and in a race the car will be disqualified.

Throughout a weekend, sound readings will be posted as soon as possible after a session at the winner's circle results board. It is the driver's responsibility to check these readings. If you are close to the limit, please be aware that differing weather conditions or car tweaks (changing gears, carburetor jets, etc.) may put you over the limit. Judges or stewards will not seek you out to warn you that you are close to the limit. However they will seek you out to discuss the results of an infraction. Be proactive an avoid an unpleasant surprise.

HEALTH NOTE: There is one other area where you can do something about sound control. Throughout an event weekend, you and your crew's ears are subjected constantly to the highest levels of sound. This noise will damage your hearing permanently unless you protect yourself. WEAR EAR PLUGS. They will save your hearing and help to prevent sound induced headaches.


SOUND SPECTRUM ANALYSIS
I've been fascinated by spectrum analysis since I can't remember when. It wasn't until taking engineering mathematics on my way to an electrical engineering degree that I really began to understand just how interesting it really was. No need to glaze your eyes over with a discussion of Fast Fourier Transforms, but the technology now exists to do on a home PC what it used to take very expensive test equipment to do.

Couple that with my involvement in SCCA club racing as chief of sound control for the Washington D. C. Region of SCCA and you get someone who will try to answer the question, 'Why do those darn Mazdas sound so bad?' Well, here's some insight into the question. Below are some sound spectrums of a few race cars. I'll try to walk through why they sound different.

First, an explanation of what's in the pictures below. Each picture represents the sound spread out over frequency and time. To record these, I took my cassette recorder to the track and set up a professional microphone next to the track and recorded the cars as they went by. I then fed the recordings into my computer's sound board to capture a few seconds of the car passing. Then I used an analysis program from Pioneer Hill Software called Spectra Plus to generate the spectrum plots.

Along the horizontal axis is time. Think of it as the car passing from left to right (which by coincidence is just he way they went by on the track!). Along the vertical scale is the frequency with the lower frequencies (like the bass on your stereo) on the bottom and the higher frequencies (treble) at the top. The relative intensity of each frequency is indicated by the color, like the radar images of a rain storm on the TV weather reports. The scale is on the right, but suffice to say that the red is the highest intensity. It's a relative scale that is adjusted to make the spectrums show up well, not all of the cars were equally loud.

Now let's start looking at some pictures. First is a Formula Atlantic. This is an open wheel, single seat race car that is a stepping stone to Indy cars. It uses a two liter four cylinder engine that puts out about two hundred horsepower. People generally say that this engine sounds good. Notice that there is a strong fundamental frequency with a few quickly diminishing harmonics, or overtones in musical terms. This results in a relatively pure sounding note. For those of you with a technical background, any continuous sound can be broken down mathematically into a fundamental and a series of harmonics. For example, a square wave consists of a fundamental and odd harmonics, and sounds harsh and raspy. A sound made up of even harmonics will instead sound smooth. If you look at the frequency scale at the left, you can see that the harmonics have even multiples (but a little of both). Therefore you might expect a Formula Atlantic to sound pretty good! Notice also that there is not much significant sound energy above about four thousand Hertz (or cycles per second).



By the way, the little downward slope in the plot from left to right is a result of the Doppler shift, the change in frequency you hear as a car approaches and speeds away. The truly curious can measure the car's speed from this plot!

Now let's look at one of those Mazdas. The next picture is a recording of an RX-7 built to SCCA GT2 specifications by Kearney Motorsports. Note the number of harmonics of all sorts extending higher up in frequency than the Atlantic, and not all of these lines are harmonically related to the fundamental. People generally think that Mazdas have a raspy high pitched sound and indeed it shows up on the spectrum. Notice that sound energy goes up to almost nine thousand Hertz. Having so many spectrum lines is what we hear as an 'unpure' sound. If these individual frequency components are unrelated to each other as multiples of each other then it begins to sound like noise and not a pure tone.

Now let's take a look at some good old 'merican iron. Below is a GT1 Corvette with a nice sounding V8. Again there is a strong fundamental with a rich set of harmonics that don't go much above two thousand Hertz. Consequently, it has a low pitched sound but the rich harmonics give it a quality that just can't be described, it has to be felt!


That's all for now. I hope you got something out of this little discussion. I've enjoyed sharing it with you. My thanks to Pioneer Hill Software for the fine job in creating a truly impressive tool.

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