92db limit - how do they test, how can we test before coming

With the 92 db sound limit, how is it tested at the track? More importantly, is there a way to test this yourself, simply and cheaply?

Thanks,
Mike

Satch,

Thanks. I'm thinking about my M1 with its Stebro exhaust. It's a long way to haul it, then get caught by the hidden microphone.

Mike

It appears instruments may be had from any variety of sources, including Ebay - testing methodology, well, if we don't have any soundwave electronics engineers piping right in, I suppose you might ask the track for more specifics - at what distance their microphones are from the cars, for instance.

Steven,

Thanks, I downloaded the app Decibel 10th. Will give it a shot and post results for other interested members.

Mike

Laguna Seca measures 50 feet from the driving line, using the a-weighted scale on a sound meter.

Eric

You might discuss this with a local SCCA sound monitoring worker, as most tracks require them for Club Racing. While it isn't my specialty, I can tell you that all kinds of things will affect it, including water on the track (more reflective), the day's humidity, and the temperature. One simple expedient is to have on hand 'turn-down' tip(s) for the pipe(s). It's a well-known fact that turning the exhaust outlet away from the measuring microphones will attenuate it. Because sound measurement is so subjective, this is not so much a solution as it is a diversion; but that's the way the game is played.

Here's my experience when I used to run with SpeedVentures at Laguna

One of the many tricks (at the cost of lap time) is to back off the throttle at the sound check stations.

They will flag you down with a donut flag and then ask you to pit and "make adjustment" to your car/your driving style.

So is it less than 92db from 50 feet away, or less than 92 db at the exhaust, but they measure from 50 feet away and extrapolate the lost sound levels?

I want to make sure my Dinan will pass. It's not the loudest, but I'd hate to drive all that way and then find out I have issues.

So, a car running at WOT on the Laguna Seca track is likely quieter than the Harley owned by the local Yahoo. Interesting

So, does this mean no charity rides in race cars? If so, my wife says, "What's the point in going?"

This. I'm also a race worker and an ex racer. If you have a car that is close to the limit, you can be legal at one track and not at another. You can be legal at the same track one day and over another depending on conditions. That being said, I was racing a Dodge Neon with a gutted cat and a straight pipe and only hit 93 once here in the midwest.

The other thing I do have to comment on is that it does seem like the limit may vary depending on who is using the track. I really don't think some of the cars at pro races or vintage TransAm or CanAm cars would be under that. But that are the rules the contract says so we will have to play along.

At Calabogie in Ontario they measure sound with the car stationary in the paddock. The sound guy stands about 50 ft behind the car while you rev it to some rpm (I forget what). If you pass a sticker goes on the windshield. At least with that method you know if you're good to go before the first session.

Everyone,

I think I can help with this one now that the distance from the measurement device to the track has been provided.

Sound Pressure Level (called SPL, measured in dB's relative to a pressure of 20 microPascals) varies inversely with distance (i.e. the closer you are, the louder it sounds). The relationship is well defined and can give you an answer at any distance based on a known dB level at a known distance. There is even a handy calculator that someone put together on the web located here:

http://www.sengpielaudio.com/calculator-distance.htm

From this calculator, one can find that to get a SPL of 92dB at the measurement station, the car would measure ~125dB at a position 1 ft from the noise source.

Some caveats though:
1. The sound will be directional meaning you will have to measure it at several angles around the car to find the loudest.
2. Taking the readings of a stationary car will not include the effects of tire and wind noise which will add to the drive-by reading. Thus, don't think you'll be safe if you're just under with your measurements. Give yourself some margin and remember that every 10dB is a 50% increase in the perceived loudness.
3. Unless you're using a chassis dyno to load the engine, you probably won't produce the maximum noise with your car by just revving it. You might do better with drive-by measurements at WOT.
4. I haven't seen any information on whether the measurement is time- averaged in any way. Either would affect the measured results. Your home testing device would need to be set up similarly (A-weighting and with the appropriate averaging) to produce accurate results.
5. Be wary of 'apps' that measure sound. The mics in cell phones are tuned to speech frequencies and may not be up to the task of making this measurement. Although not exhaustive, a good monograph on this can be found here: http://blog.faberacoustical.com/2009/ios/iphone/iphone-microphone-frequency-response-comparison/

Maybe the local chapters can pony up for a decent meter and do a pre-Ofest check as a service to Club members. It might not be 100% accurate, but it would give people an idea if they were close or not.

FYI, a nice paper on exhaust system design can be found here:
http://acoustics.asn.au/conference_proceedings/AAS2005/papers/34.pdf

I hope this provides some answers and also spawn some further questions. FWIW, my day job is developing noise control technologies for jet engines although I'm much better at applied aerodynamics!

Cheers,
Brian

Considering all the variables that go into the sound reading at Laguna Seca, will we have a chance to do a test run on our first lap to see where we stand? That way we know if we should lift or short shift past the sound station. I have heard of this happening at driving events there.