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Dec 04, 2007

Savant comments on “plainly audible” noise ordinances under consideration in Boulder County communities:

“To me, this clears the way for some smart aleck to propose that the objective speed limits should also be done away with on the roads in their communities. Since equipment and training for measuring vehicle speed is expensive and imperfect, it should be left to the law enforcer’s judgment as to whether someone was driving excessively fast. Right?”

In a similar case, Heather alerted me that Hartford is looking at a similar noise enforcement proposal. Springfield amended their noise code in a similar fashion (eliminating the objective standards in the process) but that’s another post for another time.

I agree that objective standards are preferable (and I’ve written about this many times) but I can understand a municipality’s desire to go this way because measuring sound is hard - it’s not as simple as “point the sound level meter at the noise source and read the number.”

It starts with equipment vendors who convince officials to purchase the $5,000 kitchen-sink sound level meters rather than the simple ones that would make due in most cases. Then officials on the street have to parse their way through the various settings — A-weighting or C-weighting? SEL, Lmax, Leq, SPL? What’s the difference between background noise and ambient noise? Field calibration? Windscreen (you do know that the wind can affect measurements, right)? Diffraction/reflections from the operator or nearby ground/wall surfaces? Is your microphone random incidence or frontal incidence (meaning to you point your mic toward the noise source or away)? Is the noise source at least 10 dB above your background level?

Let’s not even talk about the practicality of using a sound level meter to measure transient sources like a moving car that may be 10 miles away by the time the official fetches his SLM from his vehicle.

ANSI S12.18 (a standard that describes the preferred procedures for outdoor sound level measurements) is 18 pages long, not counting the various normative standards that S12.18 references. Is your enforcement officer going to be completely familiar with those procedures after a 4-hour training session? Here’s a hint - I’ve been doing this 13+ years and I still have to refer to those standards from time to time.

As long as noise control engineers focus all their attention on big-money items like transportation and industrial noise control, small-budget municipalities won’t have much choice but to trust their ears.

For what it’s worth, I do have some ideas about practical instrumentation for municipalities, but I don’t have the resources to really develop those ideas on my own. The big vendors I’ve discussed this with have shown a tiny bit of interest, but I still get the feeling that they would rather sell $5,000 meters. Again, that’s a post for another day.


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