More on SPL measurements

First, there is very good information on A-weighting in sound measurements on wikipedia. http://en.wikipedia.org/wiki/A-weighting Most important is the shape of the curve. Compared to the C-weighted curve (the other most commonly used weighting curve), it is not nearly as flat and reflects more closely the human hearing characteristic. The D curve is also very interesting, which approximates the inverse of the equal loudness contour first determined in the 1930s. http://en.wikipedia.org/wiki/Equal-loudness_contour

By the way, please don't mess with Wikipedia... it's a great resource, even with the understanding that it may not be 100% accurate all the time. I've seen people playing around, one time a page was edited with "Sean is gay" for a few minutes, then was replaced with "You should not be reading this" and then the nonsense disappeared from the page. At least it didn't stick.

Next, I mentioned the inverse-squared law in relation to the measuremnts. This is well known within the wireless industry, and should be equally understood in acoustics. But what I did not realize was the effect of air resistance on the attenuation of sound. This is explained in my favorite reference book by Scott Hunter Stark "Live Sound Reinforcement". You can find this by doing a book search on Google, it actually contains images some pages (53-54 in this case). The attenuation of sound affects higher frequencies, so again the A-weighted curve is attenuated moreso than the C-weighted curve as you move away from the source.

In a previous entry, I mentioned the purchase of a hand-held SPL meter that will be used for a residential party. The party is to be held in Poway, CA. City of Poway municipal code Chapter 8.08 "NOISE ABATEMENT AND CONTROL" states (in various places; reformatted):

7. “Disturbing, excessive or offensive noise” means:
a. Any sound or noise which constitutes a nuisance involving discomfort or annoyance to persons of normal sensitivity residing in the area;
b. Any sound or noise conflicting with criteria standards or levels as set forth in this chapter for permissible noises;
c. Any sound or noise conflicting with criteria standards or levels established by Federal or State government which are applicable in the City.

20. “Sound level meter” means an instrument for the measurement of sound, including a microphone, an amplifier, an attenuator, networks at least for the standardized frequency weighting-A, and an indicating instrument having at least the standardized dynamic characteristic “fast,” as specified in American National Standard Specifications for Sound Level Meters S1. 4-1971 or the latest version thereof.

Zone or Land Use Designation / Applicable Limit One-Hour Average Sound Level (in decibels)

OS-RM, OS, OS/1du, RR-A, RR-B, RR-C, RS-2, RS-3, RS-4, RS-7, and Specific Plan, PRD and PC regulations with a density of 11 dwelling units or less per acre.
10:00 p.m. to 7:00 a.m. 40
7:00 a.m. to 10:00 p.m. 50

PF, RA, RC, MHP, and Specific Plan, PRD and PC regulations with a density of 11 or more dwelling units per acre.
7:00 a.m. to 7:00 p.m. 55
7:00 p.m. to 10:00 p.m. 50
10:00 p.m. to 7:00 a.m. 45

We will ProActively monitor and adhere to these limits and have a hard copy of the municipal code on-hand during the event. In addition, the placement and directivity of the speaker cabinets will make it almost trivial to avoid even coming close to these limits in this case... There is only one direction of concern (i.e. one offending neighbor) and this is NOT the direction that the speakers will be pointed anyway. In addition, this particular neighbor does not fit the description as set forth in the municipal code "persons of normal sensitivity residing in the area" and this fact has been documented over several years.

ProActive Sound can provide the same research, testing and preparation for YOUR event in YOUR municipality with sufficient lead time.