ESTABLISHING A SOUND LEVEL REGULATION

Topics covered here are:

Location of Regulating Microphone
Establishing a Regulation Sound Level Limit

Location of Regulating Microphone

Ultimately a decibel limit is applied to the sound level in the nearby community.   So the immediate reaction is to install microphones in the community.  However, this is neither to the community's or the amphitheater’s benefit.  The problem is that sound levels in the community from nearby sound sources, such as vehicles, mowers and heat pumps can, at times, be louder than the music.  Yet the music sound is still perceived as too loud.  If the music were permitted to be as loud as these other sources it would be even more unacceptable.  This is not to the benefit of the community. From the amphitheater point-of-view, it cannot accept responsibility for sounds recorded by the microphone over which they have no control.  Consequently, locating a microphone in a community microphone produces contaminated sound data that is not acceptable to any party.

Fortunately, there is a solution to the problem.  A microphone location is established at a closer location which is dominated by music sound.  An independent acoustical consultant measures the concert sound at this location and at the same moment in the community.  The difference between the two decibel readings represents the attenuation of the sound as it travels into the community.  This effort permits the higher sound limits to be established at the closer-in location, and at the higher sound level. This process establishes reliable sound level data.

Establishing a Regulation Sound Level Limit

Another item of concern is establishing the acoustical parameter to be used for the sound level limit in the community.  Experience has shown that the very long time-average do not reflect the subjective response of citizens. Citizens prefer short-time averages. 

Amphitheaters generally prefer long-term averages. Often a compromise time-average of 10 - 30 minutes is selected.  This longer period allows the amphitheater time to be proactive in using their Grozier system to control sound to within the regulation limits. Using the second-by-second data from the Grozier system, the amphitheater has time to correct the problem before community limits are exceeded and fines are levied.

The decibel value used in the regulation needs to be understood.
1. Review the difference between the two weighting filters dBA and dBC . For heavy rock music, the same regulation limit is 10 dB higher for dBC than dBA. So a limit of 100 dBC is the same as a limit of 90 dBA.
2. Understand how the time average period effect the regulation limit. The characteristics of heavy rock sound are such that 1 second average sound maximum is 5 dB greater that 1 minute average and 8 dB greater that 1 hour average.
3. Add the effects of 1. and 2. For example the dB limits given in the table below are all approximately equivalent for heavy rock music.

Note that there is almost a 20 dB difference between two of the values shown.