WHICH METRIC?
dBA or dBC

A comparison of the dBA and dBC weighting curves show that the dBA filter attenuates the microphone response at low frequencies. 
The A-weight curve has been used over many years to mimic the response of the human ear.  But since the response of each human is different and the response of the ear depends on the loudness of the sound, this curve can only be judged to be an approximate average.

The A-weight curve tells us that, for example, the ear is 20 dB (10 times) less sensitive to sound pressure at 100 Hz than at 1000 Hz.  Or conversely, the sound pressure at 100 Hz must be boosted 20dB to give it the same apparent loudness as at 1000Hz.  This phenomenon explains, in part, the need for bass boost in audio systems.

In spite of the approximation of the A-weight curve, this metric has been used for many decades in noise regulations.  The sound from trucks, power tools, air handling units to name a few, are regulated using dBA.  In many music venues this metric is also used in spite of the strong bass content of the high-energy music.

For those communities that are far (e.g. >half a mile) from the music venue, this metric may be quite acceptable. Low frequency sound grazing over open land is naturally attenuated.  When low frequency attenuation occurs, female vocals become the most prominent sound.  E.g. communities south of New World Music, IL (now named Tweeter).

However, for those communities that are closer, residents receive the unattenuated low frequency sound.  E.g. Chastain Park, Atlanta.  In addition, the low frequency sound travels readily through the house walls and closed windows, changing the spectral shape of the music sound to one with the equivalent of strong bass boost.  Also interior house furnishings absorb low frequency sound poorly, adding to the dominance of low frequency sound.  In these special circumstances, there seems to be a good reason to use dBC as a metric.  The Grozier system at Hawaii Convention Center also uses dBC.

Because of local variations in layout of music venues relative to communities, each situation needs to be evaluated on is merits.  Typically, an experience acoustical consultant can effectively evaluate the situation.

***** Copyright 2009 *****