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In part one (CPM October 2007), we talked about how volume affects worship, and how hard it can sometimes be to keep it pulled back, even when it's the right thing to do. In this edition, we'll talk a bit more about the meters, the numbers, hearing, good volumes, and our responsibilities as sound engineers.
Meters
Shortly after moving into the new Willow Creek Community Church nearly four years ago, we self-imposed a 95 db(C)-SPL-peak-fast limit. If you don't know what any of that means, go to Radio Shack and pick up an analog SPL meter for less than $50. We have been measuring on several meters and software programs to learn their subtle differences. Your SPL meter will have a "weight" or filter switch, typically offering "A" and "C" as options. These are EQ filters that color the way the meter's mic responds to sound. To oversimplify, (A) passes mostly midrange, and (C) is closer to flat.
Most of the research for noise exposure and hearing conservation has been done using the (A) scale, which has a slightly heightened sensitivity to the ear's most sensitive range of frequencies. Using (A) to measure music to protect hearing has proven consistently suc-cessful and (A) has been adopted by several governments as a standard of measurement for hearing protection and conservation. Therefore, set your meter to (A)-slow for documenting your services (yes, your church should be documenting, as you'll soon read). Meters are our friends, not techno-geek weapons aimed against the artists. It's OK for that number to be over to the side where others can see it. Things feel very different between a PM service and a 5:30 am. conference rehearsal. Glancing at a meter when your head is still numb is sometimes the only way to know if you are in the ballpark in an empty room.
OSHA and A/C Weighting
OSHA (Occupational Safety and Health Association of the USA) is a government organization that places regulations on noise exposure in the work place. Its specifications for time exposure limits use the (A) Weighting filter. Observe Fig. 1. For all the reasons above, Willow made the change from (C) to (A). dB SPL(A) reads lower than dB SPL(C) because dB SPL(A) is looking at the midrange. Our studies reveal that for our services and the kinds of music, and the quality of the PA (four years ago), dB SPL(A) can read between 4 and 11 dB lower than dB SPL(C). The average for music seems to be 5 dB lower. If you have deep sub bass and bright highs, the difference for music between (A) and (C) might be closer to 11 dB. So as we migrated from (C) to (A), we were keeping under 90 dB SPL(A) Slow-response, (approximately equivalent to the 95 dB SPL(C) fast peak ceiling we had been operating under). By switching to slow, we were finally paying a little less attention to instantaneous peaks, and more to the mix.
Keep in mind that this is a full range 90, with sub support down to 32 Hz. The amount of lows and highs still make a difference on a meter reading (A), just not as dramatic as if it were on (C). A reading of 90 out of a quality PA is quieter and more musical than 90 out of a mid-rangy, "honky" system, which would seem much louder, more annoying, and less musical to our mid-range-sensitive ears.
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Chris Gille is also known as “Papa Audio,” or audio & systems director in the production department of Willow Creek Community Church.
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