Tuesday, April 5, 2016

Use C-weighting to monitor levels in live sound

I continue to encounter live sound "professionals" who have not fully educated themselves on the science of sound and the frequency response of the human ear.  The topic evokes responses of borderline religious or political fervor, where discussion invariably devolves into personal attacks when defenders cannot produce technically accurate rebuttals. It's understandable that someone who has been doing sound for a long time would become defensive when learning they may have been doing something wrong the entire time.

I have seen all sorts of explanations about the correct application of A- and C-weighting.  Some professionals think that the difference has to do with the sampling rate, or think that C-weighting is a completely flat measurement.  Many people continue to use A-weighting to measure loud performances which is incorrect.

All this in the face of what is abundantly clear:  Many live sound "professionals" (and particularly the "DJ" subset) permanently raise the hearing thresholds of their audiences by running at high levels.  Each generation of concert-goers, once their hearing has been damaged, goes on to demand high levels from the next generation of live sound professionals, and the cycle never ends. Maybe we need one or two "reasonable sound level" shows during a run for audiences that don't want their hearing damaged.

I have personally resigned from productions when forced to exceed what I have set as my maximum sound level based on my own research. Not surprisingly, the individuals requesting higher levels had on prior occasions disclosed they had some form of hearing damage.  I urge you to do your own research and monitor your levels with an SPL meter. 

I don't want to hear excuses like "even though I'm at the mixing console I'm not the one who decides how loud the levels will be" - this sounds eerily similar to the excuse "I was just following orders" - and we know that whether your finger is on the trigger of a gun or on the master fader of a mixing console that you ARE responsible for the damage you cause.  As professionals we absolutely have the right to negotiate our contract however we want, and to exercise our right by walking off a production.

Here is a list of what facts you need to accept in order to reach my conclusions:
  1. The frequency response of the human ear is not flat
  2. The frequency response of the human ear changes at different overall sound levels.  At low sound levels, such as those of a quiet room, the frequency response curve is different than when in a loud concert.  (Fletcher and Munson)
  3. The A and C curves closely match human hearing at 40 and 100 phons, respectively.
  4. OSHA's inappropriate selection of the A curve for measuring loud performances happened prior to 1983. Once a standard is created, it is difficult to change to something better
  5. The appropriate curve to use on your SPL meter is based on the loudness of the performance (quiet lecture or loud concert)
My argument is that if you use the C scale and stay under Yamaha's limits you will certainly stay under OSHA's limits using the A scale, with the added benefit of causing LESS hearing damage.


Given the sensitivity characteristic of the ear, the 'A weighted' curve is most suitable for low level sound measurement....In the presence of loud sounds, such as rock concerts, the ear has a "flatter" sensitivity characteristic...In order for the measured sound level to more closely correspond to the perceived sound level, one would want a flatter response from the SPL meter.  This is the function of the B and C weighting scales.  In apparent conflict with this common-sense approach, O.S.H.A. and most government agencies that get involved with sound level monitoring continue to use the A scale for measuring loud noises.  Since this causes them to obtain lower readings than they otherwise would, the inappropriate use of the A scale works in favor of those who don't want to be restricted...It is beyond the scope of this book to detail the 'danger' levels, but anyone responsible for sound reinforcement should be cautious about delivering levels above 95 db(C) SPL to an audience for any prolonged period.
Davis & Jones. pp. 30-31 Yamaha's Sound Reinforcement Handbook. 2nd ed. Milwaukee: Hal Leonard, 1989.
C-weighting is optimized for full-bandwidth sources at levels exceeding 85dB. A-weighting filters out the high and low frequencies and is optimized for lower volumes.
Gibson, Bill. p.46 The Ultimate Live Sound Operator's Handbook. 2nd ed. Milwaukee: Hal Leonard, 2011.
Pro audio equipment often lists an A-weighted noise spec − not because it correlates well with our hearing − but because it can "hide" nasty hum components that make for bad noise specs.
...the C-weighting correlates better with the human response to high noise levels.
Gracey & Associates
On a sound level meter (SLM)...and we are forced to choose between A and C, we should pick C.
Doctor ProAudio
When dealing with the uninformed, live sound folks can "cheat" the rules (or some poser's cursory knowledge) by using the A-weighted filter so that measurements appear to be lower in SPL than a standard or rule that is based on a Flat or a C-weighted limit.
Pro Sound Web

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