Questions About Sound Levels
The articles in the last several TFWM Audio Newsletters have raised questions from some of the readers which I will attempt to answer here.
"What if there is a big discrepancy between the “A slow” levels and the “C Slow” levels? Our “A” levels may read 95 - 98 db (yes I know we are a loud church when we get going, and we are addressing the issue) but the “C” levels could very well be at 100 – 105 db. I know the filters is what makes a difference in the readings; but isn’t pressure from those frequencies still harmful to your ears? We do use a lot of low end in our mix."
You are correct that there can be a big difference between "A" and "C" weighted measurements. For example look at the following chart:
At 50 Hz there is almost a 30 dB difference between the "A" and "C" weighting curves. Therefore if your music has a lot of bass it is easy for you to see "C" weighted levels that are 10 dB or greater higher than the "A" weighted levels.
"A" weighted measurements correspond fairly well to how annoying a loud sound is, and most governmental sound level regulations are based on "A" weighted levels. Therefore in order to compare your readings with exposure limits such as those published by NIOSH or OSHA it is important to use "A" weighting. "A" weighting probably underestimates the potential for hearing damage from loud low frequency sounds, but "C" weighting probably overestimates the potential for hearing damage.
Please remember that the exposure limit tables such as those published by NIOSH are based on the average sound level not the loudest sound you measure. For example NIOSH recommends that workers should not be exposed to an average level of 94 dB A weighted for more than an hour during the entire day. Exposure to an average level of 97 dB A weighted is allowed for no more than 30 minutes, and 100 dB for 15 minutes. The peak levels can exceed these numbers as long as the average levels don't exceed the limits.
Another reader asked:
"Are there specific standards or guidelines for church sound levels? I usually have my meter set to fast/A weighting and run 87-90db on Sunday. What I would like to find, is a paper or article that defines acceptable levels… low and high. For some (I would include myself) it’s never loud enough!! And for some, over 85db is “TOO LOUD”."
There are no legal limits on sound exposure in churches (at least in the USA) but that does not mean we should ignore the subject. As Doug Jones pointed out in his article we have a responsibility to God if not to the State not to damage the hearing of the congregation.
At the low end of the level scale we need to provide enough level that the spoken word and musical lyrics can be clearly understood over the background noise in our House of Worship. Conversational speech may be in the 65 to 70 dB range, and if the background noise in your HOW is no greater than about 40 dB A weighted, then spoken word at 65 or 70 dB (around 25 dB above the background noise) may be loud enough to be clear. If your room has greater background noise, or if you have a noisy congregation, then somewhat louder levels may be required. The danger is if the speech gets too loud it may start to get annoying even if it is not loud enough to do hearing damage. If you have a preacher with a large dynamic range you may need to ride the gain (or use a compressor) to keep the loudest parts from getting too loud.
Usually it is the music that will push the upper limits of sound levels. Fortunately the music is usually not the entire service time and the quieter portions of the service will help keep the average sound levels within control. The best way to determine the average level over the duration of the service is with a meter that can measure Leq (Level equivalent) or the time weighted average over the length of your service. Using such a meter you hit a start button at the beginning of the service, and the meter then displays the running average level as the service progresses. At the end of the service hit the stop button and record the average level for that service. With some experience you will find that it is still possible to have good musical dynamics while still keeping the average level under control so you are not damaging hearing.
Ray A. Rayburn
Ray@K2Audio.com
http://www.K2Audio.com
