Good sound reproduction requires balance. Many auditoriums have at least two sound systems. The house system covers the audience, and the monitor system covers the stage.
On stage, balance must be achieved between the various instruments to satisfy the musicians.
The resulting stage level must be balanced with the house system level to satisfy the audience.
Let’s use an analogy to clarify things. If I need gray paint, I can mix a little black in with a lot of white until the correct shade is attained.
But if I start with a lot of black paint and add white until I reach the desired shade, I can end up with WAY too much paint. Balance depends on what you start with!
This is not unlike the battle between the main and monitor levels in an auditorium. What you start with determines where you end up.
Primer On Ratings
The loudness (level) of a sound is stated in decibels. Decibels are numbers that describe how loud something is relative to something else.
The “something else” used for sound pressure level specifications is the threshold pressure sensitivity of human hearing.
In other words, whenever you see something like “x” dB-SPL the number is indicating how loud the sound is relative to the lowest possible audible sound.
A few other factors must be thrown in to clarify things, including a frequency-weighting term and a meter response time.
An “A” weighting means that the level was measured through a filter that removes a great deal of the bass (sorry about that bass players).
Think of it as a mid-range measurement. Slow response time means that the meter is ignoring the short-term transients and responding to the average level that it is being exposed to (much like the human auditory system).
Combining all of these, the sound pressure level at a listener position could be correctly stated as 95 dBA-Slow. Note that other weightings and meter response time are possible, but dBA-Slow is very relevant to tracking human perception of sound.
Note also that in most cases a meaningful specification of sound pressure level must include where it was measured. One meter is a handy reference for quoting the levels of arbitrary sound sources.
So, it doesn’t mean much to say that the level of the sound system is 100 dB. But it is very meaningful to say that the sound pressure level is 100 dBA-Slow at 10m.