Now, the house system has been designed to provide even coverage of the audience, so we’re probably miking the band to get the same sound to everyone.
For the house system level to dominate what the audience hears, it must be louder than the sound that is coming from the stage.
If the level is 105dBA on stage, we can estimate the level in the house. This is where it gets a bit complicated.
First, the level from the stage is not coming from a single location – it is spread out. We might treat it as a very large sound source.
For sound sources that are large relative to the listener distance, the sound level falls off at less than the 6 dB-per-distance-doubling predicted by the inverse-square law. We will use 3 dB-per-doubling, which is probably still conservative for much of the audience.
The directivity characteristics of the on-stage sources will not be considered – not a far stretch since there will be sources pointing in all directions.
Using one meter as a reference distance, and assuming a front-of-house mixer distance of 8 meters (25 feet), the reference distance will double roughly four times en route to the front-of-house, for a level drop of 12 dB.
This means that the monitor level at FOH will likely be about 105 dBA – 12 dB = 93 dBA.
Adequate Signal To Noise
Research has shown that for one level to mask another, the level differential should be on the order of 10 dB. This means that the minimum SPL for the house system to dominate the monitors in the house at the mix position is 93 dBA +10 dB = 103 dBA.
For the purpose of discussion, we will consider the level from the mains to be the “signal,” and the level from the monitors to be the “noise.” What we’re after here is an adequate signal-to-noise (S/N) ratio.
Assuming even coverage from the house system, the S/N ratio will be about 10 dB at the mix position. It will be less than 10 dB at listener seats closer to the stage (a good reason to not site there), and more than 10 dB for listeners that are beyond the mix position.
Research has shown that the maximum SPL that a broad age-group audience will tolerate without complaints is about 95 dBA-Slow.
This is validated by the author’s experience as well as informal surveys of the Syn-Aud-Con Listserv. There’s no way that we are going to get our desired 10 dB S/N with the stage sound at its current level.
We can conclude that the use of a live drum kit on stage makes it likely that the minimum level that the house system can be operated at is over 100 dBA-Slow and beyond the comfort zone of a broad age-group audience!