Why Your PA Will Never Beat Physics

Live sound goobers delude themselves into imagining that with enough of the right equipment they can overcome room acoustics.( As a baseline reminder, these are the same people who can’t deliver a decent mix in an outdoor environment where they have absolutely no excuse for their incompetence and performance failures.) This is one more place where not knowing the basic concepts of a profession creates gross amateurs who do far more damage than good in their existence. If fact, sound goobers are often far greater performance offenders than enhancers. (See Snarky Puppy, for example.) 

The problems with a typical performance venue is that the RT₆₀ time (reverberant energy from the loudest point to where the reverberant energy has degraded by 60dB) is measured in, at least, substantial portions of one second (1S). Due to room resonances, the decay is not typically a nice clean exponential curve, either. In fact, due to room modes the actual decay curve can be pretty long and lumpy. So, for a simple drum beat at 120bpm, it is likely that the constant reverberant energy in the room might be within 3-6dB of the original sound source. A highly counter-intuitive fact of reverberation is that, except for room resonance variables, reverberant energy is equal everywhere in a room. If the measured reverberant energy is 90dB_SPL at the back of the room, it will be very close to that at every microphone on the stage. If the sound system is delivering 125dB_SPL at the room’s critical distance (where the reverberant energy and direct sound energy are equal, the “noise level” (unwanted reverberant energy) will be very near 120dB_SPL everywhere in the room. 

So, keeping that fact in mind, you might realize that the sound pressure level (aka noise level) at the element of every microphone on a stage in my last example (above paragraph) will be 120dB_SPL. So, to achieve any level of isolation/discrimination/intelligibility at each microphone, at least 6dB and ideally 20dB or more of signal (direct signal) will need to be delivered to the microphone. This problem is neatly, if not particularly musically, resolved with the use of direct injection (DI) boxes. There is no acceptable fix, except for atrocious microphone technique, for vocals and instruments that require a microphone. The reason musicians use microphones like the SM57/58 and its lo-fi clones is because of that instrument’s undeserved reputation for high Max_SPL capability. There is a reason that Shure does not specify a “maxSPL” value and it is not because it is a happy marketing story. The SM57, for example, is selected for close mic’ing of loud guitar amplifiers because of the additional distortion it introduces. There are many far better choices for that job if you are not looking for the distortion contribution of the microphone. In the kind of high volume environment we’re talking about in the previous paragraph, hyper-high max_SPL specifications are a must: 140dB_SPL and above, for example. 

140dB_SPL and above sounds like an impossible sound pressure level, but if a loud voice is typical 88dBSPL at 1 foot, every halving of that distance results in a 6dB increase in sound pressure, eight half-steps would result in a 142d_BSPL at the element. Since many pop singers insist on planting their lips right on the microphone grill (even though there is typically some space between the grill and the actual element) it is not inconceivable that sound pressure levels greater than 130dB are typical. That would provide at least a 12dB signal-to-noise range for the microphone in my worst case example; assuming the microphone can function in that hostile environment. 

Signal-to-noise ratios sum exponentially, though. If you have two channels combining where each channel has a 12dB S/N ratio, combining the two channels will cost you half of your S/N ratio. If you are going for any sort of quality sound, the key will always be to drive the average sound pressure level down, not up. I suspect, if you are a typical amplified music performer you are going for something other than quality sound. What that is I can’t guess.