Sound Reinforcement with Social Distancing
August 11, 2020
It is early August and it has been a while since my last blog. Unfortunately Covid-19 is beginning to spike again, and therefore most live performances are on indefinite hold and even the few scheduled performances may need to be cancelled. Nevertheless, many local musicians would like to have some safe “jam” sessions. That is to say: a limited number of musicians, everyone wearing a mask (except of course the woodwind and brass players), and everyone at least 6 feet from any other musicians. And oh yes, the session is outside. This works fine for 4 musicians or less, but when there are more musicians there is a problem … the musicians are so far apart that they cannot hear each other very well.
My wife asked me to come up with a solution. Of course, the simple solution would be to close mic every musician in the session, but that is both time consuming to set up, and from the musicians point of view … let’s just say not very spontaneous, more like working in an outdoor studio. I came up with a far simpler method that is good enough, even if it is an imperfect solution. Here is how it works:
The musicians are placed in groups of (ideally) 3. Each group has a single condenser cardioid microphone and a monitor. The members of the group are arranged in a semi-circle around the mic and monitor. All groups face the center. A simple example is of 6 musicians in two groups, call them group A and group B. The monitor source for group A is from the group B mic and the monitor source for group B is the group A microphone. The assumption is you can hear those in your group just fine, but the other group is too far away to be heard well.
If the cardioid mic had a perfect null to its rear, and there were no sound reflections, and the monitor speakers also had a perfect null at the rear of the speaker, all would be great. However, the nulls are not that good and there are sound reflections. In short, mic A hears group A, but it also hears the other groups attenuated by 10 -15 dB. Ditto for the group B mic. Put another way, there is a lot of crosstalk. Since each mic is supposed to hear 3 performers, each 6 feet from the mic, the mic gain needs to be pretty high. Add to this that the monitor is right next to the mic. To avoid feedback the monitor gain (loudness) has to be kept fairly low. In practice, the speakers add about 6 dB to the overall volume. You don’t really “hear” the monitors, but if you turn the sound reinforcement off, the ability of the musicians to hear each other is noticeably diminished.
Since we are very near the feedback limit, we need to do things to reduce feedback potential. First having one mic 180 deg out of phase from the other helps a lot. Second, using the EQ to eliminate “hot” frequencies also helps. Using an automatic feedback detection (AFD) unit does not work very well, Why? Because this scheme has the feedback potential that is broadband, and so the AFD turns on nearly all its filters to compensate. If you look at your gain setting you seem to be able to play the music maybe 6 dB higher than without the AFD unit, but not really. Measure the actual sound level just before feedback without using the AFD. Now turn on the AFD and add the extra gain its use allows. Now measure the sound level. Basically it is the same as not using the AFD unit. Why? The AFD unit used so many filters that it lowered the overall sound level by say 6 dB. It is sort of taking away sound with one hand and then putting it back with the other hand … no net gain.
Still using your EQ you will find a few frequencies with high feedback potential and placing a few notch filters at these particularly sensitive frequencies will help. If your performance area is a large lawn with no nearby structures, you can likely just leave the EQ off. In either case, you will need a somewhat aggressive low cut filter.
Since these sessions are outdoors, AC power can be a concern. I have a number of Bose S1 battery operated speakers and so the only power I need can come from a deep cycle marine (or RV) battery and a pure sine wave inverter. My “board” only requires 15 W. If you don’t own battery operated speakers, you can use small efficient monitors and the pure sine invertor. I did some testing using a number of small monitors. At the sound levels we are talking about, these monitors average about 30 W . A typical marine/RV battery has about 80 amp-hours of charge. However, to avoid shortening the life of the battery, you don’t want to discharge the battery by more than half, and so the usable change in the battery is 40 amp hours or about 500 Watt hours (12.5 volts * 40 Amp hours). For sessions of 1 to 2 hours you can use regular monitors, a marine/RV battery, and a pure sine wave inverter.
I have used this setup a number of times for groups of musicians of up to 10. This very moderate sound reinforcement is greatly appreciated by the musicians. The groups I have worked with have played on open lawns or under a free standing carport (to provide shade). As I have said, from an acoustic point of view, the open lawn works best, however a session under a carport also works fine.