Wednesday, June 20, 2018

Why your sound system rings, and how to fix it

1.       How to ring out a room
I am often asked as a sound technician about why certain sound systems seem to ring a lot. There are several reasons for this. If you have an understanding of all of these problems it might save you some time and money when you are having problems with your sound system.

Problem 1 – Not understanding the Gain and the Volume Controls
                Gain and Volume are not the same thing. Gain (sometimes labeled Trim) is the amount of sound that you are allowing into the microphone that is on the stage. Let’s consider this scenario. You are running sound for a recording session, and the artist wants a particularly open and far away sound. You could just add a lot of reverb, but that would sound like you’ve added a lot of reverb. Another way to get this sound is to place a mic in a far corner of the room, and crank up the gain, and have the artist stand all the way across the room, or even in the next room and yell at the mic. By cranking up the gain, the mic now has a wide pattern, and is picking up sound from all over the place. Remember mics have patterns to their pickup ability. Cardioid mics have a bit of a heart shaped pattern. This “heart” can grow and contrast as you turn the gain up or down. A shotgun mic has a pretty straight pattern, and picks up signal along a lengthwise pattern. Some mics are “omni-directional” where they pick up sound from all directions. Some are “uni-directional” where they only pick up sound from one direction. The thing to remember is that adjusting the gain changes the size of this pattern, no matter what the pattern is.
                Now consider this, if you are on a stage, and there are lots of instruments blaring away, and you crank up the gain all the way on a singers mic, you’ve adjusted the amount of sound that the mic can pick up. If you adjust the “cardioid” pattern wider than the singers head, then now you have whatever is behind the singer coming into the mic as well. So gain is something that you want to probably keep low, and often times is the first thing that causes ringing in a room.
                Since we are talking about gain, we need to also go over volume. Volume is controlled by two or more controls on a mixer. The first volume control is the main channel fader, usually these are numbered 1 through however many channels you have on your board. Another volume knob is usually located above the main volume fader and is usually labeled 1, 2, 3, or 4, or Aux 1, 2, 3, 4. These are the volume knobs for the different Auxiliary monitor signals. You may have only 1 or you might have 2 Aux sends on your board, or you might have four depending on how advanced your board is. These knobs are the monitor signals. Now something that might cause ringing is if you have the gain up on the mic, and it starts to pick up the signal from the monitor. This will cause a feedback loop, and will cause the earsplitting ringing that inexperienced soundmen are known to cause. It’s wonderful when this happens in a church full of old people. (No, I’m just kidding.) Anyway, this is another reason why you want to be careful about cranking the gain wide open.
Problem 2 - Not Understanding the Volume Faders
                The main volume fader is labeled in a strange way, it has a line for zero, and then it counts up in decibels, and down in decibels. Many people think that they have to have the volume fader set at zero. A really common thing for soundmen to do, is to ignore all those markings, and pretend that when you are pulled all the way down, that is zero. And then blend in the volume from there. Sometimes no matter what you do you will experience a little bit of ringing. A trick my mentor taught me was to take a piece of gaff tape, or labeling tape, and find where the channel rings, and then put a piece of tape so the fader cannot reach that point. That way when you are in the middle of a show, you won’t accidentally turn up that fader too much.
                A lot of boards have bus faders. These are volume knobs that can control a whole bunch of channels at once. You can assign which faders you want to control by pressing the right buttons. The trick with these is to set all your volume knobs first, and then use the bus to bring the whole section up or down.
                The final group of faders are the mains. These are the left and right channel. Typically, at most shows, unless there is a specific need for stereo, I run these in mono in the left channel, and sometimes have even used the right channel as a monitor send if I didn’t have enough monitor sends. This gives me one monitor that has the full mix (somebody always wants that mix anyway) and then I can use the aux sends for the other mixes. On a two aux send board, this gives me three monitor mixes, and for a small band that is perfect usually. Anyway, just like the other volume faders, you want to make sure you find out where everything starts to ring and tape them so you don’t bump up your volume too much. 

Problem 3 -  Not Adjusting your Powered amps and speakers volume knobs properly
                All your powered amps and speakers will have volume knobs too, if they are too close to microphones or each other they will cause ringing. So make sure they are not feeding into any live mics.

Problem 4 - Not ringing out the room ahead of time
                Ringing out the room – this is an important topic that I want to end with. Every mixer has at least two EQ settings, a high setting and a low setting. Some mixers will have a mid setting, and some will have two mid settings. Then some mixers have built in graphic eqs, with several settings. But I find that having a separate eq for each of the mains channels, and each of the monitor sends is important. Here is the problem, certain frequencies are just tricky and will cause ringing when you try to bump the volume. If you use a spectral analyzer (frequensee is a good app for your phone) you can see exactly which frequencies are ringing. Once you get the room to ring (by playing white noise through the system) you find where the first ringing is, and turn down that frequency on the eq for that speaker. Then you turn it up some more until it rings again, and then you locate that frequency and turn it down. Then you do this until you can crank the volume all the way up. (Like I mentioned before some rooms are just tricky, or you might not have enough eq faders to mess with to do a perfect job. This is a good reason to go ahead and splurge on the EQ with a ton of faders instead of buying the cheap one. Anyway do your best, and then mark the board if you can’t get all the ringing out.) Just repeat this process for each one of the speaker systems you set up. If you run your mains in mono, then EQ them first. Then turn off the mains, and EQ each of the monitor lines you set up. Then turn everything on and double check it all to make sure everything is going to work.
                Once you’ve rung out the room, and set up all the sound, then bring in the band to do a sound check. If there is anything ringing after that, it’s probably something that can be adjusted with the eqs that are built into the board itself.
                Hope this helps.