You can also see my other instructables on sound:
How Sound Works
The Concept Of Sound Pressure (SPL)
Before we go into how to fix problems there are a few things that I strongly believe about sound that you should know.
1. You can't program sound.
2. If it can fail, it will. If there is no way possible for it to fail... then it sill might.
3. You should know your equipment better than the back of your hand before attempting to troubleshoot.
With that in mind, let's talk audio!
Step 1: Feedback
You can fix feedback by turning the volume on the mic down (the fader on the sound board), turning the gain down (usually the knob at the top), moving the mic behind the speakers, or turning the main fader(s) down. The preferred way to fix it is to turn the gain down, or the main fader(s). (The main faders affect your gain structure.) If you don't know what mic the feedback is coming from, you should use the main fader(s). To prevent feedback keep your volume and/or gain as low as possible.
Step 2: Humming
One of the most common causes of hum is grounding issues. The soundboard and amps should in an ideal world have a dedicated electric line and ground. The rest of the sound equipment should also be isolated from the rest of the electric. If you have computers in your tech booth, try putting a ground lift on each of the computers and see if that helps. A lot of hum can come from a computers ground. Whatever you do, do not leave a ground lift on a computer. Use it for no more than 10 minutes for troubleshooting, and then figure out a way to isolate the ground. Most ground lifts have a contact to easily redirect the ground.
Another cause of hum is interference. Even if you are not using wireless technology in your sound system, cables still have problems with RFI (Radio Frequency Interference). Mute all of the channels on your mixer and bring your mains down. If you still hear hum, that means that it is coming into your line past the soundboard and it isn't one of your inputs. If you don't hear the hum anymore, make sure your cables are properly shielded, and start removing cables one by one to see which ones are giving you trouble. Once you find where you are having problems, try to replace the cable and see if that helps. When a cable starts to go bad, it can start to hum.
Yet another fix would be to have as little unbalanced signal in your system as possible. Unbalanced signal is very prone to humming. The difference between balanced and unbalanced signal is for another discussion, so for now I will just go over the basic cables and whether or not they are balanced. If I forget to address a cable, please leave it in the comments and I will address it as soon as possible. XLR is balanced, 1/4 in. and 1/8 in. (normally used for guitars and headphones) are normally unbalanced, and RCA is unbalanced. Normally the rule of thumb is if it has two contacts for every channel, it's unbalanced. If it has 3 for every channel, it's balanced.
A product for fixing grounding hum is called Hum X, a product for 1/4 connections is HE-2 Hum Eliminator (You could also try a Direct Box A.K.A. DI Box), and a more universal product is DTI Hum Eliminator.
There are more ways of fixing hum, but this covers the basics. If you have more advanced problems, consider hiring an AV consultant. I personally have worked with All Pro Sound, and really like their staff, knowledge, and business ethic. I would highly recommend calling and talking to any one of their staff about any problem you might have.
Step 3: Clipping
You can compare a "healthy" waveform to a "clipped" waveform in the pictures.
Step 4: Wireless problems
A good way to get the mic closer to the receiver is to move the receivers to an area backstage. Many facilities already have wire run from their backstage areas to their tech booth.
Step 5: Conclusion
If you are serious about sound, I suggest looking at your local library to see if they have any books on sound. There is no "right" way to do sound. It is an art form. As long as it sounds good, then you are doing it the "right" way. If Picasso did everything the "right" way, we wouldn't be able to view artwork in the same way as we do today.
Please note that I do claim copyright to the information. I did not use any specific sources when compiling this information, all of this is from my personal experience.
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