It won’t take long before something unexpected will happen. Here are a few of the most common problems in sound systems.
There are many things in audio that are called feedback, but most often it refers to that lovely squealing sound that gets louder and louder until everyone is miserable. This happens when a microphone picks up the sound coming out of a speaker, which is sent back through the speaker and amplified, until everyone in the venue turns around and glares daggers at the sound tech.
The quickest way to fix feedback is to quickly turn down the offending channel (which sometimes means turning down the master fader if the offending channel can’t be found). The best way is to determine at what frequency the channel is feeding back and remove that with EQ. If many microphones are feeding back at once, it might mean EQ’ing out the frequency on the master channel, either via its own channel EQ if it has one or an outboard graphic EQ. Many graphic EQs have “feedback finder” lights to help this process.
Before a show, it can be helpful to “ring out” a system by turning all available microphones on and turning up the master volume until feedback occurs, EQing out the offending frequency, and repeating until feedback occurs at multiple frequencies. This can have a negative impact on the overall tone of the system, and the benefits of increased headroom before feedback must always be balanced against overall system quality.
Phase issues can be the hardest to hear, but are also surprisingly common. Phase problems are why microphones need to be spaced at least three times as far away from each other as they are from the source. Phase issues happen when sound waves interfere with each other (either acoustically or electronically) to increase or decrease the volume of a signal, usually in interesting and unhelpful ways. Because of the nature of waves and wavelength, different frequencies will cancel or boost each other differently, leading to very odd effect. (This is the theory behind phaser effects.)
The most common kind of phase problem happens whenever subwoofers are placed apart from each other, as in a stereo setup. The different times at which the signals arrive at the audience creates a “power alley” exactly between the two speakers where the subwoofer volume will increase dramatically, while falling off in a “comb” pattern of loud and quiet sections varying by frequency in a process creatively called “comb filtering”. At higher frequencies, the wavelengths are small enough that these comb teeth are not noticeable, but the longer wavelengths of low frequencies can mean that a step a foot to the left or right can drastically change the low frequency response of a system.
Electrical hums and buzzes usually happen when something isn’t grounded or connected correctly. Anything that gets too close to an electrical cable without the proper cabling type or an improperly connected ground can pick up a buzz starting at 50Hz or 60Hz (depending on your region’s electrical line frequency) and at every octave above it. That makes this kind of buzz incredibly difficult to remove. Thankfully, many direct boxes and other similar equipment will likely have a “Ground lift” switch which disconnects the audio ground, often solving minor buzzing in a system.
You may be tempted, if that doesn’t work, to use an adapter which removes the electrical ground from a system, or even pull the ground pin off an electrical connection. DO NOT DO THIS. Removing electrical grounds from audio equipment will exacerbate system problems, possibly even introducing line voltage onto the audio ground, which can cause injury or even death. Never remove electrical grounds or otherwise perform electrical work without consulting with a licensed electrician, both for your own safety and for the safety of those attending or performing at your venue.
I add this section not because it’s complicated, but because of how easy it is to forget about. If a microphone or direct box isn’t working, always check if it requires phantom power to operate and, if it does, that it is on. I can’t tell you how many times I would swap cables and run all around trying to find the solution to a microphone not working, only to finally realize I forgot to turn phantom power on for that channel.