Audio Distortion: Finding the Source and Clearing the Air

Is this type of distortion good or bad?
Photo provided by ba1969

The word distortion has different meanings depending on who you ask.  For some, they immediately think of “distorting words or phrases” wherein someone takes what you have said and twists it into something else.  For others, they picture a distorted image.  A guitarist sees distortion as an effect for applying to their guitar’s sound.  A sound tech sees distortion as a bad sound reflecting a problem in the audio system.

Reasons for distortion

Audio distortion occurs because;

  • A microphone or sound source, like a computer, is being overloaded with sound.  Regarding microphones, the microphone can’t handle the volume level which it’s detecting and thus distorts the sound that it’s sending into the sound system.  In the case of other sound sources like a computer, you might push the volume level within the computer software past a point which the computer’s hard can handle.  Thus, it sends out a distorted sound.
  • Speakers are being pushed beyond their limits.  Working in live audio production, you might have the occasion of working on equipment not set up to handle loud volumes that your gig requires.  Pushing speakers to produce louder sounds than they were meant to produce will lead to a distorted output.
  • Faulty equipment.  Equipment can fail in a variety of ways.  For example, an effects processor could fail and you’d no longer hear any sounds passed through it.  However, it could also fail and you’d hear a distorted sound passed out of the processor.

Stopping / preventing distortion

Sound source distortion

The most common reason for distortion is an input overload like the microphone overload mentioned above.  Mic’ing an instrument, or even a vocal, is more than sticking a microphone right up to the sound source.  You have to use the right type of microphone for the job.   You also have to set it up in a way that best captures the sound.  A distorted sound can be resolved by placing a greater distance between the sound source and the microphone.  Or, it could be resolved by switching the type of microphone.  For example, placing a condenser mic up to a kick drum can cause distortion so you could either swap it for a dynamic microphone or, in the case of large-diaphragm condensers (LDC), move them a few feet away from the microphone. I’m not saying the live environment is the right place for an LDC but you do see how the microphone type and location can stop / prevent distortion.  Check out The Six Types of Kick Drum Microphones.

Distortion from a sound source, like a computer, usually happens when the sound within the source (computer, cd player, etc.) is driven to a higher output level than what the hardware is able to handle.  I find this can easily happen with computers because there are two volume settings which can get changed; the operating system’s volume control and the volume control of the software used to play the sound.  You can even go one level deeper and look at the volume of an individual track in a multi-track audio program.  Start by checking the operating system’s volume is at the normal level, then go to the software program and then down into the individual track volume.

Speaker distortion

Audio speakers are designed to handle a certain level of volume.  Once they pass a certain level, they will distort the sound and if prolonged, will eventually fail.  The latest newsletter goes into more detail on speaker failure.

Regarding preventing speaker level distortion, you need to know a few pieces of information; the maximum volume your speakers can handle, the average volume your church service or other gig runs, and the level of the loudest part of the service / gig.  You can then determine how much headroom you have in your production.  This headroom is the measurement of the difference between the average sound produced out of the sound system and the loudest output level the system can handle. If your average volume level is 90 dBA and your speakers can only handle 120 dBA then you have 30 dB of headroom.  You can prevent speaker distortion by watching your sound meter readings when you get to the loudest part of your event and reducing the board volume to keep it under the limit.  You can also use a compressor to do this effectively as long as you aren’t overdriving your compressor which can also lead to a distorted sound.

A common question I receive, when I hold a training session, has to do with the channel / board level peak lights.  The question comes in the form of “is it ok if the clipping light comes on” or “isn’t it bad if the channel is clipping.”  Clipping is a sign the signal level is too high.  A channel with occasional clipping isn’t an issue.  A singer suddenly belts out a word louder than the others or a drummer let’s his sticks hit a bit harder.  That’s ok.  Most of the time, this sort of clipping isn’t going to be noticed.

A channel showing consistent clipping means you need to lower the channel volume or if you caught it during the sound check, you can reset the gain structure for that channel.  Board clipping works much the same. Occasionally, it’s ok but anything more than that and you have to re-evaluate your board input levels.  I would go as far to say if your board level is clipping in any form, then you should look for the source and see what you can do to eliminate it.  Clipping is an extra audio process that can negatively affect your sound.

Faulty Equipment

Distortion produced by faulty equipment isn’t a matter of “how do I stop it” as much as it is a matter of “what do I do know.”

Consider the impact of removing the equipment from service.  Will its removal have a drastic affect on the quality of the sound?  I’m talking about “will the congregation notice the difference.” With this information in mind, you should evaluate your existing equipment and determine what happens if that particular piece of equipment fails.

Should you;

  • Repair it?
  • Replace it?
  • Have access to a spare for swapping in?

The Take Away

Audio distortion is a sign an input volume is too high, a speaker is doing more than it’s rated, or a piece of equipment has failed.  Each reason for distortion has a solution which can be easily implemented.  The best part of all, barring equipment failure, you can resolve most problems during the sound check….of course we are talking “live sound” where anything CAN happen at any time.

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Comments

  1. Quaid says

    This article from Peavey,

    http://www.peavey.com/support/technotes/concepts/clippingrevisited.cfm

    mentions that different mixer manufactures test for distortion at different points. One unnamed manufacturer doesn’t test for distortion after the channel faders. So, depending on the positsion of the fader, you could be causing distortion BEFORE the channel’s clip led lights up.

    It goes on to say that a way to test this is to set the fader at maximum, and then raise the channel’s gain while a signal is being produced into the channel. If you begin to hear distortion before the clip led lights, then you have a mixer that, as the article says, “…cheating the clip indicator to understate how much a signal is clipping.”.

    I have ran into this problem with our portable system configurations, where no clip leds are lit anywhere in the system, but still, distortion is audible.

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