Audio Distortion: Finding the Source and Clearing the Air

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.  And that last one is what I’m hitting today.

What is distortion?

Distortion is the sound heard where there is an unexpected change of the original signal to something else.  It can also be the result of a corruption of the signal at the source.  We usually identify it when we say something like, “that doesn’t sound right.”  And we usually are right in that assessment.  For some reason, the sound coming out of the system has been negatively altered and we need to fix it.

Noise versus distortion

An easy mistake to make is hearing noise in the system and assuming it’s distortion.  Therefore, to make it clear:

• Distortion is heard as the result of a change of the original signal to something else.
• Noise is an external (random) signal added to the original signal.

Think of it like this, if you scream into a microphone and the mic can’t handle the loud volume, then the audio signal will be distorted.  If you run a power cable next to an unbalanced cable, you’ll hear noise (interference) in the signal.  Another way to look at it is the loss of clarity is distortion, the addition of interference is noise.

What’s the ultimate cause of distortion?

Typically, clipping is the reason for distortion as distortion is merely the audible detection of clipping.  Clipping occurs when an audio component can’t provide enough power supply voltage to cleanly handle the signal.  This can happen for varies reasons.  For the needs of this article, I’ll leave it at that.

This can be seen on the channel level, mixer level, and even the amplifier level.  It’s one thing to over-drive speakers, it’s another to over-drive an amp.  Clipping, in this case, occurs when an amplifier is pushed to create a signal with more power than its power supply can produce.  It will amplify the signal only up to its maximum capacity, at which point the signal cuts or clips at the maximum capacity of the amplifier.

Reasons for distortion

Audio distortion can occur for a number of reasons.  Common reasons include:

• A microphone or sound source, like a computer, is overloaded with sound.  For example, 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.
• An instrument sends too hot of a signal into the system. I’ve seen this when a bassist uses a SansAmp on stage and is boosting their signal so much that it overloads the input.
• 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 louder volumes than your gig requires.  Pushing speakers to produce louder sounds than they were designed to produce will lead to a distorted output.  This isn’t distortion through clipping as much as it is the speaker’s hardware is unable to reproduce the signal it’s sent, though it’s still about voltage.
• Amplifier clipping. This happens when, as mentioned, the amp is overdriven.
• 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. This may or may not be related to the component’s ability to handle the signal as much as the result of busted internals.

Identifying the source of distortion

When you hear a “bad sound” in your system, you need to identify the source of the sound and take corrective measures.  Consider these steps.

1. Identify if it’s distortion or noise.  If it’s static-y or occasional, chances are it is noise and a review of cabling and wireless receivers is in order.
2. Review the amplifier and check for clipping.  This will tell you if the problem is at the amp-level or before.  If the amp is clipping, check to see if any channels are clipping.  If they are, the problem is with a channel. If they aren’t, the problem is with the overall signal-output.
3. Review the mixer’s output meter for clipping. This can easily be the sign of a channel problem.  It’s also possible, though not likely, to be a combination of all the channels summed together and creating a louder output that the mixer can’t handle.
4. Review the channels for clipping.  The LED or meter lights or clip/peak light can clue you into the specific channel.
5. Review the stage equipment.  It’s possible that the distortion is occurring at the component level on the stage.  For example, the bassist’s Sansamp is overdriving the signal that’s going into a stage input box or something else that can’t handle the voltage.  That equipment then passes on a distorted signal without a sign that anything “looks” wrong with the signal on the mixer.
6.  Check the speakers.  If everything else checks out, it could be a blown speaker.
   • Check out this article for how to check your speakers.

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.

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 event runs, and the level of the loudest part of the event.  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 a sound meter readings when you get to the loudest part of the event and then 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 the compressor which can also lead to a distorted sound.

If you do need to replace your speakers and are unsure about using passive or active speakers…

• Check out Episode 9 of the Behind the Mixer Podcast.

Channel clipping

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 lets 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.

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 now.”

Consider the impact of removing the equipment from service.  Will its removal have a drastic effect 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?