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Troubleshooting: Hum & Awe

Identifying and correcting troublesome noises in sound systems

One of the most common questions that comes up on many live sound forums is how to stop noises in a sound system.

I’ve been doing a lot of experimenting on this subject over the past several years, plus I’ve been battling sound system noise such as hum in the field for more than 45 years, so here are my observations on sound system noise and what to do about it. Before having any chance at stopping any kind of noise in a sound system, first we need to define what it is. Here’s a short list of noises that often appear in live sound systems and what we call them.

Hum – A steady-state bass noise at 60 Hz (cycles per second) without any significant harmonics. It sounds like humming with your lips closed, and is very close to low B-flat on a bass guitar with all the treble turned off.

Buzz – A steady-state noise that’s typically at B-flat as well, but it also includes a lot of harmonic overtones with a lot higher pitch. It sounds like a noise you would make with your tongue at the back of your teeth through open lips. Or just touch the tip of your guitar cable while it’s plugged into a stage amp that’s turned on. The sound you hear will be a buzz.

Hiss – A steady state white noise that sounds like rain on the rooftop or steam coming from a kettle. You can simulate it by blowing air through your lips without quite making a whistle. This is actually white-noise, not the pink-noise you might use to tune the sound of a room using a real-time analyzer (RTA).

Hash – A pulsating “buzzy” noise that can sound like bacon frying or buzzes being modulated. Sometimes it’s steady state, while other times it will have a modulation period depending on the cause.

Some Fixes

Believe it or not, you can make a pretty educated guess as to what causes any of the above noises in your sound system by listening to the actual quality of the Hum, Buzz, Hiss or Hash. So here’s what usually causes each type of noise in a sound system and simple steps to correct them.

Hum is most often caused by some sort of ground loop current between two different pieces of audio gear.

This typically occurs when you’re forced to plug stage and sound gear into multiple power outlets around the room. For instance, if the main sound system power amplifiers are plugged into outlets next to the stage, while the mixing console is plugged into a convenient outlet at the back of the room, the small difference in ground voltage between the two outlets can cause a rather large current to flow in the shield of your XLR connector. And that current can get into any amplifier or powered loudspeaker and make a lot of hum.

A quick fix is to run a long extension cord from the amplifiers’ outlet to the mixing console’s outlet. A longer term solution is to add an Ebtech Hum Eliminator inline between the output of the console and the input of the amplifiers, which will break the current loop and stop the ground loop hum. This same ground loop current can flow between DI boxes on the platform/ stage and the mixing console. So that’s the time to use the ground-lift switch on the DI box to lift the ground loop hum. (I’ll focus much more on detecting and correcting this particular type of noise in my next article.)

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Is this an instrument or loudspeaker cable? Better check and find out or a “big buzz” in the system might be the result.

Buzz is most often caused by an unshielded audio cable connecting two pieces of audio gear. For instance, use of a non-shielded loudspeaker cable to connect between a bass guitar or keyboard and its stage amp will likely produce a really big buzz. Thus double-check to make sure you haven’t accidentally used a loudspeaker cable instead of an instrument cable.

Note: Buzz can also be caused by a floating safety ground in your sound system that can become a shock hazard if you don’t pay attention to this warning sign.

Hiss is usually caused by mismatched audio levels between interconnected gear. There’s a general design rule that most sound gear’s input and output level controls should be run about 75 percent of the way up. (This implies between 7 and 8 on a scale of 10.)

So if you look at any mixing console’s faders you’ll probably see a highlighted area right around three-quarters of the way to the top marked as Unity Gain or 0 dB.

That’s the sweet spot to shoot for. If the input faders are near the top in order to attain enough sound level, it implies that you didn’t give the pre-amp enough gain.

On the other hand, if the output faders are down very low to keep from overdriving the amplifiers, that’s a hint that you’re likely distorting the internal mixing bus within the console. This level mismatch can occur after the signal leaves the mixer.

For instance, if you use a line-level output of an aux send to feed the mic level input of a video camera, you’ll be forced to turn the aux output level control down very low to avoid distorting the camera input.

That will usually cause all sorts of hiss to show up in the sound going to the video feed.

Hash is typically caused by switch mode power supplies contaminating the audio ground.

For instance, many laptop computers use internal switch mode power supplies to generate a bunch of different voltages for the screen, CPU, hard drive, etc. In fact, many times you can hear this hash change frequency based on hard drive access.

The best way to correct this is to either use an external USB audio port such as a Whirlwind pcUSB, or connecting the computer’s 1/8-inch headphone output to a special transformer-isolated DI designed for the output of a computer, such as a Radial Stage Bug.

The Radial Stage Bug SB-5, a transformer-isolated DI designed for interfacing computers and sound systems

With careful listening and a little detective work, you can make any sound system noise-free. Identifying the type of noise you’re hearing will put you on the path of discovery. Of course, there are likely to be multiple types and causes of noise in larger sound systems. In these situations, turn everything off, and then turn on each component one at a time, slowly. When you turn something on and the noise begins, that’s a hint as to its origin.

Correct that noise, then continue turning on more things, one at a time, until you encounter the next noise. Fix that and continue until the system is quiet with all components turned on. Then it’s time to make noise of a different kind!

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Next time I’ll detail more advanced testing procedures using a clamp-amp meter to look for the causes of ground loop hum in your building’s electrical system. And we’ll also discuss video hum bars that show up on your monitor screens, also with much more.

 

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