If your sister is humming, it might be taken as a sign of happiness. But if your sound system is humming, it can be taken as a sign that someone has made an improper connection or placed wires where they don’t belong.

The two most common causes of sound system hums are ground loops and induction.

First, a discussion about ground loops. It starts with the Service Entrance Section (SES), that big gray box outside buildings that delivers electrical (AC) power. If you take a look at an SES, you may well see a heavy copper wire that is connected to a long copper rod (called a Ufer ground) driven about six feet deep into the ground. As you might have guessed, this is how a building's electrical system is grounded.

Now, for purposes of discussion, let's assume the SES is located outside the northeast corner of a church. Moving inside, let’s assume the mixing console is also positioned in the northeast corner, and is plugged into a circuit fed from the interior breaker panel only 10 feet away.

Let’s move along to the stage, which hosts an electric keyboard. The keyboard has high impedance 1/4-inch outputs (like a guitar cable), and one of them is plugged into a 1/4-inch wall plate that a well-intentioned (but ill-informed) individual installed so the keyboard signal could be routed to the mixing console. After all, our snake or floor pockets only have XLR inputs for microphones, right?

When we fire up this system, there's a very audible hum. Evaluate the hum on a piano and you’ll find that it is just below a B flat in pitch. The U.S. uses 60-cycle electrical power, and this 60-cycle hum being generated by the system is called a ground loop. (By the way, B flat is about 63 cycles.)

Here is what is happening. The mixer is plugged into a circuit that is about 10 feet from the Ufer ground, while the keyboard on stage is plugged into a circuit that is about 125 feet from the Ufer ground.

Because these two devices are connected together, there is a difference in ground potential in the circuit due to the unequal lengths of distance to the ground. This difference in ground potential is equalized through the cable that carries the audio signal from the keyboard to the mixer, which produces our audible hum.

It's something that happens about 95 percent of the time when connecting audio devices on distant circuits with high impedance unbalanced cables. Sometimes we get away with it, but rarely.

The proper way to make this connection is quite simple, and works for keyboards, computers, boom boxes or any other AC powered device: take the high impedance audio output from the device (in our example, the keyboard) and plug it into a direct interface box (also known as DI box, DI, and direct box). They are readily available at most music/audio stores, and not at all expensive.

The DI converts the high impedance signal to a balanced low impedance signal via the use of a transformer, and the output of the DI uses a simple XLR (microphone) cable of any length to plug into a snake or floor pocket.

Because the low impedance signal is balanced, not only is the ground loop eliminated, but there's also the benefit of running the signal the entire length of the building without any appreciable loss. General industry recommendations point out that running high impedance signals further than 18.5 feet will cause signal loss and also produce an increase in noise.

The second most common cause of hum is improper placement of audio cables. All AC electrical power cords, including extension cords, are surrounded by a magnetic field created by the current within the cord. Lay a microphone or guitar cable at 90 degrees to a power cord and cross over it, and there probably won’t be a problem, but lay a mic/guitar cable immediately parallel (and adjacent) to a power cord and the result is usually inductive hum. The magnetic field of the power current is impacting the signal in the audio cable.

Therefore, check to see that audio cables running from the stage to the mix position are not tied to the AC conduit in the building (a very common problem), or that they are not simply laying parallel and too close to AC power cords. Another very common culprit is the rat’s nest sometimes found behind the rack at the mix position - audio cables should be neatly routed away from power cords, usually tied on opposite sides of the rack.

One last thing about ground hums, and it’s really crazy. I learned it from an elderly US Army field radio man in Arizona 15 years ago. I had installed a system in a church, and it had worked well for about 3 years. One day, however, someone from the church called, explaining that there was now a hum in the system.

I talked him through a litany of changes that might have taken place that would introduce a hum into the system, but we came up blank. So I went to the church and spent several hours troubleshooting everything I could possibly think of.

After several hours of frustration, the elderly man walks into the sanctuary and asks what I’m doing. I told him of my search for the source of the hum and how I couldn't find the cause.

His comment? “We’ve been in a drought for the last two years. The Ufer ground is too dry. Pour some water on it.” So we took a bucket of water outside, dumped it on the Ufer ground, and the hum instantly disappeared.

The soil moisture level had dropped so low that the ground rod was no longer able to maintain its “earthed” status. The introduction of moisture reestablished the ground connection.

The gentleman explained that the field radio guys were taught to urinate on the ground rod if the radio wasn’t operating well. Although quick, simple and cost-effective, I don’t necessarily promote this method with your clients.

This topic usually creates quite a few questions, so please feel free to contact me and I’ll be happy to continue.

Have a great day!