The phenomenon of 'hum loops' is unknown to many musicians, at least by name. Most people only know this diffuse humming in the boxes which is there and then gone again. We often blame humming on poorly made devices, on stubborn pickups or we simply accept it. After all, we play live louder than the humming anyway, right? Nevertheless, the hum loop is a problem when we either want to record material or our songs have quiet parts where the hum is unpleasantly audible. Time for a trace.
Janosch Held is a sound engineer and is regularly on tour with well-known scene stars from rock and pop, most recently with the rock band Eisbrecher. Janosch knows which knobs to turn to get the best sound out of the PA system.
A hum in a live or rehearsal room setup can have many different reasons. Most of the time, the hum is caused when interference and audio signals mix unintentionally.
The sound from guitar and amp is routed via an audio cable to the mixer or computer and from there to the PA system. Your audio signal flows through your cable.
If you experience a hum due to a ground loop, an interference voltage mixes with your audio signal which is then audible in the PA system.
And this is what happens: If the audio source (e.g. a keyboard) has an internal power supply that is connected via a 3-pin plug with earthing contact to a socket that is also 3-pin, both the housing and the signal ground are usually connected to the protective ground. If you now connect this grounded audio source with a cable to a device that is plugged into a different electrical outlet (for example a mixer), it can cause an unpleasant hum. This is because you have connected two sockets and two groundings via one audio cable.
In other words: the mixer is plugged into a electrical outlet. The keyboard is plugged into a different electrical outlet. Both are grounded and both are connected via the audio cable. This can produce a hum that is audible at worst at 50 Hz or 100 Hz.
The reason: the housing and signal grounds feature different potentials electrically. This causes equalizing current to flow via the ground/shield of the instrument cable. However, nothing else should 'flow' through the instrument cable, apart from the audio signal. The equalizing current generates an interference signal that overlaps with the audio signal. Thus a humming occurs.
The hum in your loudspeaker or PA system is therefore virtually an audible current.
That also explains why many bands only get to know this phenomenon on stage. In the rehearsal room, the amps and the mixer are often in the same socket anyway.
How to prevent a ground loop
The good news: It does not cost much effort or material to prevent a ground loop. This is how it goes:
All devices go into one socket. Thus, use multiple sockets and extension cables.
DI boxes also correct hum problems. The signals from guitar amps, bass combos and keyboards are fed from the line out into a DI box. Then it goes from the DI box line out to the mixer. You need a simple but reliable DI box for each audio channel.
- If there is a switch on the device, you can use a ground lift to disconnect the signal ground of your devices from the housing ground and thus, from the protective earth. Bass combos, for example, usually have ground lift buttons.
Safety warning: never, never, never disconnect the grounding of a technical device. This is life threatening.
Another cause of humming on stage, in the rehearsal room and anywhere a lot of current and audio signals flow is electric smog. Every current in a current carrier (conductor) generates a magnetic field. Many people colloquially refer to this magnetic field as 'electric smog'. This electric smog affects audio signals if they are close enough, because it mixes with the transported audio signal and therefore becomes audible.
Electric smog can also indirectly cause humming. The magnetic field can scatter into the capsule, more precisely into the coil of a microphone, and thus create a hum. Example: a dynamic microphone is placed in front of an amplifier. Its power supply unit emits electric smog. This electric smog is converted into audible noise in the microphone and transmitted to the PA system.
The sound pickups of instruments can also be susceptible to electric smog.
Similar to the microphone, the magnetic field acts directly on the coil in the pickup, thus producing the annoying humming noise. If this cannot be prevented by keeping guitars and power sources far enough apart, a noise gate can help. It suppresses quiet signals up to a certain level, including quiet noises from sound sources. Therefore, it must be carefully adjusted. The noise gate 'opens' again when the guitarist strikes the strings. Because then the vibrations are stronger and the gate recognises them as intentionally. Digital mixers always have noise gates integrated into the channels.
Thus, you can prevent electric smog from being audible at the PA system
The influence of electric smog on the audio signal is not easy to prevent, especially on small stages. The only thing that helps is a consistently separated and structured cable routing:
Do not lay power cables parallel next to audio cables, crossings of these cables preferably at a 90° angle
Do not lay audio cables directly past power supplies
Buy high-quality power supplies that are well insulated
The effect is particularly bad with power cables for stroboscopes or dimmers. The sound system and the lighting system must be supplied with separate power anyway but you should also separate them as much as possible.
- Use balanced audio cabling whenever possible
These are the two most common causes and solutions when your setup is humming.
Photos © The Hirsch Effekt