087 – Electromagnetic Induction
Electromagnetic induction creates current from a changing magnetic field. This can mess with our audio systems.
Written by Scott Adamson
Electromagnetic induction is the basis of how alternating current is generated. It was first discovered in 1831 by an English physicist named Michael Faraday. Even though it was also independently discovered by Joseph Henry shortly after in 1832, Faraday was the first to publish the results of his experiments.
The term magnetic field describes how magnetic force is dispersed in the area surrounding a magnetic object. You probably know that a magnet has two poles and, depending how they are situated, two magnets can either attract or repel each other. The magnetic field is a vector quantity that describes the area where this force exists by measuring the strength and direction.
When an alternating current runs through a conductor (an object that allows the flow of current, such as the copper wire of a cable), it creates corresponding magnetic fields. Conversely, fluctuating magnetic fields can generate an alternating current within a conductor. These fields directly correlate to the strength of the current. A stronger current means a stronger magnetic field. But these magnetic fields can actually induce an alternating current in another conductor!
If a magnetic field is fluctuating back and forth at the standard US AC frequency of 60Hz, and then we run another cable through those magnetic fields, as long as it’s part of a closed circuit, AC is induced into that copper wire!
Here’s a simplified example from TPA’s Show Power course. When we spin a magnet and have three coils placed 120° apart from each other, the fluctuating field from the spinning magnet induces alternating current into the coils, which correspond to the hot legs for our three-phase power.
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In live music, we run a lot of alternating current all throughout the venue — to lights, sound, video displays, and motors for rigging. This means that each of these cables are producing small magnetic fields. When it comes to something like a 5-conductor power cable, which carries a lot of current, this can cause an issue!
If audio and power cables are run alongside each other, alternating current can be induced into the audio cable. We experience this as an unwanted hum. For this reason, it’s best to keep power and audio signals separate.
