It’s a beautiful Thursday afternoon and you’re practicing for the upcoming weekend gig with the band. Everything seems to be going perfectly — you had the gear set up just right, and the sound couldn’t be better. But then suddenly comes a SHRIEEEEEEK!!!!! A violent, ear-piecing squeal that has your dog running for cover. Within a few minutes of sleuthing, your ax is singled out as the culprit.
Of all forms of undesired feedback you might experience as a guitarist, nothing quite compares to microphony. Also known as microphonic feedback, this happens when your guitar’s pickup system goes out of control and cooks up a sudden high-pitched shriek. You’re left with no option but to fold up the performance and try rectifying the problem.
Why Do Guitar Pickups Go Microphonic?
Like the name indicates, microphony happens when the pickups on your guitar start behaving like microphones. They pick up unwanted sound from outside and mash it up with the signal coming from your guitar. It’s this concoction that turns into a squealing screech as it travels down the signal chain and into your speakers.
So, why exactly do guitar pickups go microphonic? We’ll find out in a bit, but let’s first take a quick look at how pickups work. This way, you’ll be in a position to understand what goes on when you experience microphonic feedback.
Anatomy of a Guitar Pickup
A guitar pickup is a transducer at its core; that is, a device that converts energy from one form to another. It transforms the vibrational energy produced by guitar strings into sound that can be amplified, recorded and/or transmitted. In that sense, a pickup can be categorized as a sonic transducer. This family includes all devices that convert sound into electric current and vice-versa; microphones, speakers, etc.
All sonic transducers are electro-magnetic in operation, leveraging Faraday’s Law of Induction. This states that an electric conductor exposed to a changing magnetic field will have a current induced in it. In other words, a wire wrapped around a magnet will have an electric current flowing ‘inside’ it if there’s a disturbance nearby. A disturbance like the one created by the movement of a guitar string when you pluck it.
Of course, not every magnet-and-wire combo can capture this disturbance properly and turn it into the twangy sound you hear through the speakers. Guitar pickups use magnets made of alnico/ferrite. Both materials produce strong magnetic fields and retain their potency even when exposed to harsh conditions. The permanent magnet will then have a finely enameled copper wire wound around it in turns numbering in thousands.
It’s this coil that picks up changes in the magnetic field that occur when you strum the strings. The current registered as a result is then transmitted to the amp, where it’s boosted and sent down the chain. For a good measure, the strings are made of a ferromagnetic material (i.e. one that becomes magnetized when exposed to another magnet) like steel. The result is rather obvious — you have an additional magnetic field inducing a current in the coil when the strings vibrate.
Microphonic Feedback: When Pickups go Rogue
You’ll recall that guitar pickups are classed as sonic transducers; microphones are yet another member of the family. So you can expect a few similarities in anatomy — some types of mics use a setup similar to that of guitar pickups. You have a diaphragm that moves when subjected to sound waves, with a wire-wound magnet located close by.
There’s also a bit of overlap on the functional side. Point a powered mic to your ax as you play it, and it’ll definitely pick up some sound that you can then amplify. This is something you can take advantage of when playing an acoustic on stage — you’ll want to mount the mic on a stand to free your hands.
How about substituting a mic with a pickup? No, afraid not. While a microphone is designed to ‘hear’ any kind of sound, guitar pickups are tailored to only register the intricate vibrations of guitar strings. You might recall from your physics class that sound needs a medium to travel through (solid, liquid, or gas). Vibrations can travel without this hindrance. And that means a guitar pickup will work even in a vacuum — the same cannot be said of microphones.
But what gives pickups the magical ability to pick up the strings’ subtle movements while shutting out all other vibrations? It’s down to the way they’re built. Unlike microphones, pickups don’t have any components moving as they operate. They’re often dipped in wax to fill up all the tiny spaces between the windings, leaving little (if any) room for movement.
And so, microphonic feedback is usually a sign that your pickups are beginning to lose their original shape. What happens is that the windings become loose enough to vibrate under the influence of external forces. These secondary vibrations disturb the magnetic field, creating a signal that’s sent through to your amp and loudspeaker. This comes in the form of a high-pitched howl because the wire on your pickups is extremely thin, and thus vibrates at a very high frequency. To add salt to injury, the unwanted sound will inevitably cause the coil to vibrate further — you don’t have to be a genius to guess what happens from there.
This also explains why microphonic feedback varies so wildly. When you’re playing at a low-to-moderate volume, the sound waves coming from your speakers aren’t strong enough to initiate secondary vibrations. But crank the volume up, and the feedback becomes a screeching nightmare — one that also has enough energy to sustain itself.
Dealing With Microphony: What’s Your Best Bet?
We’ve just hinted that you can rein in your squealing pickups by starving them off external energy — you know, avoiding high-gain situations, turning the volume down or stepping away from the speakers. But these measures might not always be effective or practical. As such, you’re better off having your pickups repotted with wax. This will be pricey, but it’s the only way to prevent the problem from growing out of hand.