ELI5 : Why do guitar amplifiers give off bad feedback? guitars in giant rock concerts don’t normally do this, how they can be so loud without feedback?


ELI5 : Why do guitar amplifiers give off bad feedback? guitars in giant rock concerts don’t normally do this, how they can be so loud without feedback?

In: Technology

The guitar amps/setup at rock concerts are different to your typical bedroom 30watt fender amp.

Firstly, at most large concerts, amps are ethier mic’d up on stage, or off stage in an Isolation Cabinet. The musicians will probably have ‘in ear monitors’ (head phones essentially) for listening to all the audio they need to hear. This essentially means that its much harder for a feedback loop to occur as any amp noise isn’t being directed to the guitar.

Secondly, a lot of feedback is caused by noise, a few things can remedy this.

1) A noise suppressor – This acts as a gate where low-level noise such as power-hum isn’t let through to the amp, as soon as anything louder is detected (for example playing the guitar) the gate is opened and sound is let through to the amp. This makes feedback harder to occur (in simple circumstances)

2) Power conditioners – These help prevent ‘noise’. Which i think is commonly referred to as ‘ground hum’ ?

Any large concert will have (high end) noise gates and power conditioners to help prevent unwanted noise/feedback.

Note : A lot of musicians will have their ‘rig’ off stage, however they’ll have an amp or speaker on-stage so they can intentionally trigger feedback by walking up to said amp/speaker and dry-humping it with the guitar. This is a very intentional setup which prevents feedback in 99% of the regular performance.

Feedback is caused by an amplified signal finding a sympathetic resonance with the guitar producing that signal, getting louder and louder until it no longer resembles the original tone. It happens most commonly when the amp is absurdly loud for 2 reasons: 1) a cranked amp applies a LOT of gain to the signal. 2) A loud amp is more likely to vibrate the strings or internal components of the guitar’s pickups.

That said, in most concerts it’s not actually that loud on the stage. Even playing small clubs as a rock band you’ll more often see amplifiers mic’d up and put through the house PA and not cranked to fill the room. The PA speakers being in front of the stage and pointed away from the band so the majority of their sound goes towards the audience can can’t cause feedback. Stage volume is actually downright reasonable in most cases unless the sound system isn’t operated correctly or undersized. And even then, if you turn your back to the amplifier a lot less sound gets to your guitar because your body is in the way, which is why when you do see guitarists try to create feedback on purpose, they’ll turn around and face their guitar towards the amp.

And if you think it’s disappointing to go on stage with a half stack and not really turn it too far past like…3. In your stereotypical “giant rock concert”, most of those amps [are fake](http://www.metalsucks.net/wp-content/uploads/2013/07/Immortal-fake-cabs-604×453.jpg).

You might have one cabinet mic’d up or in more modern cases…they’ll use a direct output from the their amplifier “head” and run it directly through the house PA with no actual guitar amp on stage. Think…plugging your phone into the car’s AUX input or using bluetooth connection to the onboard stereo system.

I don’t know how they do it, but I work in the software industry and we have a technique to remove feedback loops: you cancel it.

This is done using adaptive feedback cancellation.

Basically, the algorithm learn the correlation between the sounds it emit and the sounds it receives back. It depends on the frequencies, the shape of the room, the distance and orientation if the speakers and microphones. Once this is learned, during the concert it can predict the feedback it will receive and subtract it from the sounds received on the mike.

The adaptive part is because you probably want to continuously adapt to possible change in feedback response because your singer moved the mike, or the sound comes louder than during testing, etc…