Weird harmonic thing on my banjo...

[5stringDNA]

I have no idea what iscausing it, but I noticed that every once and awhile my banjo would ring really loud when I was playing my bass. It started driving me nuts and I realized that it only occured whenever I played an E at any octave. The banjo sits on a stand btw. I just thought that was really odd. I didn't noticed until a few days ago, so I'm curious as to what is happening.

[Munjibunga]

It's called constructive feedback. The banjo's E string is ringing in sympathetic resonance with the sound waves you're introducing into its environment.

[geoffkhan]

Yeah, try going to a piano and holding down the sustain pedal and then playing an instrument. Instant reverb!

[5stringDNA]

I figured it was something like that, but the funny thing is that Banjo's don't have an E string. It's tuned G-C-G-B-D

[wulf]

Sympathetic resonance. One or more of the pitches produced by the bass cause the banjo to resonate. It doesn't necessarily have to be be the same as one of the banjo strings and it also doesn't necessarily have to be the fundamental pitch of the note you're playing (the low E is about 41Hz but also produces overtones at roughly 82Hz, 123Hz, 164Hz, etc).

You can observe the same effect on a snare drum, especially if the snare isn't damped and even if you're playing your bass through an amp that can't quite cope with it (whereas I believe bass amps normally have a resonant frequency below anything you're going to produce).

Sympathetic resonance can be used musically, eg, the extra strings on a sitar. However, if you don't want it, you'll probably be able to apply a bit of damping to the banjo (eg. a small towel pushed gently between the strings and the head).

Wulf

[Flanders]

I used to live in a house where the very walls would move every time I played an F#. It seemed like everything in the house was attuned to that frequency. Weird.

[5stringDNA]

Yah- I've noticed teh effect on snare drums before, and a banjo is essentially a snare with strings. Snares seem to be set off by a larger variety of frequencies though.

[Brandt Scanlan]

The second harmonic of E is B (right after the octave).