# Electromagnet produces sound from hollow pipe

I have wrapped around a 80cm metal pipe some copper wire and attempted to produce an electromagnet. The electromagnet works nonetheless from the extremities of the metal pipe a sound is being produced which may alter with the frequency of my current. The frequencies are too high to measure with my instruments (kHz scale) but I have made some measurement which show it is not the same frequency as that of the current. What is happening ???

• These can be sound waves in a metal pipe. If there is air in the pipe, then sound waves are excited in the air column. Base frequency for air in the pipe ~kHz – Alex Trounev May 8 at 15:55
• What is the frequency of your current and how much higher is the sound frequency (eg., twice or 100x)? – Keith McClary May 9 at 4:49

Congrataulations! You have invented a new kind of musical instrument.

Seriously, any pipe has a natural resonant frequency, and the sound you are hearing will be at that frequency, or one of its harmonics. It will be loudest when the natural frequency matches with the frequency of your electrical source.

Compare with a trumpet or trombone, where the sound coming out is based on the raspberry noise that is made into the mouthpiece, but which is 'tuned' by the piping of the instrument. A simple change to the geometry of any brass instrument creates a change in pitch, without needing a change by the player.

• Thank you very much for the reply nevertheless I was more wondering as to why it produced a sound in the first place, what is vibrating and how is it vibrating, is there an equation to relate the two frequencies together ? – Mathematician Pure May 8 at 18:12
• @MathematicianPure If you are interested in equations, then you need to describe the installation in more detail. – Alex Trounev May 9 at 0:30
• If you want equations, then you probably need to look into the Fourier equations, but as I said, the frequencies you hear are purely down to harmonics, so will be fairly simple multiples of the the input frequency, depending a lot on how 'pure' that frequency is. The source of movement is mainly the interaction between the coil and the core, but eddy currents in the copper itself may come in to play too. – Mike Brockington May 9 at 9:16

The real question is why the pipe's resonant frequency is being excited by the electromagnet in the first place.

The adjacent loops of wire in an electromagnet exert an attractive force upon one another when current is flowing in the coil. If the coil is loosely-wound and not locked in place with an adhesive, the wraps of wire will vibrate very slightly at twice the frequency of the signal they are carrying as they attract one another and then stop attracting one another and attract, etc.

In essence, your system is a very low-efficiency loudspeaker, which makes the tube vibrate very slightly. when the tube's resonant frequency or a harmonic thereof is close to the driving frequency of the current in the coil, the acoustic output is magnified and you can hear that sound coming out of the ends of the tube.

• That's one possibility. Another is magnetostriction, which is the usual explanation for why mains transformers hum. – Solomon Slow May 8 at 17:36