An electromagnet surrounds itself with a magnetic field when connected with a current. The strength of that magnetic field changes according to the strength of the current, and the polarity of the field is determined by the direction of the current’s flow. In other words an electromagnet can either pull or push on a permanent magnet, depending on the polarity of its magnetic field, and it can do so strongly, weakly, or anywhere in between. Because of this, when an electromagnet is placed near a magnetic object capable of vibrating, and the current is generated in such a way as to represent sound, the magnetic object will vibrate and we (hopefully) hear sound!
This is can be seen as a graphical representation of air pressure. It is pressure waves such as this, traveling through the air, that we experience as sound. In order for your computer (or your stereo) to reproduce these sounds with the help of a speaker, it generates an electrical current based on the waveform. A speaker connected to the computer audio output via speaker wire receives this current. Inside the speaker is an electromagnet, and surrounding the electromagnet is a ring-shaped permanent magnet attached to a speaker cone. The current controls the strength and polarity of the electromagnet, which pushes and pulls on the surrounding permanent magnet. As a result, the speaker cone moves in and out – vibration. These vibrations create the pressure waves in the air that we experience as sound. Here is a nice image of the inside of a speaker.
Of course we can remove the electromagnet from the speaker cone system and place it over something entirely different, such as a piano string. In this case, the magnetic field generated by the electromagnet fluctuates just as it did before, but now it is interacting with a piano string, which will vibrate just as the speaker cone did (almost). If on my computer I generate a sine tone at a frequency of 440 (A4 – above middle C) and send it to an electromagnet positioned above piano strings tuned to A4, those strings will start to vibrate and you will hear… A4!
There are two problems introduced when using a piano string like this instead of a speaker cone:
Problem 1 – A speaker cone is attached to a permanent magnet (which is, obviously, magnetic), whereas piano strings are made out of steel (which responds to a magnetic field, but is not itself magnetic).
Problem 2 – A speaker cone is free to vibrate, whereas a piano string is not only fixed at each end but under a huge amount of tension, and very rigid.
The use of permanent magnets on the EMPP (solution to Problem 1):
An electromagnet can pull AND PUSH on a permanent magnet – just hold two magnets together in different ways to see how polarity affects the interaction of two magnetic bodies. Piano strings, on the other hand, are steel. A magnet, permanent or electrical, can only pull on metal such as steel, not push. However if a permanent magnet is held next to the electromagnet, it serves to pull the metal string toward it just a bit. It turns out that that slight offset of the string’s “at rest” position is enough to compensate for the fact that it can’t be pushed by an electromagnet. For the EMPP, permanent magnets are glued to either side of the electromagnet in a specific configuration. This allows the piano string to respond much more precisely to the electromagnet, increasing it’s total displacement (which makes it louder) and allowing it to directly reproduce the frequency of the signal sent to the electromagnet.
Selection of specific frequencies (solution to Problem 2):
Because piano strings are fixed at each end, under tension, and rigid, they have a specific pitch and color, unlike a speaker cone. This is what makes a piano sound like a piano, and what the EMPP is all about. The EMPP is not good at accurately reproducing any sound used as a source signal, as a speaker does, but it is great at sounding uniquely like the EMPP – something of a hybrid between a piano and synthesizer. That said, piano strings have a surprisingly wide range of flexibility when it comes to making sound. They can vibrate at their own fundamental frequency, but they can also be forced to vibrate at any of their first 20 or so partials quite nicely. If I send an E6 (an octave and a 5th higher than the A4 mentioned above), that same A4 string will vibrate at the frequency of E6 – its third partial. Finally, if I play a complex audio file, such as the sound of someone speaking, through all 12 electromagnets of the EMPP, the result is surprisingly rich. The piano strings under the 12 electromagnets essentially respond to any frequency to which they are capable of responding among the complex mass of frequencies produced by the human voice. The result sounds somewhat like speech and somewhat like a piano. See the Audio Examples page, under Media, to hear this.