Here’s a lesson on harmonics by justinguitar.com. The Clear Science staff thinks this is pretty good and clear.
If you like heavy metal (and who doesn’t?), here’s Dimebag Darrell giving a physics lesson on harmonics.
We talked about frets on a guitar and how when you push the string into the fret board it raises the pitch on the note by changing the length of the standing wave on the string. Another interesting thing you can do is this: after plucking the open string, just lightly touch the string with your finger at the 12th fret. A sound like a bell or flute will result, as you create the second harmonic of the original standing wave.
Here’s what happens: by touching the string lightly while it’s vibrating, you cause it to come to rest at that point you’re touching. This results in another node, as shown above with the second harmonic. Touching the string at the 7th or 5th fret can make the third and fourth harmonics, with higher numbers of nodes.
But if you touch the string someplace that does not result in equal segments between nodes, no harmonic is produced and the string stops. This is simply math. Guitar players can tell you all the spots where harmonics can be produced, even if they don’t care too much about the math and physics behind it. (Some of them care though!)
Since we were talking about guitars and how their pickups work, the Clear Science staff wanted to point out that all kinds of physics can be illustrated with a guitar. Take for example the vibration of the strings, which produces the notes. The strings are held stationary two places: at the nut, shown by the blue line, and at the bridge, shown by the grey line.
When you pluck a string, you set up a standing wave, with stationary nodes at the nut and bridge. When you fret the string (i.e. push it down with your finger), you change the distance between these nodes, and it changes the note.
The 12th fret is exactly halfway along the string, and when fretted there will produce a note one octave higher than the unfretted string. (For example, the top string is generally an E, and the note at the 12th fret is a higher E.) The 7th fret is 2/3 of the way along the string, and the 5th fret is 3/4. These special frets are usually marked with pearling or with some dots.
Thanks, awpoops! Of course you mean when we discussed how guitar pickups work by electromagnetic induction. The Clear Science staff likes guitars, so we’re always willing to talk more about them.
(By the way, we know that’s not a real guitar on your site, awpoops.)
We talked about guitar pickups and how they work. It has to do with electromagnetic induction, which says that when there’s a change in the magnetic flux through a coil of wire a current will be induced in the wire.
The little disks under an electric guitar’s strings are magnets. Each pickup has a magnet under each of the strings. (Just for the record, there are different kinds of pickups that don’t look like this, but what we’re describing is the most common kind.) Inside the pickup, there’s a wire coil around the magnets. This wire coil is connected (after going through the tone and volume knobs) to the output socket, which you connect to the amp.
Since the strings are steel, each string becomes magnetized due to the permanent magnet under it. When you pluck the string it oscillates from its original position. Since it’s magnetized, the magnetic field through the wire coil involves both the string and the magnet, and so the string’s oscillation changes the magnetic field inside the coil. (Up above we’ve represented the magnetic field with a blue shape, rather than drawing field lines.) These changes in the magnetic flux cause electrical current to go back and forth in the wire. This signal ends up at the amp and speaker, where it becomes sound waves.
Ever wondered why you don’t have to plug in an electric guitar? It makes its own electricity.
Pickups are the strips you see under the strings on an electric guitar. They turn the physical vibration of the strings into an electrical signal. On an acoustic guitar, you hear the sound waves created by the strings’ vibration. On an electric guitar, you hear the electrical signal after it’s been run through an amplifier and comes out of a speaker. (Although the Clear Science art director couldn’t have an amp when he was a kid, so would put his ear on the horn of the guitar and hear the sound waves traveling through the solid guitar body.)
Guitar pickups work by electromagnetic induction, so let’s talk about what that is. If you have a coil of wire (a copper coil is illustrated above), a magnet passing through the coil will induce an electromotive force (EMF) in the coil. This EMF will cause an electric current to flow. (Incidentally, this is also how electric generators work.)
This concept is called Faraday’s law of induction, discovered by Michael Faraday. The EMF is equal to the time rate of change of the magnetic flux through the coil. Magnetic flux means the amount of magnetic field per area per time. Said plainer, this means changes in the magnetic field will cause some kind of current.
