Brian Carr

Guitar Strings and Sound Waves

IntroductionFor quite some time, I have found sound waves very

interesting. Maybe this is due to my great love and appreciation of music. Whatever the reason, imagine my level of interest when I discovered that, when observed in the right conditions, a guitar string provides a visual representation of a sound wave. This is fascinating to me on multiple levels. For one, sound waves are not visible. Secondly, the guitar is my favorite instrument. To be able to visualize a normally invisible phenomenon through something that already holds a high level of interest for me made me want to pursue a further understanding of the subject.

Sound Waves

If you look across a surface of water and drop in a pebble, the resulting waves resemble a basic sound wave. Although there are fundamental differences between the types of waves, the water comparison can help us to visualize sound waves.

VocabularyThe wavelength is the distance of one individual wave

cycle, or between two identical points between consecutive waves.

Amplitude is the height of the wave from bottom to top (or trough to crest). A sound wave’s amplitude is heard in the loudness of the sound.

Frequency is the number of individual waves that occur per second, and is measured in a unit called Hertz. Frequency is related to the pitch of a sound, or how high or low the sound is. For example, a string bass has a lower pitch than a flute, so its frequency, or number of waves per second (hertz) is lower than that of the flute.

An Invisible PhenomenonSound waves travel through the air. While doing

this, they can move around objects in their path, reflect off of objects, or are absorbed by some things, but none of this can be seen. So how do we know about sound waves?

There are ways to visualize sound waves. An oscilloscope is a machine that transfers a sound into a visible wave (see photo on next slide).

Sound waves can also be made visible through experiments. This video, from the Discovery Channel series Time Warp, is really cool and contains a great explanation of what is happening.

Sound waves from a flute, clarinet, oboe and saxophone playing the same note.

QuestionsWhat are the nodes that are mentioned on

the video?

Why do the waves on the previous slide look so different than the wave on slide five, or a water wave?

What does all of this have to do with guitar strings, anyway?

NodesIn the Discovery Channel video, Jeff Lieberman mentions nodes. They are the points of the wave that are not part of a crest or a trough – they aren’t up or down, but are essentially points along the wave that have an amplitude of zero. Think of them as the points where the wave crosses the red line in the diagram on slide five. Although there is no amplitude at these points, the sound waves are moving so rapidly that we cannot hear breaks in the sound.

HarmonicsThe sound waves in the oscilloscope photos look different for a reason. To produce a basic, even wave such as the one from the fifth slide (where we studied the parts of a wave) requires a mechanically produced sound of only one frequency. The sounds we hear every day, however, are more complicated. They consist of a fundamental frequency (one that would look like those even, symmetrical waves) plus other frequencies called harmonics. The harmonics are multiples of the fundamental. The combination of the fundamental and the harmonics form waves like those we viewed from the oscilloscope.

Harmonics, cont.The combination of fundamental frequencies and harmonics occur in almost all sounds, but the exact combination varies from sound to sound. The variation in sound wave combinations is the reason that various objects sound different from each other. The oscilloscope readings are of the same fundamental frequency, but the strength of the various harmonics varies in the different instruments, resulting in a different sound and waves that differ in appearance.

Guitar StringsNow to my favorite part! In my research, I found that many people use a guitar string to illustrate the concept of a sound wave. Think of a plucked string like a wave with node points at both ends of the string. If the string is pressed down on the guitar neck at a certain point, the wave runs from the pressed point to the bridge, creating a shorter wave and a higher pitch. But as we learned, this long wave is not the only wave happening – there are also the harmonics to consider. Check out the diagram on the next slide.

On the vibrating string, all of the above harmonics are happening at the same time. Now check out this video of an actual vibrating guitar string.

Guitar Strings, cont.Pretty cool, eh? This exploration shows the science behind a guitar technique called, appropriately enough, playing harmonics. If a vibrating string is touched at a node point, or the finger is placed lightly over a node point and the string is plucked, a higher, chimey-sounding note is sounded. The effect is tremendous, both on electric and acoustic guitars.

ConclusionThis is just a short overview of sound waves. The topic can be pursued to a greater extent in the classroom. For example: sound waves continue into the concept of resonance, in which one vibrating object can cause another object to vibrate if they have similar fundamental frequencies. I’m not sure what grade level introduces sound waves, though – it might be beyond elementary school. Of course, I could expand upon the guitar aspect to a much greater degree, but such detail is probably not called for in a general classroom. It is a fascinating topic, though!

Resources Discovery Channel.com (n.d.). Retrieved December 5, 2009 from

http://dsc.discovery.com/videos/time-warp-good-vibrations.html Hewitt, P., Suchocki, J. & Hewitt, L. (2008). Conceptual Physical

Science. San Francisco: Pearson Addison-Wesley. Morgan, S. & A. (1994). Designs in Science – Using Sound. New

York: Facts on File, Inc. Nodes – Wikipedia, the Free Encyclopedia (n.d.). Retrieved

December 8, 2009 from http://en.wikipedia.org/wiki/Node_(physics) Runde, C. Advanced Digital Pathways Audio 1 (n.d.). Retrieved

December 5, 2009 from http://adpaudio1.blogspot.com/2008/04/sound-fundamentals-part-2.html

Thomas, R. (n.d.). Plus Magazine (website). Retrieved December 5, 2009 from http://plus.maths.org/latestnews/sep-dec08/stellarchoir/

YouTube: Broadcast Yourself (2009). Retrieved December 8, 2009 from http://www.youtube.com/watch?v=Qxqt_RGrpPQ