Study of modes in an Electric Guitar<br />Introduction<br />Guitar pickups allow musicians to switch between different sounds. This means the guitarist can control the timbre of their instrument; it is the position of the pickups which affect the timbre. They are placed along the string to be more sensitive to certain frequencies. By studying the vibration of a guitar string, this can be demonstrated. <br />Practical work <br />For our practical we first zeroed the desk to make sure dangerously loud volumes did not apply. We set up the analyser in logic which would allow us to observe the harmonic content of an audio signal. At first we tried three different sine waves so we could see a pure waveform that consists of one frequency.<br />As we can see there is no harmonic content, the signal is a pure waveform.<br />It is the harmonic content that gives an instrument its own unique sound which is known as the timbre. The timbre of an instrument allows us to tell the difference between a guitar and a flute, for example. Subtle frequencies produced by the instrument that are multiples above the fundamental frequency. Different instruments create different harmonics that make their own unique timbre, which allows us to distinguish the instruments apart.<br />The guitar we used for the experiment was a Stratocaster model which consists of three single coil pickups. The switch allows us to change between these pickups which will change the timbre of the instrument.<br />http://upload.wikimedia.org/wikipedia/commons/3/31/Stratocaster_pickups.jpg<br /> <br />When a guitar string is plucked the string vibrates along the string both directions. When it encounters an obstacle, such as the bridge or nut, it will reflect back. Vibrations build up at certain frequencies which are known as normal modes. These normal modes create the fundamental and harmonics. <br />http://img.ultimate-guitar.com/_img/columns/zeg/3-4-1.jpg<br />The diagram shows the first point of vibration which is the fundamental, the second point is the second harmonic, the third point is the third harmonic etc.<br />An antinode is the point of maximum amplitude and a node is the point of no vibration. If the pickup lies close to an antinode it will be strong in the picked up single therefore if the pickup is close to a node then the signal will be weak.<br />http://www.igdb.co.uk/pages/techniques/images/harmonic_content.gif<br />Results<br />For our experiment we played a D string on the guitar using each pickup. The analyser in Logic would then plot the fundamental and harmonics of the signal.<br />1st pickup<br />5thFundamental2nd harmonic<br />Here we can see the fundamental frequency and each harmonic. The second harmonic in the spectra has a very strong signal and would suggest that this harmonic lies near to an antinode. The fifth harmonic has hardly any signal at all; this would suggest this lies very close to a node.<br />2nd pickup<br />6th<br />The harmonic that stands out from the spectra is the sixth harmonic. This has a very small signal so it would suggest that this harmonic lies on a node.<br />3rd pickup<br />Most of the harmonics are fairly strong in the spectra so this would suggest the third pickup does not lie on any nodes.<br />To interpret our results we drew out the neck and pickups of the guitar. This then allowed us to draw the string vibration by drawing the fundamental note, then the 2nd harmonic, then the 3rd harmonic etc.<br /> <br />By using this sheet I can see if the spectra match the results from the pickup positions.<br />After comparing results from the first pickup I can see that my prediction was correct. The 2nd harmonic did lie close to an anti note giving it a strong signal. The 5th harmonic did lie close to a node. This would explain why that harmonic had a low signal. The third pickup was placed mainly near to anti nodes which was why none of the harmonics had a weak signal.<br />Conclusion<br />I found out pick up placement is important when designing a guitar. It is important to avoid nodes if you want the harmonic to be picked up clearly. After further research I found out that the bridge pick up is angled for a reason. This is so the higher strings can pick up more treble and the lower strings can pick up more bass.<br />Conducting this experiment showed me how harmonics in a signal make the timbre and characteristics of the sound. This helped me determine why musical instruments sound different to each other even if they are playing the same note.<br />References<br />Strat collector: How Guitar Pickups Work [Online]Available at: http://www.stratcollector.com/pups.html<br />[Accessed 1st December 2009]<br />Wiki answers: Why do different instruments have different frequencies but the same note? [Online]Available at: http://wiki.answers.com/Q/Why_do_different_instruments_have_different_frequencies_but_the_same_note<br />[Accessed 1st December 2009]<br />Connexions: Timbre: The Color of Music [Online]Available at: http://cnx.org/content/m11059/latest/<br />[Accessed 2nd December 2009]<br />Atlantic quality design: How do Guitar Pickups Work? [Online]Available at: http://www.aqdi.com/pickups.htm<br />[Accessed 2nd December 2009]<br />Till: Response Effects of Guitar Pickup Position and Width [Online]Available at: http://www.till.com/articles/PickupResponse/index.html[Accessed 2nd December 2009]<br />Ultimate Guitar: Part III - Chapter 4 "
Technique - Harmonics"
[Online]Available at: http://www.ultimate-guitar.com/columns/the_guide_to/the_ultimate_guide_to_guitar_chapter_iii_4_technique_-_harmonics.html[Accessed 2nd December 2009]<br />