Course:PHYS341/Archive/2016wTerm2/AcousticsoftheGuitar

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Acoustics of the Guitar


The acoustic guitar is a hollow-bodied six-stringed musical instrument in which sound is transmitted from the strings to the body and then to the surrounding environment. The sound waves from the strings of the instrument transfer energy through the bridge then to the guitar’s body. Upon contact with the body, the vibrations result in an internal resonance made by the body and soundboard.[1] The soundboard is the part of the instrument that radiates sound as the stings control the pitch of the sound. The way in which the acoustic guitar radiates sound resembles a Helmholtz Resonator in which the air oscillates from the cavity to the surrounding environment, carrying the sound waves and projecting the energy out. The sound waves of the acoustic guitar are the result of the radiation from the vibrating body and not the vibration of the stings or vibration of the body.



Fig.1 example of an acoustic guitar




Structure of the Guitar

Fig.2 Acoustic guitar-en diagram


The acoustic guitar is composed of the head, neck and body. The head has the tuning pegs, and nut. The neck is composed of the fretboard on which the strings run from the head to body. The body has the pick guard, bridge and the sound hole. Most acoustic guitars have flat backs and nylon or steel strings.

The strings of the guitar control the pitch of the sound produced. Pitch is determined by the mass, tension, length and mode of vibration of the strings.[2] The mass of the string affects the pitch as the more massive the string is, the slower it vibrates resulting in a lower pitch and the lighter or thinner the string is the higher the pitch.[3] For a guitar, the tension of the strings are approximately equal as uneven string tension would lead to distorting torque on the instrument.[4]The length of the individual strings on the guitar are of approximately equal length, it is in playing the instrument that the player can manipulate the vibrating length to change the pitch. Placing fingers on the fretboard shortens the length of the string, resulting in a higher pitch.[5] The mode of vibration of the string is plucked as to produce sound, the musician must pluck them. To increase the loudness of the sound though the strings is to increase the energy input, therefore plucking the string harder. However, by doing so increases the chances of string vibrating and hitting the frets which results in fret buzzing, compromising the sound.

The Soundboard and Body of the Guitar


The main function of the soundboard or top plate is to increase the loudness of the instrument. The strings of the guitar hardly make any noise as they do not create a large air disturbance.[6] The soundboard increases the surface area of the vibrating area. The sound waves transfer from the vibration of the string then to bridge and the saddle to the top plate of the guitar’s body. [7] The vibrations from the strings transfer the energy to the bridge and then to the to the top plate of the guitar which is made of a thin piece of wood which vibrates up and down.[8] Often, the top plate is made of a light springy wood such as spruce and has the thickness of 2.5mm which allows it to vibrate. [9] Inside of the guitar’s body there are a series of braces on the flat plates which strengthen and keep the plate flat. The top plate is much more important to the sound of the guitar than the back plate as it does provide the mechanism for the resonance of sound. The back plate is less acoustically important as it is held against the player’s body. The sides of the body do not vibrate much in the direction of the radiation of sound. [10]

Helmholtz Resonance


Fig. 3 Brass Helmholtz Resonator
Fig. 4 Helmholtz resonator sketch

A Helmholtz Resonator is a container of gas, usually air, with an open hole, [11] such as a large vase, beer bottle or whistle. The air around the open hole vibrates due to the “springiness” of the air inside the container. Depending on the amount of air inside the container, the pitch of the Helmholtz Resonator can change. For example, in a beer bottle when one blows on the top of neck; a higher tone will be perceived if the bottle is filled with more liquid than when it is empty.

The Helmholtz Resonance in a guitar is the result of the air by the sound hole oscillating from the force of the springing of the air inside the body. Air has a spring-like quality such that when the air is compressed; the pressure will increase and the air will move to stabilize the pressure inequality by returning to the original position. Therefore, when the string excites air molecules near the entrance to the sound hole, it compresses the air by the sound hole which push into the sound hole and creates pressure. Then the air inside the cavity pushes out the excited molecules out of the hole. This push and pull results in oscillations which vibrate like a mass on a spring until the air rests at its normal position.[12] The phenomenon occurs at low frequencies and the vibration of the air in the body and outside are vibrating in or out of phase with the strings. This results in the amplification of the soundwave by increasing or damping various harmonic tones. However, the main difference that separates the Helmholtz resonators and guitars is due to the considerable motion both the guitars front and back plates that make the mechanism more complex. [13]While the Helmholtz resonance is applicable to the lowest frequency of the instrument, approximately 100 Hz. [14] The low frequency of the resonator smoothes and extends the guitars bass response.[15] If the frequency was greater, the guitar sound would result in a lack of a bass response and a honky sound.[16]



To summarize, the acoustics of the guitar are determined by several mechanisms of resonance, pitch, sound waves and vibrations which come together to create one instrument. The strings produce the pitch of the sound while the transfer of energy through vibrations cause the soundboard to resonate. The soundboard radiates sound waves more efficiently than the strings, due to the surface area which is large and flat. Resulting in increased system energy transfer efficiency. The body of the guitar acts like a Helmholtz Resonator in which air molecules oscillate back and forth from within the instrument to the surrounding environment.

See Also

Reference

  1. ↑ Billington, Ian. "The Physics of the Acoustic Guitar". Retrieved 16 March 2017. Unknown parameter |Website= ignored (|website= suggested) (help)
  2. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  3. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  4. ↑ "Acoustic Guitar". Retrieved 16 March 2017.
  5. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  6. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  7. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  8. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  9. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  10. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  11. ↑ "Helmholtz Resonators - Basic Analytic Devices." Helmholtz Resonators - Basic Analytic Devices". Retrieved 16 March 2017.
  12. ↑ Wolfe, Joe. ""How does a guitar work?"". The University of New South Wales. Retrieved 16 March 2017.
  13. ↑ Rossing, Thomas D. (2010). The Science of String Instruments. New York: Springer New York. p. 25. ISBN 9781441971104.
  14. ↑ Rossing, Thomas D. (2010). The Science of String Instruments. New York: Springer New York. p. 25. ISBN 9781441971104.
  15. ↑ Relph-Knight, Terry. "The Acoustic Guitar Body – Part 2". N.p. Retrieved 04 April 2017. Check date values in: |access-date= (help)
  16. ↑ Relph-Knight, Terry. "The Acoustic Guitar Body – Part 2". N.p. Retrieved 04 April 2017. Check date values in: |access-date= (help)
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