We all heard that acoustic guitar body acts as the amplifier of the sound created by wire plucking and strumming. This is because an acoustic guitar body is some kind of resonator.

Every resonator amplifies just certain frequencies while it inhibits all others. So why then when an acoustic guitar is played we hear all notes amplified and not just certain ones? Shouldn't there be some notes louder and some other notes quieter?

  • $\begingroup$ Not sure wikipedia agrees with you. en.wikipedia.org/wiki/Sound_box The sound box typically adds resonances at lower frequencies, enhancing the lower-frequency response of the instrument. $\endgroup$ – user171879 Oct 28 '17 at 14:41
  • $\begingroup$ This article and the second part might have some of the detail you require. acousticmasters.com/AcousticMasters_GuitarBody1.htm Some sound bodies do amplify the some of the harmonics of certain notes more than others and this produces what is called a wolf tone en.m.wikipedia.org/wiki/Wolf_tone $\endgroup$ – Farcher Oct 28 '17 at 15:09
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    $\begingroup$ The body of an acoustic instrument does not amplify the sound. That is to say, it does not add any energy to the energy that the user inputs by plucking the strings. The difference between an acoustic guitar and an un-plugged, solid-body electric guitar is that the acoustic instrument does a much better job of coupling the vibrational energy of the strings to the air. (see also, en.wikipedia.org/wiki/Impedance_matching#Acoustics). $\endgroup$ – Solomon Slow Oct 28 '17 at 18:02
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    $\begingroup$ This post will help you. Acoustic guitars do not amplify and the fact that the body is a resonator isn't necessarily why the body helps make the sound louder. $\endgroup$ – DanielSank Oct 29 '17 at 0:30
  • $\begingroup$ 'Every resonator '. No. Only tuned resonators do that. A guitar body is an untuned resonator, or perhaps a resonator with multiple resonant frequencies. @Farcher The Wikipeaida you cite is self-contradictory. Either the frequency is the same or it isn't. $\endgroup$ – user207421 Oct 29 '17 at 9:18

Every resonator amplifies just certain frequencies while it inhibits all others.

This is true only for very simple resonators. The shape of the guitar body is such that it has a different size at different angles. This corresponds to different resonant frequencies. In addition, the top has a supporting bracing that is very different on different models and is critical for the sound.

Furthermore, a guitar is nor necessarily a resonator, but a converter of the mechanical energy of the strings to the acoustic energy of the air. You are absolutely correct that a strong resonance at a certain frequency would help this frequency at the expense of other frequencies and thus would be detrimental for the sound of a broad spectrum instrument. Therefore the objective of the guitar design (counter intuitively at first) is not to create, but to avoid strong resonances.

There are two parts in a guitar relevant here. One is the top made either of softer ceder for a warmer sound or harder spruce for a brighter sound (sometimes a two-layer top has both). The purpose of the top is to move air with its large area thus converting the mechanical energy of the strings to the acoustical energy of the sound. However, the top radiates both forward and back, plus the sound radiated back is out of phase with the sound radiated forward.

The second important part is the body. Historically the most popular material for the body had been the Brazilian rosewood before Brazil prohibited exporting it. Similar results are achieved with the rosewood from India, Madagascar, and other areas. Mahogany is also popular along with other wood species.

The main purpose of the body is to reflect forward (through the sound hole) the sound emitted back by the top and do it while inverting the phase, so that the sound emitted by the sound hole would be in phase with the sound emitted forward by the desk. Thus the guitar body is not a resonator, but a phase inverter very similar to common speakers with a PORT. Phase inversion depends on the internal dimensions of the body and is a compromise in order to optimize all frequencies equally. Although this is more important for lower frequencies (again, similarly to speakers).

The materials, shape, and bracing have been perfected by generations of guitar makers resulting in specific sound signatures. One example is the 1A model with a ceder top by Jose Romirez. Its sound is mesmerizing.

Bass Reflex

  • $\begingroup$ +1 this is Countto10. My favourite violin piece is "lark ascending" by Vaughan Williams (yeah, I'm old :) but this article might interest you re the hype over violins and their value sciencemag.org/news/2017/05/… $\endgroup$ – user171879 Oct 28 '17 at 16:44
  • $\begingroup$ @User171879 What happened to your user name? :) $\endgroup$ – safesphere Oct 28 '17 at 17:54
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    $\begingroup$ The guitarist's and luthier's term for what you call the "deck" here is the top. Many would say that rosewood (Brazilian or otherwise), while being the most popular body wood, is not objectively the best. It lends a classic, balanced sound, especially to the most popular body sizes and shapes. Jumbo guitars and guitarists who would like more clarity may prefer maple, and that's just one example. There are many effective tonewoods and guitar manufacturers of all sizes offer selections of different body styles and tonewoods. +1 $\endgroup$ – Todd Wilcox Oct 28 '17 at 19:38
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    $\begingroup$ @ToddWilcox Thanks, I've updated the answer. I knew it sounded a bit off :) $\endgroup$ – safesphere Oct 28 '17 at 19:50
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    $\begingroup$ For anyone with an EE background, the purpose of a guitar body is impedance matching. The strings are a high-impedance source, and air is a low-impedance medium, so you need a transformer (the soundboard) to get much energy transfer. Or an amplifier, but then you've got an electric guitar :) $\endgroup$ – hobbs Oct 29 '17 at 1:10

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