Sound due to guitar

Question

My teacher told me that quality of sound depends on shape and size of guitar and its resonator.

How does quality depend on that?

Answer 1

The timbre of an instrument is defined by what combinations of frequencies that it produces when it plays a note. Aside from a pure sine wave, all sounds are composed of multiple frequencies.Therefore, the timbre of a guitar is dependent on what frequencies that are amplified and which ones are muted. If you start singing the letter 'o' [as in "Oh boy"], then transition to singing the letter "e", they sound different, right? The difference is the shape of your mouth, and not the vocal chords. Your vocal chords do basically the same thing throughout the singing. Your mouth acts as something called a resonant body and changes the frequencies of sound that are amplified or muted.

The body of the guitar acts as that resonant body. It's material and it's shape both go into what frequencies it resonates with. If you smack a metal bar, it makes a high pitch sound, and if you smack a wooden spoon it makes a lower pitch sound. That's because harder materials tend to resonate at higher frequencies than soft materials like wood. That's part of why materials like ebony, rosewood, applewood and other hard woods are used instead of balsa.

Answer 2

First, we should agree on what quality means, and that might be a little bit difficult. So, for this answer I would say that quality is the timbre of the sound, that is sometimes termed as "tone quality".

I will center my answer in acoustic/classical guitars since in electric guitars there is also a big portion related to amplification and effects …

The sound from a guitar can be depicted with the following figure (ref 1).

From this figure, guitar behavior can be described as follows: At low frequency, guitar transmits vibrations through the bridge to the top plate, which displaces fluid inside the cavity and induces pressure changes that cause soundhole radiation. Vibrational energy is also transmitted to the back plate via both the ribs and air cavity pressure changes. At high frequency, most of the sound is radiated by the top plate and the role of the bridge becomes more relevant (ref 2).

Then, we have three major parts involved

1. Strings;
2. Body; and
3. Enclosed air.

Strings are important because they are the source of the sound. Steel strings give a different sound than nylon strings, for example. It is also important how the strings are perturbed/played: bowed, fingered, plucked. This is because they would have different spectral composition.

The other parts, can be viewed somehow as filters that you apply to your signal, and you would end up with a different spectrum, that is, a different timbre and therefore, a different tone quality.

References

1. N. H. Fletcher and T. D. Rossing. The physics of musical instruments. Second Edition. Springer, 2010.
2. Rodríguez Gómez, S. E. (2012). Numerical analysis of the modal coupling at low resonances in a Colombian Andean Bandola in C using the finite element method (Bachelor's thesis, Universidad EAFIT).

Answer 3

The speed of wave on a string is given by a relation between tension and inertial mass property of the string, and that's why the quality of sound depends on structure of guitar. The exact formula is $\sqrt{\tau/\mu}$.