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How Acoustic Guitars Work

The Guitar's Sound

Cool Fact
A piano has 88 keys stretching through more than 7 octaves. The lowest note on a piano vibrates at 27.5 Hz and the highest vibrates at 4,186 Hz.

Have you ever noticed that a piano, a harp, a mandolin, a banjo and a guitar all play the same notes (frequencies) using strings, but they all sound so different? If you hear the different instruments you can easily recognize each one by its sound. For example, anyone can hear the difference between a piano and a banjo!

An acoustic guitar generates its sound in the following way:

  1. When the strings on a guitar vibrate, they transmit their vibrations to the saddle.
  2. The saddle transmits its vibrations to the soundboard.
  3. The soundboard and body amplify the sound.
  4. The sound comes out through the sound hole.

The particular shape and material of the sound board, along with the shape of the body and the fact that a guitar uses strings, give a guitar its distinctive "sound."

Photo courtesy Gibson Guitars
The body of a Gibson guitar during construction

There are a number of different ways to modify sounds to get the particular voice of the instrument. For example, if a guitar produced a pure tone, a guitar's 440-Hz A note would sound like this:

  • Click here to hear a 440-Hz tone. (At the dialog select, click "Open.")

Here is what that tone looks like -- it is a pure 440-Hz sine wave:

One modification that a guitar makes to that tone is to add harmonics to it. For example, when you pluck one string it plays the pure note, but the string also rings at harmonics like two-times, three-times and four-times the pure tone. Other strings also pick up the vibrations from the saddle and add their own vibrations as well. Therefore, the sound you hear from a guitar for any given note is actually a blend of many related frequencies. To get an idea of the effect this sort of blending has, here is a 440-Hz tone with an 880-Hz tone (at half the amplitude) added to it:

  • Click here to hear 440-Hz and 880-Hz tones. (At the dialog select, click "Open.")

Here is what these two blended tones look like together:

A guitar also adds an envelope to any note it plays. The note doesn't just start and stop abruptly -- it builds and trails off. Over the course of the note, the amplitude (loudness) of the note changes. For example, a guitar's envelope might look like this:

Here is what this envelope sounds like when applied to the blended tone:

  • Click here to hear an envelope applied to the 440-Hz and 880-Hz tones. (At the dialog select, click "Open.")

This does not sound anything like a guitar, but it is much closer than a pure 440-Hz tone is. To make it sound exactly like a guitar you would have to:

  • Get the right set of harmonics at the right frequencies and amplitudes and blend them
  • Get the right envelope
  • Get the right tonal modifications: The body of a guitar favors some frequencies (amplifies them better) and discriminates against others (does not amplify them as well). You would need to apply that distortion to the tones.

If you did all of that, you could artificially create a note that would sound very guitar-like. That is what synthesizers do when they emulate the sounds of different instruments.