What is sound?
A vibration that travels through the air as changes in pressure, and how the guitar produces and projects it.
Essential
Content under musical review. It may contain inaccuracies.
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A vibration that travels through the air as changes in pressure, and how the guitar produces and projects it.
Essential
Sound is a vibration that travels through a medium, usually air. When a string vibrates, it pushes the nearby air and creates zones of slightly higher and slightly lower pressure that spread out in every direction. When they reach your ear, they move the eardrum and your brain interprets them as sound.
A single sustained note:
Compare a low note with a high one. The low one vibrates more slowly; the high one, faster:
A string fixed at both ends vibrates in wave patterns. Press the different modes and watch the points that don't move, the nodes:
In the sound lab you can see the waveform and change it. Start with a sine wave, the simplest, then switch to a sawtooth:
Sine wave Sawtooth waveEvery sound wave is described by a few properties, and each matches something you hear:
| Physical property | What you perceive |
|---|---|
| Frequency (vibrations per second) | Pitch: low or high |
| Amplitude (size of the pressure change) | Loudness: soft or loud |
| Waveform and spectrum | Timbre: the "colour" of the sound |
| Envelope (how it starts, holds and fades) | Attack and duration |
In air at about 20 °C, sound travels at roughly 343 metres per second. A young human ear hears roughly from 20 to 20,000 vibrations per second.
On an acoustic guitar, the string alone moves very little air. The vibration passes through the bridge to the top, which is much larger and pushes more air; the body and soundhole reinforce certain frequencies. On an electric guitar, the pickups turn the string's vibration into an electrical signal, and the amp and speaker produce the sound.
A guitar note has a very fast attack, when the pick or finger releases the string, and then it fades gradually. That's why it sounds different from a violin, which can sustain the note with the bow. Plucking with the nail or the flesh changes the attack, and with it your sense of the timbre.
On a string, waves travel to the ends, reflect and overlap: the result is a standing wave with fixed nodes. Fletcher and Rossing explain that a string radiates very poorly on its own because it's so thin; the top acts like a transformer that couples the vibration to the air. The resonances of the top and the air cavity give each guitar its characteristic response.