Musical Instruments

Classical Mechanics Level 1

All stringed musical instruments—like guitar, violin, etc.—have a hollow box behind the strings. (See the sound hole on the hollow box of a guitar, circled red in the picture.)

What purpose does this hollow box serve?

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The strings vibrate the air inside the hollow box to produce a louder sound The hollow box absorbs the sound coming from the strings and thereby produce a low sound To reduce the weight of the musical instrument The hollow box allows the string to vibrate freely

3 solutions

Ram Mohith
Jun 18, 2018

A vibrating sound itself produces a very weak sound which cannot be heard at a distance. Therefore all musical instruments such as guitar and violin are provided with a hollow box (called as sound box ). The box is so constructed that the column of air inside it, has a natural frequency which is same as that of the strings stretched on it. So when the strings are made to vibrate , the air column inside the box is set into forced vibrations, the frequency of which is same as that of the string. Since the sound box has a large area, it sets a large volume of air into vibration, so due to resonance, a loud sound is produced.

Intuitively, based on my experience as a pianist and guitarist, I disagree. The strings through their attachment to the soundboard cause it to vibrate. As a plane surface, it has a much larger interface with the air, and thus a much greater effect on the air enclosed within the instrument than do the strings themselves, which are completely outside that enclosed space. Perhaps part of the confusion in providing solution is due to the failure to differentiate between the soundboard and the sound box, as evidenced in the attribution to the sound box having a large area. It is the sound board, being a two-dimensional surface, which has an area.

David Thomson - 2 years, 11 months ago

The answers need more thought. The term 'low sound' seems confused, ambiguous, and is apparently misleading. Most of the energy from string vibration is coupled to the sound box front surface by the bridge, on the front surface, thence directly to the contained air, and indirectly via the hourglass shaped instrument edge to the back of the instrument and thence also to the air inside. The hole couples & focuses the sound in the air in the box to the air outside the instrument. Some of the sound is also radiated from all of the outside surfaces of the sound box. There is no 'natural frequency' of the air inside, or it would sound horrible. In fact, the complex mechanical structure encourages frequency alacrity, and generation of the rich harmonics desired in such instruments. (There are several other significant but subtle sound paths which add to the complexity and personality of the instrument.)

J B - 2 years, 10 months ago

100% truth. Guitars aren't meant to be limited to one tuning, having the sound-box resonate with only some frequencies would be an absolute pain in the bottom for guitarists.

Louis Welch - 2 years, 8 months ago

A Former Brilliant Member
Mar 11, 2020

A sound wave produces so called nodes and antinodes. When sound transport through air it makes air particles to come close together and produces higher pressure (antinode) and spread out again (nodes) repeatedly. When the sound wave hits the wall of the hollow box inside the guitar, it ends with an antinode (more about this if you read about waves in physics). This reflects the wave out of the guitar and at the same time this reflecting wave will meet a new incoming sound wave. The reflecting and the incoming wave are in exactly the same phase and will therefore interference with each other. The interference of these two waves produces a wave with a bigger amplitude. Let’s say the reflecting wave has amplitude A=1 and the incoming wave has an amplitude of A=1. When these both waves are added together the end result is a wave with an apmlitude of A=2. This is called constructive interference, when two waves are in the same phase, meaning the both waves produces their antinodes and nodes at the same time. A wave with a bigger amplitude has more energy and makes a louder sound.

Barry Pace
Sep 18, 2018

Electric guitars are NOT hollow, so, all string instruments are not hollow.

Musical Instruments

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