Free Form: Music and the Voice

As a singer in the Radcliffe Choral Society, I thought it might be interesting to discuss the physics of music and the voice. We have already discussed light waves and light interference to some extent here. Sound waves work in a very similar manner, though they are different from light waves through their manner of propagation. While light waves are electromagnetic waves, sound waves are mechanical waves, meaning that they propagate via pressure and displacement through mediums like air or water. The volume of the sound is proportional to the amplitude of the sound wave, and the pitch is dependent on the frequency or the wavelength of the wave. Higher pitches correspond to higher frequencies and lower pitches to lower frequencies.

One thing that is interesting to consider is how harmonies work. The octave of a note corresponds to half or double the frequency of that note. In this case, the nodes of the sound waves lines up, as shown in the image below:

Image Credit: http://www.musicez.com/images/octave.gif

We say that octaves have a frequency ratio of 2:1. Other common music intervals include the major third and the perfect fifth, which correspond to frequency ratios of 5:4 and 3:2 respectively. Interestingly, your ear can feel differences in frequency if there is a lot of dissonance between two notes. Dissonance can occur when two notes are off by a small interval and their nodes don't match up. Dissonant sounds are often described as 'grating' or 'unstable.' You can hear the pitch waver due to the misalignment of the phases of the sound waves. If you click here, you will find an application for a virtual piano where you can create chords. You can play around with it and see if you can figure out what note combinations make dissonant chords.

Having explained a little bit about how sound and music works, we can take a look at how the voice creates sound. To make sound, your body begins with the lungs and the diaphragm, where it pushes air up to the vocal folds, located within the larynx, with enough pressure to cause the folds to vibrate. The vibrations of the vocal folds cause them to open and close, chopping up the air that is pushed through them into little pulses of compressed air. These pulses of air are what make up the sound that we hear. The image below shows how the vocal folds work.

Vocal folds releasing pulses of compressed air.
Image Credit: http://hyperphysics.phy-astr.gsu.edu/hbase/music/voice.html

Needless to say, the loudness of the sound is controlled by the amount of air pushed up by the lungs and diaphragm. A greater amount of air means more compressed air pulses, or in other words, larger amplitude of sound waves. We are able to adjust the pitch of the sound we make by using muscles in our larynx to stretch our vocal folds tighter or less tight, creating vibrations with higher or lower frequencies, and therefore, higher or lower pitches. The reason men have lower voices than females in general is because men have larger vocal folds. Men's vocal folds range from 17mm to 25mm in length, while women's vocal folds range from 12.5mm to 17.5mm. This causes the vibrations in the vocal folds of men to have lower frequencies than those of women.


Citations:
http://en.wikipedia.org/wiki/Sound
http://en.wikipedia.org/wiki/Music_and_mathematics
http://en.wikipedia.org/wiki/Interval_ratio
http://en.wikipedia.org/wiki/Consonance_and_dissonance
http://hyperphysics.phy-astr.gsu.edu/hbase/music/voice.html
http://en.wikipedia.org/wiki/Human_voice