Scientifically, the difference between musical and nonmusical sound, is that the former is a steady, constant sound consisting of periodic vibration, while the latter is an unsteady sound consisting of non-periodic vibration, which is continually varying. (Taylor, 1883:48-50)
The easiest way for a musician to understand the difference between the periodic and non-periodic motion is illustrated by Buck (1918:15):
Sound waves have a special type of repeated motion which goes “back and forth.” This is called a vibration or an oscillation. Moravcsik (2002:23) gives picturesque examples of these movements. For instance, a person going to work in the morning and returning home in the evening can be said to oscillate between the workplace and home. Other he lists:
As it is with Buck’s example, if a sound wave oscillates with its motion repeated equally through time, it is periodic (see fig.1), if not, it is non-periodic (see fig.2).
Figure 1. Periodic sound wave
Figure 2. Non-periodic sound wave
As Helmholtz (1895:8) mentions, periodic musical vibrations can be seen:
“Although they may be too rapid for the eye to follow them singly, we easily recognize that a sounding string, or tuning-fork, or the tongue of a reed-pipe, is rapidly vibrating between two fixed limits, and the regular, apparently immovable image that we see, notwithstanding the real motion of the body, leads us to conclude that the backward and forward motions are quite regular.”
It can also be felt:
“… player feels the trembling of the reed in the mouthpiece of a clarinet, oboe, or bassoon, or of his own lips in the mouthpieces of trumpets and trombones.”
From my personal experience, if you play violin without shoulder rest, you can often feel the vibrations on your collarbone. Also, this reminded me of Beethoven. Although a lot of stories, such as him cutting the legs off his piano, then lying down on the floor to sense the notes, or him putting a stick between his teeth to better feel the sounds, are most likely just apocryphal tales (Lewis, 2010:94), I do believe it’s not impossible to imagine him, due to his hearing loss, searching for various tactile vibrational sensations to aid his playing or composing, even if just subconsciously, starting from those found beneath one’s fingertips when playing an instrument.
“Periodic vibration results in definiteness of pitch, and so a musical sound is considered, scientifically, as one that has an ascertainable pitch…” (Buck, 1918:24)
Here we have pitch as the dividing line between musical and nonmusical sounds. However, as Buck goes on, there are doubtful cases. For example, when a cork is pulled out of a bottle, it results in a sound of clear pitch. The bottle can even be tuned by pouring different amounts of water, thus giving different pitches when drawing the cork. Hammering metal or even throwing down the wooden sticks of suitable dimensions on a table can give definite pitches. (Rayleigh, 1894:4)
As a matter of fact, a lot of sounds in nature can happen to be of recognizable pitch. Few years ago, I went to Dun Huang region with my family in China. Among many places, we also visited Ming Sha Mountain (see fig. 3). Ming Sha (鸣沙) can be translated as singing sand. Before going, I read that if a person slides down from the mount top, the sand will produce a melodious sound with falling. (Wang and Hu, 2013:42-43) I wanted to try this by myself and it turned out to be true.
But, even if pitch was the only criteria, would these be considered musical sounds? What about the sounds of indefinite-pitched percussion, that are without definitive pitch and are used for musical purposes?
As Helmholtz (1895:7) states, musical and nonmusical sounds intermingle in various degrees. Even scientifically, the only that are widely separated are the extremes, such as the difference between a note of a pianoforte and the rustling of the leaves.
So in reality, we see that the scientific demarcation line leaves out other criteria that also need to be considered.