Wind chimes produce clear, pure tones when struck by a mallet or
suspended clapper. A wind chime usually consists of a set of individual alloy
rods, tuned by length to a series of intervals considered pleasant. These are
suspended from a devised frame in such a way that a centrally suspended clapper
can reach and impact all the rods. When the wind blows, the clapper is set in
motion and randomly strikes one or more of the suspended rods– causing the rod
to vibrate and emit a tone.
The pitch of said tone is governed by the length of the rod, but the
perceived loudness is affected by many determinants: the force of the clappers
impact, the alloy’s density and structure, and the speed and direction of the
wind (to name a few). Also affecting the loudness is the lack of resonating
chamber or hard connection between rods and frame. The chime would certainly be
louder, for instance, if the rods were built with the inclusion of small
chambers containing a volume of air whose fundamental harmonic was the same as
that of the rod– when struck, the rod would transfer vibration to the enclosed
air as well as directly to the atmosphere, resulting in a louder tone. A hard
connection between rods and frame would also accomplish this result somewhat;
the vibrations of each seperate rod would be commuted to the others, resulting
in more vibrating surface area (and hence, more volume).
The transmission of the chime’s sound without the abovementioned
alterations is quite simple; each rod releases longitudinal waves radially from
it’s longest axis (excepting deviances caused by deformation or impurity of the
metal), which travel until they are absorbed or reflected by an independent
surface. These waves travel at a speed governed by the temperature of the
atmosphere– the colder the air, the more immediate the transmission.
The waves that are not absorbed can be perceived by the human ear; of
equal importance to the directly intercepted waves are those reflected before
interception, as these allow an animal or human to identify the physical
relationship of self to sound-emitter. These intercepted waves (reflected or
not) are processed by the ear in an amazing process.
Sound waves vibrate the ear-drum, causing the minute movement of three
microscopic bones (hammer, then anvil, then stirrup) in the middle ear. The bone
chain, having transferred air vibration to physical vibration, systematically
disturbs the fluid (perilymph) in the inner ear (cochlea). Hair cells along the
basilar membrane (which runs the length of the cochlea) perceive the
disturbances and interpret them as auditory signals to be transmitted to the
nervous system. With pure tones such as those created by a wind chime, certain
groups of hair cells are agitated more than others– and the position of that
group along the basilar membrane can be directly correlated to the relative
pitch of the tone.