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.
|