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.
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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 separate 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 radically from its 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.