Spectrum Analysis and Directivity Pattern of a Transducer-Driven Conch Shell
The sound of a conch shell is very unique in nature and has been used in different cultures around the world, mainly in India, Greece and Japan. It was used to mark auspicious occasions, for signaling purposes and on rare occasions as a musical instrument.
The conch shell is considered as one of the earliest “horn instruments” used by mankind. However studies on the acoustical properties of the instrument is scarcely found in literature. In order to shed more light on the nature of the conch shell sound, we obtained X-ray images of two different conch shells. The images were processed for analyzing the geometry of the spiral cavity. We tried finding a spiral geometry that best matches and describes the spiral cavity displayed in the X-ray scans. Furthermore, the directivity pattern of a conch shell driven by a small loudspeaker and an electro-pneumatic transducer are measured experimentally. It is found that the shell radiates sound uniformly in space at frequencies near the cavity’s resonance. The shell's sound spectra in three different excitation cases are also compared (lip excitation, loudspeaker and electro-pneumatic transducer). As expected, the sound spectrum of the lip-driven shell contains clear peaks at a fundamental frequency and its harmonics, giving the sound a musical tone. In contrast, the overtones of the loudspeaker and electro-pneumatic transducer driven shell are not harmonics of a fundamental frequency.
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