Conch Shell as an Omni Directional Speaker Pratik Desai (ID #6742866) 9/12/14 Submitted to Dr. Rama Bhat.
CONCH SHELL AS A OMNIDIRECTIONAL SPEAKER Conch Shell is the protective cover of long sea snails and possesses unique spiral profile which is quite similar to exponential horn, produces a diffuse sound field with shorter overall length. This research is aimed to explore the property of conch shell in anechoic chamber. It is proposed to drive the conch shell with an electro pneumatic transducer in order to produce an intense sound field. The device will be used in anechoic chamber and the sound field in the room will be measured. Key words: Conch Shell, Speaker, Directional Pattern, Reverberation Chamber. Introduction: Conch shell, widely used on various occasions since centuries in various communities over the world, has a spiral gradually increasing cavity made of calcium carbonate which is outer cover of the mollusk snail, is to serve the purpose of protection. Conch shells are available in the abundant quantity over the various sea shore of the world. Hard shell and aesthetics in look has made it very useful in various aspect such as decoration, ornaments or jewelries or to make cameo. Some used it as weapon by putting the up-turned conch shells over the fencing to serve the purpose of security where as some used it for communication or passing coded message i.e. natural hazards like flood or fog, warning of war or celebration of victory while some used it as the music instrument. 1 Not only that according to Hindu mythology, spiritual and destructive sound coming out of the conch shell while blowing the air form the one end of conch shell is powerful enough to drive away evil spirits, and holy enough to use as a water containers in the temple for the purpose of worship. In many Hindu temple conch shells are sounded to notify the beginning and ending of worship. 2 Literature Search: One of the adorable use of conch shell is to use a musical instrument. Conch shell is unique wind instrument due to growing cavity around the columilla commonly known as a central pillar around both axial as well as transverse direction which forms the void spiral cavity as shown in Figure 1, Figure 1 X-Ray Pictures Of Conch Shell From Center (Left Side) And Top View (Right Side) This mystery was revealed by Bhat 2-6 and collogue using X-ray tomography technology Research on conch shell was initiated by Bhat
in 1992 3, and he reported fundamental frequency of conch shell and the higher frequencies are integer multiples of fundamental frequency and he added the relation between shell cavity depth and the fundamental frequency 2, and in 2005 7, R. Rath and P. Naik, discover the Fibonacci pattern of the conch shell structure, on the basis of X-ray topography they conclude that structure grows in both spiral and transverse direction following the pattern shown below. f1 = f2 = 1; fn+1 = fn + fn-1.. (1) Which is called Fibonacci pattern and the ratio of successive pair is called as the golden ratio which can be defined as below. Ø = fn / fn-1...(2) They discovered that golden ratio is 1.618 and the golden angle is 222.49 or its supplement of 137.51. Along with X-ray tomography pictures Rath and Naik 8 represent mode locking frequency in their research work. The geometry of conch shell is varying as it is available in many countries but the structure remains same which grows around a central pillar in both axial and transverse directions and thus spiral structure will be formed. The cavity is closed at blowing end and open at mouthpiece section. Seiji Adachi 9 classified the musical instruments as stringed instrument and wind instrument. He divide Wind instrument further in to woodwind and brass instrument. Based on his classification, conch shell belongs form brass family of wind instrument which also includes trumpet, French horn, trombone and tuba. Considering this fact, in 2009 Samuel D. F 10. explained how family of brass instrument works. He mentioned Brass instruments are musical instruments which produces sound by vibrations of air column in its cavity and hence they are also defined as lip vibration instruments and Conch shell is also one of them. There are many factors that affects the production of good quality of pitch and sound. This is the one of the reason for having good training to play with conch shell in a proper way. As shown in Figure 2 8, players keep one end of the instrument Figure 2 Model of Wind Instruments
in their mouths and blows the air. As soon as player will blow the air, pressure difference will be generated between the lips and which will create oscillations. Following this method acoustic pressure will be generated and which will be fed to musical instrument as an Input. Because of modulation of airflow, vibrations will be generated in the cavity therefore air pressure difference will be generated across the lips in wind musical instruments and thus this vibration will be results in to the spiritual sound. 8 Playing Conch Shell is an art and expert player can play various melody using their hand and lips oscillations. Helmholtz 11 Martin 12, Yoshikawa 13, did well on the motion of lips and conclude that in order to get oscillation in conch shell, lips should follow sinusoidal motion. Later in 2010, P. Cook 13 came up with a very innovative idea of using shell a speakers. He used Stroumbus shell as a speaker and found its directionality pattern. Directionality pattern is nothing but the distribution of sound in different directions from the source. He tried to reveal the effect of sound all over the different planes and thus placed various microphones and took measurements. As shown in figure 3, upper left graph represents right and front plane, lower left shows right and up plane, and upper and lower right plane shows front and up planes. This pattern reveals that using stroumbus shell as a speaker, distribution of sound in each direction is not uniform in other words listeners who are not within the range of the stoumbus shell, cannot feeling the sound as much as those who are within the range of the shell. After that in 2013, Bhat and Shah 14 performed same experiment but they used white conch shell, and operate with electric transducer. They came up with an exciting results and conclude that conch shell has the exact directionality pattern than any other musical instrument so far we were using. Figure 4 Directionality Pattern of the Conch Shell Figure 3 Radiated Energy in Various Planes. In above figure 14, graph at left side shows the directionality pattern when conch shell is placed as vertical and at right side shows the
directionality pattern when conch shell is placed as horizontal. Term directionality of sound is referred as the distribution of sound wave in all the direction. In order to get full intensity of sound in all direction, sound source must have to be omni directional. Sources which are omni directional serves the equal intensity in all direction which results in equal sound effects in all the direction. Getting inspiration from their work we finally aimed to perform the same experiment but instead of driving conch shell by electric transducer, we aimed to use pneumatic transducer and measured sound intensity keeping conch shell in the center and measured intensity level all around in the anechoic chamber. EXPERIMENTAL SETUP and RESULTS: Our aim in this experiment was to get the directionality pattern, so we kept the conch shell at the center of the anechoic chamber (9 X 13 X 7 ) on top of the tripod (3 5 ) and placed the microphone at the distance of 4 5. Then to get the results, rotating conch shell at 45 respectively in all over the direction and took the measurement. ANECHOIC CHAMBER: Anechoic chamber is the one which is been used for most of the acoustic experiments because it is made of unique walls which serves the purpose of absorbing sound energy instead of reflecting back. Main advantage of using such chamber is that, when any sound is propagated from a sound source, it provides the natural artificial environment which can be consider as similar as the open quite field where there is no possibility of sound reflection. To fulfil our requirements the chamber we used was semianechoic chamber which had unique walls possess small pyramids of 25cm long made of polyurethane. We measured the initial background noise level which was found 36.6 db. Figure 5 Set Up Of Experiment in Anechoic Chamber As shown in figure 5, A indicate the position of the conch shell, and circle drawn in red indicates the periphery of the microphone, where black dot B indicates the position of microphone at equal distance
on circle at 45. Also pyramids shown in black color indicated the sound energy absorbent. The conch shell was driven using pneumatic transducer which was controlled by single generator. Figure 6 shows the experimental set up in the anechoic chamber. Figure 6 Set Up of Conch Shell Mounted as Horizontal Rotation in an Anechoic Chamber. Figure 7 Mounting of the Conch Shell in Tripod Figure 6 shows the mounting of the conch shell on the tripod, in entire experiment, instead of placing microphone at different angle, we placed the microphone fixed and we gave 45 rotation to tripod mounted with conch shell, every single time.
Based on the above setup, we note down the measurements. Measured intensity of sound at different angle of rotation and at both horizontal and vertical position is shown in tabular form on the right hand side. Based on the results, we plot the polar diagrams which reflects the directionality of the Conch shell, and the graph shows that while driving conch shell at resonance frequency (308 Hz), it has equal intensity distribution in all the direction in both longitudinal and transient direction, which is shown in graph below. Table 1: Measured Sound Intensity at 45 of Rotation Degree Of Rotation ( ) Intensity (Db) In Horizontal Direction Intensity (Db) In Vertical Direction 0 55.1 55.1 45 53.7 53.9 90 52.9 52.7 135 53.1 52.4 180 53.9 53 225 55 54 270 55.7 55.1 Figure 8 A) Directionality Pattern of Conch Shell in the Horizontal Direction and B) Directionality Pattern of Conch Shell in the Vertical Direction.
Above shown figure 8, indicates the directionality of the conch shell in both horizontal as well as in vertical direction. Circle indicated with blue shows the reference circle while circle indicated with orange shows the circle which was obtained from the resultant measurements HARMONICS IN CONCH SHELL: Before Bhat and Shah performed experiment on conch shell, Bhat and Taylor 2 carried out one experiment on Indian Conch shell. They measured the sound from lip driven conch shell and found the fundamental natural frequency as well as five overtones as an integer multiple of fundamental frequency with smooth decreasing amplitude. He considered conch shell as open ended duct and hence the equation for harmonic series is defined as f = n*c / 2* L.(3) Where n= 1, 2, 3.. c= speed of sound L = acoustic length The main objective is to implement conch shell for industrial applications, mainly as a blow horn. In previous chapters we discussed the directionality of conch shell which shows that it has good directionality pattern. EXPERIMENTAL SETUP: To give the sinusoidal in order to get the resonance frequency in conch shell instead of blowing air from mouth, we fed air, function of sinusoidal wave driven by pneumatic transducer, with the help of function generator. Further, the microphone which we used to get the measurements, was placed 4 5 far from the conch shell tripod Microphone was attached to the FFT spectra analyzer and output was recorded in floppy drive. While blowing the conch shell at its resonance frequency which was 308 Hz, we got very exciting results. The obtained result is shown in below figure 10. As it can be seen from figure, that sound spectrum do contains harmonics while blowing the shell at its fundamental frequency. This measured sound follows the same spectrum as the sound measured from a mouth driven Greek conch shell which is shown below in figure 9. When the shell is driven by mouth, the lip vibration excites the pressurized air flow from mouth and hence the spectrum displays a fundamental and several harmonic frequency peaks.
1 24 47 70 93 116 139 162 185 208 231 254 277 300 323 346 369 392 415 438 461 484 507 530 553 576 599 622 645 668 691 Impedance IMPIDANCE Impidance vs Frequency 140 120 100 80 60 40 20 0 0 500 1000 1500 2000 2500 3000 3500 FUNDAMENTAL FREQUENCIES Series1 Figure 9 Harmonics of Greek Conch Shell Mouth Blow 100 90 80 70 60 50 40 30 20 10 0 Impedance vs Frequency Fundamental Frequency Series1 Series2 Figure 10 Harmonics of Conch Shell Air Driven Figure 10 shows the harmonics of the transducer driven conch shell, here series 1 indicates the harmonics of conch shell mounted as a horizontal while series 2 indicates the harmonics of conch shell mounted as vertical. Also graphs clearly show the 1 st 5 fundamental frequencies, irrespective of position of mounting while blowing at fundamental frequency.
DISCUSSION of RESULTS: Based on the experiment and literature, comparing other musical devices like loud speaker and even stroumbus shell do not possess omni directional characteristics. It is significant that the only conch shell have the omni directionality pattern. Furthermore, pattern is found almost identical to the reference circle, and also identical with two distinct type of mounting position which conclude that irrespective of mounting position and direction, conch shell spreads equal and uniform sound intensity in all direction. integral multiples of the fundamental frequencies. Overall, for the industrial application of conch shell such as blow horn is preferable if any arrangement is done to blow the air in the cavity of the conch shell. FUTURE WORK: Further comparing harmonics of Indian conch shell with Greek shell, it concluded that frequencies are in the multiple integer of fundamental frequency and sound spectrum identically follow the sound spectrum of mouth driven Greek shell. CONCLUSION: From the research work it is concluded that conch shell provides better directionality pattern then loudspeakers and strombus horn. Moreover, conch shell has Omni directional pattern irrespective of direction. In addition to that harmonics depicts that flow of air in the conch shell is necessary for a harmonic series of frequencies as
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