Effects of Diameter of Sound Holes and Their Positions on the Natural Frequencies of Soundboards

Chua, Shyong Fei (2017) Effects of Diameter of Sound Holes and Their Positions on the Natural Frequencies of Soundboards. Final Year Project (Bachelor), Tunku Abdul Rahman University College.

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Abstract

Guitar is one of the popular musical instruments played by plucking the string. The top board, also known as guitar soundboard act as the guitar’s primary “diaphragm”, oscillates at resonance to produce the audible output, and makes the largest contribution to the instrument’s overall sound. The more the soundboard can vibrate, the louder the sound it produces. The sound hole serves as port for the enclosed air in the body, which plays the role in enhancing the radiated sound. There are researches have stated that the difference in characteristics of soundboards such as varying dimensions and geometry will affect the natural characteristics (natural frequencies, modes, and damping value) and cause the difference in the acoustic quality of the soundboard. Lots of researches have been carried out on the modal analysis of the soundboard conducted with braces, bridges, materials and investigate the dynamic behaviour of the soundboard. Moreover, there are lots of guitars available in the market, but there is no any stated standard size and position of sound hole. There are no any researches been done on the experimental modal analysis of the variation in sound hole diameter and the position along the centreline of the guitar soundboard. Hence, this project is conducted to serve this purpose. This paper is aimed (a) to carry out the experimental modal analysis of guitar soundboards for determining its natural frequencies, (b) to obtain the natural frequencies and damping ratios of guitar soundboards with the change in sound hole diameter and its position from the upper bout along the centreline of the guitar soundboard through experimental modal analysis. For the variation in diameter of sound hole, the parameters is set to be 84mm, 98mm and 106mm diameters (constant of position 193mm from top bout); for variation in position of sound hole from upper bout of the soundboard is set at 193mm, 153mm, and 114mm (constant 84mm diameter). Subsequently, Impact Testing had been conducted with LMS Test Lab Software to obtain frequency response function of the five guitar soundboards through experimental modal analysis. Roving accelerometer approach is selected as the method of this experimental modal analysis. A shaker, force transducer, and accelerometer are the main instruments for the measurement.The experimental results showed the natural frequency and damping ratio, with the mode shapes selected through frequency response functions for soundboard with sound hole of 84mm, 98mm, and 106mm diameter, and position of sound hole of 153mm and 114mm from the top bout along the centreline of the soundboard. The analysis of results showed the modification of the modes and natural frequencies as the changed in the parameter of the sound hole (diameter and position). From observation been made, the damping ratio for modes the lowest frequencies are very high, and it is eventually reduced to less than 2% after certain range of frequencies.