Musical instruments that start with the letter I include the igil, ipu, Irish uillean, Irish bouzouki, ichigenkin, inci, iya, itotele and istarski mih. The instruments originate from countries ranging from Cuba and Ireland to Croatia, Japan and the Philippines.
Bessel's functions are often used in acoustics for describing circular membranes behaviour (exploited by most of the musical instruments). They are the solutions of the wave equations using polar coordinates. Set the properties of the membrane Bessel's functions describe the vibrational modes of the membrane.
Very nearly - your instincts on this one are good. 1  For a given geometry, the normal modes (the low energy waves excited when you walk by the soda machine for instance) are described by a unique equation.  Bessel functions are the normal modes for a cylindrical geometry.