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.
In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions[1][2]) are functions of an angle. They relate the angles of a triangle to the lengths of its sides. Trigonometric functions are important in the study of triangles and modeling periodic phenomena, among many other applications.