Drums make use of circular membranes which have many modes of vibration. The excitation of the various modes depends upon where the drum is struck. The timpani is struck near the side to excite certain preferred modes. By contrast, the bass drum is struck in the center and excites the 01, 02, 03, modes. The fundamental frequency for an ideal circular membrane with no air damping effects is given by
The bass drum usually has a diameter of 50-100 cm and membranes on both ends of the cylindrical body. Although the drum does not have a well-defined pitch center, it is common practice to tune the lowest modes of the two heads about a musical fourth apart. The coupling between the two heads of the drum produces a splitting of the two lowest modes, the 0,1 and 1,1 modes.
Note that the value of the constant in the above equation for the frequency assumes that MKS units are used, as in the calculation for the circular membrane.