The Structure of the Drum
How sound is produced

Striking the head of the drum changes its shape and compresses the air inside the shell. The compressed air presses on the bottom head and changes its shape. Then, these changes are transmitted to the drum shell and reflected back, and this action is repeated, creating a vibration. These vibrations of the top and bottom heads create vibrations in the air, which become sound, and eventually, as the head vibrations are dampened, the sound diminishes.

"Drum tuning" does not mean tuning the drum to a pitch like "C" or "D" but rather to the drum's resonant frequency or a certain tone preferred by the drummer. If the tightness of the head is not uniform, the tone of the drum will change depending on where the drum is struck, and it will be a muddy tone at that. Thus, the drum head must be tightened so as to produce the same tone when struck in different places.
The top head and bottom heads are also tuned to different tensions. If both the top and bottom heads are given the same tension, the sustain of the tone is long, but the volume is low. If this condition is changed, however, the drum becomes louder. In addition, if the bottom head is tighter than the top head, it becomes louder, and the tone rings longer. However, if the bottom head is looser, the tone does not ring so long, and the tone is flatter. The greater the difference in tension between the two heads, the greater the change in tone.

It is also important to tune the drums to one another. For example, if the tom-toms are close in tone, the sound will be unclear, and so these drums are generally tuned to different tones. Percussion instruments do not have the clarity of pitch found in the wind and stringed instruments, but the more drums there are, the more important tuning is to creating a smooth, pleasing sound. Several tom-toms are sometimes tuned to a musical scale and used to play a melody.

The tone, sustain, and projection of a drum is affected by the shape of the shell.
The primary role of the drum is to resonate with the vibration of the head. The larger the volume of the resonating body, the lower the characteristic frequency, and the easier it is to resonate in the lower frequency band, while the smaller the volume, the easier it is to resonate in the higher frequency band. In other words, the larger the diameter, or the deeper the shell, the thicker and heavier the tone, and the smaller or shallower the shell, the brighter and lighter the tone.
Drummers select drums with certain diameters or depths to match the style of music they perform, and they tune the drum heads to their liking to express a rainbow of tonal qualities in their music.