Sound is a traveling vibration. When you strike a drum, for example, the drum starts to vibrate. These vibrations push and pull on the surrounding air, causing the air to vibrate. The vibrations travel through the air to reach your ear, where they cause a thin membrane—your eardrum—to vibrate. If the rate of vibration is within a certain range—from 20 to 20,000 vibrations per second—then you hear a sound.
When you rub the bamboo skewer of your cuíca with a wet cloth, the cloth sticks and slides rhythmically along the length of the skewer, creating the vibrations you hear.
Although the vibrations begin in the bamboo skewer, the skewer isn’t the only thing vibrating. As you play the cuíca, its metal-can body and plastic-lid top are also vibrating. In addition, the open space inside the can acts like the inside of a drum, creating an air pocket in which sounds can vibrate and build.
In most instruments, there’s a part that generates sound and a larger hollow space that amplifies it. In a saxophone, for example, a vibrating reed makes the sound vibrations that are amplified in the hollow, curved-tube body of the instrument. In a cuíca, the skewer and lid together generate the sound and the hollow metal can amplifies it.
You might also notice that larger cuícas make lower-pitched sounds than smaller cuícas. In fact, this rule is generally true for all instruments: Larger instruments make lower-pitched sounds than smaller ones. A giant tuba makes a lower-pitched sound than a tiny piccolo; a full-sized cello makes a lower-pitched sound than its smaller cousin, the violin. When sounds bounce back and forth across a larger space, the vibrations are slower, so the sound is lower.