Coffee-Can Cuíca

1. Remove the plastic lid from the coffee can and set it aside. Using a can opener, cut out the bottom of the can to make an open cylinder (click to enlarge diagram below). Use tape to cover any sharp edges. Set the can aside.
   
2. Use the nail to poke a small hole in the middle of the can’s plastic lid. The hole should be just large enough for your bamboo skewer to fit through, but not so large that the skewer can slip through easily.
3. Push the skewer through the hole in the lid so that a little bit (about 1/2 inch or 1.25 centimeters) is sticking out the top (click to enlarge diagram).
   
4. Cut two pieces of string a few inches (several centimeters) long. Knot one piece just above the plastic lid, and one just below it. Secure the knots in place with hot glue to keep the skewer from slipping through the lid, then cut off the pointy end of the skewer.
5. Fit the skewer-and-lid assembly back on the can. The lid should fit tightly. If it doesn’t, you may want to add a few drops of hot glue to keep it attached.

To play your cuíca, take the moistened cotton cloth in your hand, reach in through the open end of the instrument, and rub the cloth up and down along the long end of the bamboo skewer.

Depending on how hard and fast you rub the skewer, you can make everything from low-pitched croaks to high-pitched squeaks and squeals. Try pressing gently in different places on the plastic lid to see if it changes the sound.

What else can you do to make it sound different?

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.

The “stick-and-slip” vibrations that make the cuíca play are also responsible for the familiar sound of chalk squeaking on a blackboard.