Membranophones are instruments that make sound from the vibrations of stretched skins or membranes. Drums, tambourines, and some gongs are common examples of membranophones.
In this Snack, as you blow into the straw, you create pressure in the space between the outer wall of the construction-paper tube and the inner wall of the water bottle. That pressure forces the membrane to rise, allowing air to flow into the top of the tube and escape out the bottom.
As the air escapes, the membrane returns to its initial position. But as you continue blowing air into the instrument, you force the membrane to rapidly rise and fall, over and over again. If you place your finger over the top of the membrane, you can feel it vibrate. These vibrations produce sound.
Opening or covering the finger holes changes the pitch of the sound. That’s because opening a hole has the same effect as shortening the length of the “pipe” (the rolled-up construction paper). The shorter the pipe, the higher the pitch of the sound.