The Science Behind It Sound is created by vibrations. The air rushing in the side causes the rubber diaphragm to vibrate. The vibration creates an air wave in the bottle and the straw. The bottle acts like a resonator and amplifies the sound. When the rubber balloon is tight, the sound is very loud. You have created an air horn as loud as a truck air horn powered only by your blowing. Truck, train, and handheld can air horns work very similarly. Compressed air causes a diaphragm (like our balloon) to vibrate. This vibration creates the sound you hear. It is amplified by going through the horn. Manufactured air horns are louder than the one you made because they use a higher air pressure than what you can create with your lungs.