To determine why sea shells have an ocean sound. Blow across the mouth of one of the soda bottles to produce a constant sound while holding the second bottle next to your face with its opening near your ear. As you make the sound by blowing across the first bottle's mouth, move the second bottle away from your ear and then near your ear again. Note any difference in the sound when the second bottle is near your ear.
All objects have a natural frequency, or rate of vibration, depending on their size and shape.
Because the two soda bottles are alike, they have the same natural frequency. Blowing across the bottle causes the air in the bottle to vibrate, which results in vibration of the air around the bottle's mouth. The vibrating air moves outward, causing the air in the second bottle to start vibrating. This action is called resonance, or sympathetic vibration. Resonance can occur when the natural frequencies of two objects are the same.