Why Blowing in Bottles Makes Sound and Helmholtz Resonance

Hi folks, I'm going to explain why when you blow over the hold of a bottle like this you hear a specific sound, one that's different for different sized bottles, something called a Helmholtz Resonance. On a related note, I'll also explain what part the speaker had to play in this other video. In there I'd shown you how to make these bottles move using only sound from a speaker. First, Helmholtz Resonance and making sounds by blowing in bottles. The bottle has to meet some minimum requirements. It has to have a large volume, called the chamber, as well as a narrower volume, called the neck, and a hole at this end of the neck. When air is forced into the bottle, like if we just blow into the opening, the air pressure in the bottle is increased. For a while, the air in the neck of the bottle acts like sort of a massive plug, pushing inward building up the pressure. We can compare this to a spring with a mass at one end. The air in the neck is like the mass and the air in the chamber is like the spring. The air in the neck, moving inward and compressing the air in the chamber, is like the mass pushing on the spring, compressing the spring. At some point we run out of energy and the compression stops. The energy has all gone into compressing the air. At that point the process reverses and the air starts to decompress, forcing the air in the neck back out. This pressure decrease continues and even goes too far as the pressure inside the bottle becomes slightly less than the room air pressure. Since the pressure inside is less than room pressure, the higher room pressure forces air back inside the bottle. That continues until it overshoots and the air pressure inside the bottle is again compressed. It then reverses again, and so on. But each time the compression and decompression is less, with less and less energy to drive it, until finally it stops. The air pressure inside is the same as the air pressure outside. But that cycle of air going in and out happens at a consistent frequency, determined by things like the speed of sound in air, the cross-sectional area of the neck, the volume of the bottle here, and the length of the neck. Basically this formula. This frequency is called the resonant frequency. More specifically, this is Hemholtz resonance, and the bottle is called a Hemholtz resonator. If you're just blowing on a bottle to have it make a sound, the sound you hear as a sound wave is made by that air moving in and out of the bottle. And as I just said, the frequency depends on the geometry of the bottle. So that's why the sound is different for different sized bottles. What part does the speaker play in making these bottles move? It emits sound waves at the opening, at the bottle's resonant frequency, the same frequency we were just talking about. How did we choose that frequency? We chose it by blowing over the hole in the bottle and recording the sound made by the air in the bottle as it compressed and decompressed, as we described. We then examined that sound on the computer and found the frequency of that sound. We then told the computer to emit that frequency out the speaker. So the speaker is emitting sound waves at the bottle's resonant frequency. Why do we do that? Well, in this case, the first sound wave from the speaker blows into the bottle and starts the first compression and decompression going. But since the speaker is emitting sound waves at the right frequency, just as the bottle is starting its next compression, along comes another sound wave to add energy to that compression. And it again adds energy to the next compression, with the next sound wave, and so on. The speaker is keeping the compression and decompression going by adding energy each time. And that's Hemholtz resonance, why you hear a specific sound when you blow on a bottle, and the role of the speaker in this acoustic propulsion. Well, thanks for watching. See my YouTube channel, rimstarorg, for more interesting videos like this. That includes one showing how to move the bottles using sound, another on how to make a lifter or ionocraft that flies using ion propulsion, and one on how to use rainwater to power a piezoelectric crystal, and light an LED. And don't forget to subscribe if you like these videos, or give a thumbs up, or leave a question or comment below. See you soon!