During the process of boiling water, why does the sound change from loud to quiet?

This is a classic and interesting physics phenomenon! The change in sound while heating water is mainly related to the formation and collapse of bubbles. 1. Sound gets louder: Violent bubble collapse (cavitation) When water first starts heating, tiny water vapor and air bubbles form at the heat source at the bottom. * As these bubbles rise and encounter cooler water above, the vapor inside the bubbles instantly condenses into water due to the cold. * The pressure inside the bubble momentarily drops, causing the surrounding water to rush inward at high speed and collide, creating tiny 'water hammer' effects and vibrations. * Thousands of bubbles continuously forming, rising, and then 'popping' and collapsing in the water produce a rumbling or hissing sound – this is the noise we hear (commonly called 'singing water'). As the water temperature increases, more bubbles form, and the sound gets louder. 2. Sound gets quieter: Water temperature becomes uniform, bubbles rise directly to the surface When the water temperature approaches boiling (about to boil), the situation changes: * At this point, the entire pot of water is very close to the boiling point (100°C), and the temperature difference between the top and bottom is very small. * As steam bubbles from the bottom rise, the surrounding water can no longer cool them down and condense them rapidly. Therefore, the bubbles can maintain their original shape and rise all the way to the surface to burst. * Since the bubbles are no longer 'collapsing' violently within the water, the noise caused by vibrations naturally decreases significantly, and the kettle becomes much quieter.