Why do ambulance sirens sound different as they are driving away? πŸš‘

Sound travels in a wave starting from something that is vibrating and ending up in your earπŸ‘‚πŸ». When the sound wave travels through the air it makes pockets of different air densities (how packed together the air molecules are in a given space). This corresponds to alternating regions of high and low air pressure (you might have heard these terms when listening to a weather forecast β›ˆ ). The faster the air molecules vibrate, the faster each wave reaches your ear (higher frequency) and the higher the pitch we hear 🎢

So back to the ambulance sirens πŸš‘… When an ambulance is driving away from you, there is a longer distance between the air molecules that are close to the ambulance meaning that the vibration is slower between these molecules. Each wave will therefore take slightly longer to reach your ear and so you hear a progressively lower pitch. This is called the Doppler effect. We can use a simple mathematical equation to calculate the speed of the vibrations (frequency) we hear if we know the speed an ambulance is travelling away from us as well as the speed of the vibrations leaving the siren.πŸ™ŒπŸ» This principle can also be used to detect speeding vehicles using radio waves fired from a police radar gun! 🚨 ⭐️ Try at home sound waves activity: attach two pieces of string to one end of a slinky and tie the other ends of each string around one finger on each hand. Now listen to the difference in sound when dropping the slinky with those fingers in and out of your ears! Why does it sound different? 🧐
(Hint: think about how densely packed molecules are in solids compared to in the air)

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