• Question: when a police car passes you, why does the siren sound different?

    Asked by bugattiveyron to Ben, Clare, Ezzy, Mario, Sam on 11 Mar 2012.
    • Photo: Mario Campanelli

      Mario Campanelli answered on 9 Mar 2012:


      because the siren produces a sound at a given frequency, say 500 beats per second. This sound travels at the speed of 340 m/s, and when the car is coming towards you, its speed has to be added to that of sound. At 72 km/h, it means 20 m/s.
      So the beats come to you faster by about 5%, and in a second instead of 340 you will get 5% more; our ear will interpret that as a sound with a 5% higher frequency.
      When the car passes us, the two speeds have to be subtracted, so we’ll get 5% fewer pulses, and the brain will interpret that as a 5% lower frequency, for a total change of 10%

    • Photo: Ben Smart

      Ben Smart answered on 10 Mar 2012:


      This effect is known as the doppler effect.

      The sound waves in front of the car sort of get squashed together: At some point the car creates the start of one sound wave (imagine this sound wave travelling out in front of the car at the speed of sound), and then it moves forward before it creates the end of that sound wave, so the wave is shorter than it would have been if the car hadn’t been moving. If the wave is shorter then it’s wavelength is shorter, and to us a shorter wavelength sounds like a higher-pitched sound.

      The sound waves behind the car get stretched out: At some point the car creates the start of one sound wave (imagine this sound wave travelling backwards behind the car at the speed of sound), and then the car moves forward (in the opposite direction to the wave) before it creates the end of that sound wave, so this wave is longer than it would have been if the car hadn’t been moving. A longer wave means a longer wavelength which we hear as a lower-pitched sound.

      So depending on whether the car is moving towards you (squashed-together ‘short’ sound waves) or whether it’s moving away from you (stretched-out ‘longer’ sound waves), you will hear sound waves of different wavelengths, which will sound like higher or lower pitched sounds to our ears.

      Hopefully that explanation of mine made sense to you 🙂 I always found the doppler effect easier to understand with a drawing though. Please ask your teacher to draw you a diagram to explain the doppler effect, and then hopefully it’ll make more sense to you 🙂

    • Photo: Elizabeth Pearson

      Elizabeth Pearson answered on 11 Mar 2012:


      Yup. That’d be the doppler effect.

      Light does the same thing. The light from a galaxy moving away from us really quickly gets stretched out and changes its colour. The more stretched the light is the faster it’s going. The further away a galaxy is the faster it moves away from us so by looking at the stretchedness of the light we can tell how far away the galaxy is.

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