Why a Siren Drops in Pitch as It Passes
Why a Siren Drops in Pitch as It Passes — After learning the Doppler effect, I chew over anew the fact that the same thing comes to us differently depending on where and how we meet it. The siren did not change. What changed was the distance between it and me, and the direction — drawing near or moving away. Are people and matters not the same? The very same words sound warm between those growing closer and cold between those drifting apart. The same event carries a different weight as an approaching future and as a past that has flowed by. The pitch the world sounds to me belonged not to the world alone, but was shaped together by the distance between me and the world.
As an ambulance approaches, the sound is high and sharp; the instant it passes me and recedes, the sound suddenly drops. The siren itself must be making the same sound all along — so why does it sound different coming near and going away? Did the sound change, or am I, the listener, hearing something else?
In 1842, the Austrian physicist Doppler reasoned that the same principle works for both light and sound: when the source of a wave moves, the spacing of the waves the listener feels changes. His idea was soon tested by an ingenious method. A scientist put trumpet players aboard a moving train and stationed people with perfect pitch beside the tracks to judge the notes. As the train neared and then receded, the same trumpet was clearly heard at different pitches. Doppler was right.
Here is the reason. Sound is a wave spreading through air. As the siren approaches me, the crests it sends forward are pushed ever closer together. Closely packed waves mean higher frequency, and we hear that as a high sound. Toward the receding side the waves stretch and thin, so the frequency falls and we hear a low sound. The sound itself did not change. What changed was the spacing at which the sound reached me. Depending on whether the same siren neared or receded, the world sounded a different note to me.
- Police speed detectors use this principle, reading the shifted frequency of radio waves bouncing off a moving car to find its speed.
- Astronomers, seeing starlight tinged slightly red, know that the star is receding from us. That the universe is expanding was revealed this very way.
- Medical ultrasound, from the change in sound waves bouncing off blood flowing in vessels, maps which way and how fast the blood moves.
聲은 공기를 타고 귀에 닿는 소리를 뜻하니, 다가오고 멀어짐에 따라 높낮이가 달라지는 도플러 효과의 무대와 통한다.
Meet this hanja in Cheonjamun →After learning the Doppler effect, I chew over anew the fact that the same thing comes to us differently depending on where and how we meet it. The siren did not change. What changed was the distance between it and me, and the direction — drawing near or moving away. Are people and matters not the same? The very same words sound warm between those growing closer and cold between those drifting apart. The same event carries a different weight as an approaching future and as a past that has flowed by. The pitch the world sounds to me belonged not to the world alone, but was shaped together by the distance between me and the world.
