Doppler effect

Dad, in science class at school today we learned about the Doppler effect. I know from experience that the pitch of an ambulance siren changes when it's approaching and when it's moving away, but the teacher didn't explain why that happens because it's too complicated.

The phenomenon where the frequency of sound changes when the sound source is moving is called the Doppler effect, and the easiest example to understand is the sound of an ambulance siren. For example, if an ambulance is traveling at 60 km/h, then in terms of speed per second, it is 60,000 m / 3,600 sec, so the ambulance travels about 17 m per second. When the ambulance is approaching, it catches up with the sound waves in the same direction, and the sound travels about 340 m per second, so the waves are compressed by 5% (17/340), and when it is moving away, they are expanded by 5%, resulting in a 10% difference in frequency.

That's why the siren's sound changes.

The beeping sound of an ambulance siren is, I believe, 770Hz and 960Hz, which are close to the notes 'G' (783Hz) and 'B' (987Hz) on the musical scale. If the siren is sounding while the ambulance is stopped, someone with perfect pitch would probably hear it as "G-B-G-B". I explained earlier that the pitch increases by 5% when approaching at 60km/h, which is roughly equivalent to a semitone, so it might sound like "G#-C".

So, people with perfect pitch can tell the speed of something that is approaching them while making a sound and then passing by, right?

I have another interesting story to tell. Did you know that there are animals that use the Doppler effect of ultrasound to catch their prey?

Judging by sound means they can't see? ...A bat?

That's correct. Bats have amazing hearing. They use pulsed ultrasonic waves at around 60 kHz to target insects and other prey, and then observe their distance and speed.

What is 'pulse'?

These are short-duration waves. In the case of bats, they use sounds that last for a few tens of milliseconds, or about 0.03 seconds. When they approach an insect, they shorten the duration of these pulses and generate a series of pulses.

But how can we determine distance and speed by applying pulses?

By capturing the waves that reflect off the target when ultrasound hits it, we can determine the distance to the target, its velocity and direction, its size, and even the properties of its surface.

I see, you calculate the distance to the target using the time it takes for the reflected wave to return, and then calculate the velocity using the Doppler effect of the reflected wave. The Doppler effect is interesting.

That's right. While it's fascinating from a natural science perspective, phenomena like the ones we just discussed are also widely applied in industry.