Doppler effect
Oto-kun
Dad, today in science class at school, we studied the Doppler effect. I know from experience that the pitch of an ambulance siren changes when it approaches and moves away, but the teacher didn't explain why because it was too difficult.
father
The phenomenon where the frequency of sound changes when the sound source is moving is called the Doppler effect, and the easiest example is the sound of an ambulance siren. For instance, if the ambulance is traveling at 60 km/h, considering it in terms of meters per second, it moves approximately 17 meters in one second (60,000 m / 3,600 seconds). When the ambulance is approaching, it catches up with the waves moving in the same direction, and since sound travels about 340 meters in one second, 5% (17/340) of the waves are compressed. Conversely, when it is moving away, the waves are stretched by 5%, resulting in a 10% difference in frequency.
Oto-kun
That's why the sound of the siren changes.
father
The sound of the ambulance siren, which is at 770Hz and 960Hz, is close to the musical notes 'G' (783Hz) and 'B' (987Hz). When the siren is sounding while the ambulance is stationary, a person with perfect pitch would likely hear it as 'G-B-G-B'. I explained earlier that when approaching at a speed of 60 km/h, the pitch increases by 5%, which corresponds to almost a half step, so it might sound like 'G#-C'.
Oto-kun
So, a person with absolute pitch can understand the speed of something that approaches them while making a sound and then passes by.
father
Do you know that there are animals that use the Doppler effect of ultrasound to catch their prey?
Oto-kun
Does judging by sound mean that one cannot see? ... Bats?
father
Correct. Bats have incredible hearing. They emit ultrasonic pulse waves at about 60 kHz to target insects and other objects, allowing them to observe distance and speed.
Oto-kun
What is 'pulse'?
father
Short-duration waves. In the case of bats, they use sounds that are generated for several tens of milliseconds, around 0.03 seconds. When approaching insects, they shorten the duration of these pulses and emit them continuously.
Oto-kun
But how can distance and speed be determined by applying a pulse?
father
By capturing the waves reflected from the target, we can understand the distance to the target, its speed and direction, size, and surface properties.
Oto-kun
I see, you calculate the distance to the target using the time it takes for the reflected wave to return, and then calculate the speed using the Doppler effect of the reflected wave. The Doppler effect is interesting.
father
That's right. While my interest in natural sciences is endless, the phenomena we've just talked about are widely applied in various industries.