Why Can't We Hear the Jet While It Is Approaching Us?

Have you ever wondered why you can't hear a jet while it is approaching you? There are several factors at play here, including the Doppler effect, the speed of the jet, sound propagation, and the directional nature of sound. Let's dive into these elements to understand why you often don't hear the approaching jet until it is very close or has already passed.

The Doppler Effect

One of the primary reasons we can't hear a jet while it is coming towards us is the phenomenon known as the Doppler effect. As the jet approaches, it compresses the sound waves in front of it, causing the frequency of the sound waves to increase. This results in a higher pitch that we perceive. However, when the jet is directly approaching, the sound waves are compressed to the point where they reach our ears at a higher frequency but may not be audible until the jet is very close.

The Speed of Sound

Jets often travel at speeds greater than the speed of sound, which is approximately 343 meters per second (1125 feet per second) at sea level. When a jet is flying towards you at supersonic speeds, it generates a shock wave, which is what creates a sonic boom when it breaks the sound barrier. Until the jet reaches you, the sound waves it produces are trailing behind it, which is why you don't hear it until it is nearly overhead or has passed.

Distance and Sound Propagation

Sound takes time to travel through the air. If the jet is far enough away, the sound it produces may not reach you immediately. As it gets closer, the sound waves converge towards you, and once it passes, the sound waves spread out, which is when you hear the noise. This delay in sound arrival can make it difficult to hear the jet until it is very close or has passed.

Sound Directionality

Another factor to consider is the directional nature of sound. The sound coming from the jet is aimed away from you while the jet is coming toward you and aimed toward you after the jet passes you. For example, at a concert, the music is louder if you are standing in front of the speakers because the sound is directed towards you. Similarly, the sound from a jet is also directed away from you until it is very close, making it less audible.

Sound Speed vs. Jet Speed

Sound in air moves at about 760 miles per hour. Most commercial jets can move about half that speed or more. If a jet is flying toward you for ten minutes, you won’t hear it for the first five minutes. The sound of the jet is going to get to you at about twice the speed of the jet itself. If the jet is flying faster than the speed of sound, it gets worse. If the plane is doing the speed of sound, you won't be able to hear it at all until after it has passed over you.

In summary, the combination of the Doppler effect, the jet's speed, sound propagation, and the directional nature of sound all contribute to why we often do not hear the jet until it is very close or has already passed.