Why Can We Not Hear Supersonic Jets Coming?

Have you been puzzled by the absence of noise from supersonic jets when they're heading towards you? You’re not alone. Let's delve into the reasons behind this phenomenon and uncover the fascinating physics that govern the sound of supersonic jets.

Why Can We Not Hear Supersonic Jets?

Contrary to popular belief, supersonic jets are not always the ones causing a racket. In fact, the sound of these jets is often much gentler than anticipated, especially when they're flying towards you. Here are the key reasons:

No Supersonic Jets in Cities

Reason #1: Most Jets Aren’t Supersonic
While supersonic flight sounds impressive, it is not a common occurrence. The majority of jet aircraft operate at subsonic speeds to avoid issues like fuel consumption and noise pollution. Supersonic flights are typically reserved for military operations or special events, and even then, these jets are usually flying over sparsely populated areas or bodies of water.

Reason #2: Afterburners and Fuel Consumption
When supersonic jets do take off, they often do so with the use of afterburners, which significantly increase their fuel consumption. Afterburners shoot out a mixture of fuel and air to increase thrust, but they also generate a lot of noise. Therefore, even if a supersonic jet is available, it is often economical and practical to keep it subsonic over populated areas.

Sound Directionality and Jet Physics

Directionality of Sound
Sound travels in waves, and its intensity and directionality play a crucial role in our hearing experience. When a jet is flying towards you, it emits sound waves in a specific direction. These sound waves aim away from you (intensity), and you might not hear them until the jet is already close by.

Speed of Sound vs. Jet Velocity
Sound travels at about 760 miles per hour in air. Most commercial jets typically travel at half that speed or faster. Therefore, even if a jet is flying for 10 minutes towards you, you may not start hearing it until around the middle point. The sound of the jet reaches you at about twice the speed of the jet itself. If the jet is faster than the speed of sound, the sound will not reach you at all until after the jet has passed by.

Personal Experiences and Misconceptions

Myth vs. Reality
Some people have stated that they can’t hear supersonic jets at all. However, this can be attributed to several factors, including their physical condition, distance, and the environment. According to Mike Heaton, you can hear the sound of a supersonic jet, especially when it’s overhead. The sound becomes unmistakable, especially from the ground.

Point Source of Sonic Boom
If you are inside the aircraft, you are experiencing the point source of the sonic boom, which is why you don’t hear it. However, if you are on the ground, you will definitely hear it, albeit with some attenuation (decrease in sound intensity with distance).

Conclusion

The sound of supersonic jets can be surprisingly quiet, especially when they are flying towards you. The combination of subsonic flight speeds, afterburners, and directional sound waves all contribute to the phenomenon of not hearing the approaching supersonic jets. Whether you are a casual observer or a frequent flyer, understanding these factors can help dispel some common misconceptions and provide a clearer picture of aviation physics.