# Why can't you hear an aircraft traveling at Mach 0.95 until it passes?

So say an aircraft is traveling at Mach 0.95 towards you. It's almost going faster than the sound waves it's making.

Now say there's a speaker or something on the nose of the aircraft facing forward. If the aircraft is traveling at Mach 0.95, it still isn't going faster than the sound waves it's making. So that means the sound waves would still slowly pull away from the aircraft going forward, right? If that's the case, wouldn't you hear an aircraft going Mach 0.95 before it passes?

Like if you're on a highway going 60mph, and another car is going 65 and passes you slowly. Pretend the second car is the sound wave that you're emitting.

So to summarize, if the aircraft was going Mach 0.95, and the sound waves would slowly travel forward, wouldn't that make it so you could hear the aircraft before it got to you?

(In a video of a F/A-18 I saw, it was traveling at what I assume was ~Mach 0.9 speeds, because you couldn't hear any sound from it until it was really close. No shockwaves or anything so it wasn't going supersonic)

You are never hearing the noise the aircraft is making right now. Instead, you're hearing the noise the aircraft made some time ago -- where that time is determined by the speed of sound and how far away you were.

This is very much related to the Doppler effect, but instead of focusing on the shift in observed frequency, there is also an amplitude effect.

Check out this website with a Doppler simulator. Here is a screenshot from a case I played with. You should really check out the animation.

I placed the source initial position at the far left (-50) and the observer at the far right (50). I set the speed of the source at M=0.9 (it won't let you set .05 increments).

The pulses are generated at a uniform frequency -- you're supposed to observe the doppler effect, the fact that the pulses are bunched up in front and spread out behind. This results in the frequency shift you hear in a passing race car.

However, the other effect is that at the moment pictured (where the first sound wave is just about to reach the observer), you're actually listening to a sound that was generated 100 m away. The intensity of a sound drops off with the inverse square of the distance, so doubling the distance you cut the intensity by 4.

Your eyes trick you -- you're seeing the aircraft very close, but you're actually listening to the sound made when it was far away.

In our case, the sound was produced 10x as far away as what we are seeing. Which means that the sound intensity will be 1/100th as intense as we would expect for a nearby aircraft.

• I see, thanks. So if the waves that travel forward in front of the object slowly, that means that those waves will slowly ‘reach out’ if that makes sense. If this is the case, why can you only hear a jet after it reaches close to you going just below supersonic? Wouldn’t those waves traveling forward or ‘reaching out’ make it so you could hear the jet earlier? Apr 14 at 22:57
• The waves that are reaching out are the ones in front of the object that slowly progress in the direction of travel of the object, as seen in the simulation you provided. Apr 14 at 22:58
• (The video of the jet passing by I saw was an F/A-18 that was going almost supersonic and you couldn’t hear it until it was almost right beside the camera) Apr 14 at 23:03
• The waves travel at the same speed no matter what. They travel at the speed of sound. Period. Full stop. If we approximate the speed of sound as 1000 ft/s and the aircraft is traveling at M 0.95, then when you hear the sound produced one second ago, you're hearing the sound produced 1000 ft. away. Since that time, the sound traveled 1000 ft, and the airplane traveled 950 ft. So you're seeing an airplane 50 ft. away, but hearing it 1000 ft. away -- which is why it sounds quiet. The airplane is still making noise (when it is 50 ft away), but that sound hasn't reached you yet. Apr 15 at 4:53
• Ah okay, that makes sense. As you said, your eyes definitely trick you. Thanks. Apr 15 at 18:25

Why can't you hear an aircraft traveling at mach .95 until it passes?

You can.

So to summarize, if the aircraft was going Mach 0.95, and the sound waves would slowly travel forward, wouldn't that make it so you could hear the aircraft before it got to you?

Yes, that is correct.

In a video of a F/A-18 I saw, it was traveling at what I assume was ~Mach 0.9 speeds, because you couldn't hear any sound from it until it was really close.

This observation confirms your hypothesis in the second quote, and disproves the faulty assumption in the title question. You couldn't hear it until it was really close because it was traveling almost as fast as the sound waves it was creating. If it were supersonic you couldn't hear it until it was actually past you.

• Ah okay, thanks. But back to the cars on the highway example, wouldn't the faster car (the sound wave) get further in front of the slower one with time, meaning that you would still hear the aircraft even thought it wasn't close? Hope that makes sense haha. Apr 14 at 17:51
• Yes, but the speed of the car is much, much less that the the jet so the effect is quite different. Actually typing this on my phone by a highway and as i watch cars approach i really don't hear them until a quarter mile or so. Granted a jet is much louder, so at automobile speeds you'd hear it further out, but again it's way faster too ... Apr 14 at 19:35

Now say there's a speaker or something on the nose of the aircraft facing forward. If the aircraft is traveling at Mach 0.95, it still isn't going faster than the sound waves it's making. So that means the sound waves would still slowly pull away from the aircraft going forward, right? If that's the case, wouldn't you hear an aircraft going Mach 0.95 before it passes?

Correct, you hear the airplane (or the speaker) before the airplane itself passes.

Let's suppose to be at sea level in standard atmosphere. Sound travels at 340m/s (0.2 mi/s) while the airplane travels at 340x0.95=323m/s. Let's say that you are just in front of the airplane (or the speaker) and that you begin to hear it at a distance of 3000m (1.86mi). Then you'll be hit by the sound after 3000/340=8.82s and by the airplane after 9.59s. So if you couldn't see the airplane but only hear it then you would have only 0.76s to get out of the way.

• Oh okay thanks, so would the aircraft be quieter before it passes going Mach 0.95? If so, why? Apr 14 at 19:09
• Why would it be quieter? It should make about the same sound at almost any speed below mach. Apr 14 at 19:37
• Yes, it would be quieter but for the simple reason that the sound that you hear after the 8.82s is the faint sound emitted when the airplane was 3000m away. Apr 14 at 20:56