# The police car moves faster than the speed of sound?

1. Jan 13, 2014

### powermind

Hi,
Suppose the police car moves faster than sound's speed and runs away from the person on the street. Will the person hear the siren?

Regards,

Last edited: Jan 13, 2014
2. Jan 13, 2014

### maajdl

Yes.

3. Jan 13, 2014

### powermind

But the speed of the siren's sound related to the outside person=The speed of the sound relative to the dirver-the speed of the car=343-343=0!
I am talking about the sound which goes to the person.
If this is wrong, when will the person hear the siren if you have the following:
1- The speed of the car which equals 343 m/s.
2- The speed of the sound relative to the driver which equals 343 m/s.
3- The distance between the siren and the persion when the siren starts working.

Last edited: Jan 13, 2014
4. Jan 13, 2014

### Staff: Mentor

You don't need anything more than time = distance / speed.

5. Jan 13, 2014

### powermind

Do you mean that the speed of the siren's sound is not affected by the siren speed?

6. Jan 13, 2014

### Staff: Mentor

Yes. Frequency is affected, but the speed of the source doesn't change the speed on the sound itself.

7. Jan 13, 2014

### A.T.

That is wrong. The speed of sound relative to the air is 343 m/s. If you move though the air, you see different speed of sound, in different directions.

8. Jan 13, 2014

### Staff: Mentor

9. Jan 13, 2014

I'd argue that a good chunk of that was not sonic boom related and rather about the propagation of waves in air, which is relevant to the OP's question still.

10. Jan 13, 2014

### Staff: Mentor

Thanks for all the cleanup work, Borek!

11. Jan 14, 2014

### powermind

I am sure he cannot hear the siren because the sonic boom is more loud than siren :)

It means that the speed of sound is directly proportional to the speed of its source!

12. Jan 14, 2014

### A.T.

In the rest frame of the medium (air) the speed of sound is the same in all directions. In a frame that moves relative to the medium the speed of sound depends on the propagation direction. The speed of the sound relative to the driver is not 343 m/s in all directions. Especially not forwards and backwards.

13. Jan 14, 2014

### sophiecentaur

I don't think you really mean that. The speed and frequency is perceived relative to the speed of the observer (e.g. by two microphones and an on board timing device). The source velocity will affect the frequency of the sound radiated in different directions (Doppler) but, once launched, the sound travels at the asme speed in all directions appropriate to the atmosphere carrying it.

Sound laid down (the siren) by the receding car will be doppler shifted to zero frequency if the car's speed is Mach 1+. So I don't think that's an issue as the siren sound would just be infra-sonic.
Disturbances in the air, left behind by the car are less easy to analyse. Apart from the fast motion of the when it's disturbed by the car moving through it, the air will soon not be moving at Mach 1+ so you are bound to hear something. Perhaps a general low amplitude 'rumbling' due to turbulence(?) but nothing coherent like in the cone of the boom in the forward and side direction.

14. Jan 14, 2014

### powermind

Many thanks

15. Jan 15, 2014

### powermind

Does this disagree with special relativity: the laws of physics are invariant in all inertial systems?

16. Jan 15, 2014

### sophiecentaur

No. SR is irrelevant. Sound is not EM.

17. Jan 15, 2014

### Staff: Mentor

Let me expand on that a bit:
You are mixing together the two postulates. The postulate that the laws of the universe are the same everywhere applies everywhere, including here.

The postulate that says the speed of light is constant in all frames applies only to light (setting aside the broader implications of the theory that don't show-up at low speed). In other words, the fact that the speed of light is the same in all frames does not imply that the speed of sound should be the same in all frames.

18. Jan 16, 2014

### powermind

A.T, your answer will be differnet if the sound is generated inside the car which is nothing enters from outside. i.e the windows are closed?

Last edited: Jan 16, 2014
19. Jan 16, 2014

### Staff: Mentor

What about it? Sound moves at the speed of sound wrt the air.

20. Jan 17, 2014

### A.T.

The speed of sound is the same in all directions relative to the airmass in which it propagates.