The police car moves faster than the speed of sound?

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Discussion Overview

The discussion centers around the scenario of a police car moving faster than the speed of sound and whether a person on the street would hear the siren. Participants explore concepts related to sound propagation, the Doppler effect, and the implications of relative motion in different frames of reference.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that if the police car moves faster than the speed of sound, the person will not hear the siren due to the sound being Doppler shifted to zero frequency.
  • Others argue that the speed of sound is constant relative to the medium (air) and that the speed of the sound source does not change the speed of sound itself.
  • A participant mentions that the speed of sound relative to the driver is not constant in all directions, especially when considering motion through the air.
  • Some participants discuss the implications of special relativity, noting that sound behaves differently than light and that the laws of physics apply differently to sound in moving frames.
  • There is a suggestion that if sound is generated inside a closed car, the perception of sound may differ based on the relative motion of the observer and the sound source.
  • Concerns are raised about the clarity of scenarios presented, particularly regarding what is moving relative to what.

Areas of Agreement / Disagreement

Participants express multiple competing views on the relationship between the speed of sound, the motion of the car, and the perception of sound by the observer. The discussion remains unresolved with no consensus reached.

Contextual Notes

Participants highlight limitations in their scenarios, including assumptions about the closed environment of the car and the effects of relative motion on sound perception. The discussion also touches on the complexities of sound propagation in different frames of reference.

powermind
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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,
 
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Yes.
 
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.
 
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You don't need anything more than time = distance / speed.
 
Do you mean that the speed of the siren's sound is not affected by the siren speed?
 
Yes. Frequency is affected, but the speed of the source doesn't change the speed on the sound itself.
 
powermind said:
The speed of the sound relative to the driver which equals 343 m/s.
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.
 
Borek said:
I have moved the sonic boom discussion, please continue it here: https://www.physicsforums.com/showthread.php?t=732594

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
Thanks for all the cleanup work, Borek!
 
  • #11
I am sure he cannot hear the siren because the sonic boom is more loud than siren :)

A.T. said:
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.
It means that the speed of sound is directly proportional to the speed of its source!
 
  • #12
powermind said:
It means that the speed of sound is directly proportional to the speed of its source!
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
powermind said:
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!

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
Many thanks
 
  • #15
A.T. said:
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.

Does this disagree with special relativity: the laws of physics are invariant in all inertial systems?
 
  • #16
No. SR is irrelevant. Sound is not EM.
 
  • #17
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
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?
 
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  • #19
What about it? Sound moves at the speed of sound wrt the air.
 
  • #20
powermind said:
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?
The speed of sound is the same in all directions relative to the airmass in which it propagates.
 
  • #21
powermind said:
Does this disagree with special relativity: the laws of physics are invariant in all inertial systems?
Which law of physics would be different?
 
  • #22
A.T. said:
Which law of physics would be different?

You said:
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.
 
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  • #23
powermind said:
You said:
Yes, and?
 
  • #24
If the observer and the sound source are stationary relative to the air the speed of the sound is 343m/s.
But if both moves relative to the air, you said that, as I understood, the speed of the sound relative to them will be not equal 343m/s. So they can know that they are moving relative to the air!
Suppose the observer stands at the back of the car and the sound source is located at the front. In case of moving, he will hear the sound in different time.
 
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  • #25
powermind said:
So they can know that they are moving relative to the air!
Sure. You don't need sound for that. You can feel the wind in your face, or use an anemometer. What is your point?
 
  • #26
powermind said:
So they can know that they are moving relative to the air!
Obviously yes.
 
  • #27
A.T. said:
Sure. You don't need sound for that. You can feel the wind in your face, or use an anemometer. What is your point?

But I mentioned that the all windows are closed and nothing enters inside! I add you cannot see anything outside.

http://jcer.com/index.php/jcj/article/viewFile/80/78 (P 532)
Surely, we ask, he must know that he is moving! This explains the difference easily. But, said Einstein, perhaps he does not know that he is moving. Perhaps he thinks he is at rest. Perhaps he really is at rest. Perhaps it is you who are moving. How do we know? This became the essence of the first of two major postulates proposed by Einstein and which underpin his theory.
The postulate is stated as, “the laws of physics are the same (invariant) in all inertial (reference) frames.” It can equally be called the “reciprocity of reference systems.” It implies that any observer has the right to declare himself at rest and all others in motion with respect to him.

I hope you understood my point!
 
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  • #28
Sound is not light and air is not vacuum. I hope you understood my point.

You can do the analysis in any reference frame that you like, and by doing so you can change the speed of the air and the speed of the sound waves. What you cannot do is change the speed of the sound waves relative to the air, nor can a change of reference frame change the speed of the air relative to the car or whatever other apparatus or observers that you might choose to include.
 
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  • #29
powermind said:
But I mentioned that the all windows are closed and nothing enters inside!
You need to be more clear in your scenario. What is moving relative to what? What is inside, what outside the car?
 
  • #30
This thread is getting to be full of people trying to justify things they said near the top and missing the main point of the original question. It is hard to avoid, I know.
 

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