Sound waves and speed of it in different reference frames

Click For Summary

Discussion Overview

The discussion centers around the speed of sound in different reference frames, particularly in the context of a moving cop car with a siren. Participants explore how the speed of sound is perceived by stationary observers and those in motion, considering concepts such as the Doppler effect and the relationship between sound speed and its medium.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that a stationary observer in front of a moving cop car should measure the speed of sound as Vs = Vc + C, implying that sound speed changes with the observer's frame of reference.
  • Another participant counters that the speed of sound is constant (C) with respect to its medium, the air, and thus a stationary observer relative to the air would measure the speed of sound as C.
  • Further, a participant questions why the cop, who is also moving, would register the same speed of sound, indicating a potential misunderstanding of relative motion.
  • Another response clarifies that the speed of sound relative to the cop is affected by the cop's motion, suggesting a formula that incorporates the speed of sound with respect to air and the speed of air relative to the cop.
  • One participant concludes that sound will travel slower relative to the cop compared to its speed relative to the ground, indicating a difference in perception based on motion.

Areas of Agreement / Disagreement

Participants express disagreement regarding how the speed of sound is perceived in different frames of reference, with no consensus reached on the implications of relative motion on sound speed measurements.

Contextual Notes

Participants discuss the dependence of sound speed on the medium and the effects of relative motion, but there are unresolved assumptions regarding the conditions under which these measurements are made.

physics user1
Think at a cop car with a siren that moves with a velocity Vc, it emits a sound with a velocity C

Now think about a person that doesn't move, in front of the cop car, shouldn't he register that the speed of the sound emited is Vs= Vc+ C?

According to the galileian equations (true for v <<c light)

x= x' + Vc t (distance done by the sound wave according to the guy in front of the cop car)
x'= Ct (distance done by the sound wave according to the cop car frame of reference)

x= (C+ Vc) t

d/dt x = d/dt [ (C+Vc) t]

Vs= C + Vc

Is that correct? Does the speed of the wave change with the frame of reference?

Is because of that that the frequency changes? (Doppler effect)
 
Physics news on Phys.org
Cozma Alex said:
Now think about a person that doesn't move, in front of the cop car, shouldn't he register that the speed of the sound emited is Vs= Vc+ C?
No.

Realize that the speed of sound is C with respect to its medium, the air. So a person at rest with respect to the air will measure the speed of the sound as C.
 
  • Like
Likes   Reactions: physics user1
Doc Al said:
No.

Realize that the speed of sound is C with respect to its medium, the air. So a person at rest with respect to the air will measure the speed of the sound as C.

and... the cop? Why is he registering the same speed?

Ain't the cop and the person in relative motion?
 
Cozma Alex said:
and... the cop? Why is he registering the same speed?
He's not. Here's the basic formula:
Speed of sound with respect to cop = Speed of sound w.r.t. air + speed of air w.r.t cop = C - Vc.

Cozma Alex said:
Ain't the cop and the person in relative motion?
Definitely.
 
  • Like
Likes   Reactions: physics user1
Basically, it will travel slower relative to the car and the speed of sound relative to the ground
 
  • Like
Likes   Reactions: physics user1

Similar threads

Undergrad How fast does a blastwave travel?
  • · Replies 236 ·
8
Replies
236
Views
17K
Undergrad Using reference frames for derivation of Lagrangian
  • · Replies 25 ·
Replies
25
Views
3K
Undergrad Speed of a wave on a string w.r.t. which reference frame
  • · Replies 19 ·
Replies
19
Views
3K
Undergrad Why Can a Sound Wave Not Travel Faster than the Average Molecule Speed?
  • · Replies 23 ·
Replies
23
Views
4K
Graduate Confusion about the derivation of the speed of sound
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad Find period of circular-orbiting source based on max observed freq
  • · Replies 1 ·
Replies
1
Views
2K
Graduate Universal stationary frame of reference for speed, d and t
  • · Replies 7 ·
Replies
7
Views
2K
Graduate Doppler effect with moving medium but no relative motion
  • · Replies 10 ·
Replies
10
Views
4K
Graduate Can the Speed of Sound Be Measured Using Frequency and Time Delay Data?
  • · Replies 23 ·
Replies
23
Views
3K
High School Am I understanding the concept of proper frame of reference?
  • · Replies 33 ·
2
Replies
33
Views
2K