Redshift of Sound in a Gravitational Field

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Discussion Overview

The discussion centers on whether sound waves experience a redshift in a gravitational field similar to light, exploring the equations governing frequency changes and potential similarities to the Doppler effect for sound. The scope includes theoretical considerations and conceptual clarifications related to sound propagation in varying gravitational potentials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the redshift of sound in a gravitational field may be analogous to the Doppler shift for sound waves, though the concept of a "gravitational field" is questioned by others.
  • One participant notes that the velocity of sound is proportional to the square root of pressure over density, suggesting a relationship to the propagation of sound waves.
  • A later reply discusses the frequency shift of sound when emitted at one gravitational potential and received at another, indicating that the frequency change is due to different time rates at the two locations.
  • Another participant provides a specific equation for the gravitational shift of sound waves, noting that it differs from light due to the nature of the gravitational field affecting sound versus light.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the redshift for sound in a gravitational field, with some suggesting it aligns with the Doppler effect while others present alternative explanations. The discussion remains unresolved regarding the precise nature and implications of these shifts.

Contextual Notes

Participants mention various assumptions, such as the strength of the gravitational field and the conditions under which the frequency shifts occur, indicating that the effects may be negligible for conventional sound frequencies except near dense masses.

arhanbezbora
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Hi,

Do sound waves experience a redshift similar to that of light in a gravitational field?
Does anyone know the equation governing the change in frequency of the sound?
Is this the same as the Doppler shift for an accelerating source of sound?

Thanks.
 
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I believe it's just the Doppler shift for sound waves.
I'm not quite sure what you mean by a "gravitational field" though.
 
You will find that the velocity of sound is proportional to the sqrt(pressure/density).
Check the units to verify: sqrt[(kilogram-meters/sec2)(meters3/kilograms)] = meters/sec.
 
arhanbezbora said:
Hi,

Do sound waves experience a redshift similar to that of light in a gravitational field?
Does anyone know the equation governing the change in frequency of the sound?
Is this the same as the Doppler shift for an accelerating source of sound?

Thanks.

If sound waves are emitted at one location (within a medium which will propagate sound, such as air) and received at another where the two locations differ in gravitational potential, then the sound will be apparently shifted in frequency because of the different time rates at the two locations, exactly as for any other form of signal. Exactly the same shift would occur if the sound was transmitted live over a radio link instead (in which case the radio signal frequency would also be shifted).

If the field is of approximately constant strength g and the displacement in the direction of the field is h, then the fractional shift is gh/c2 (so the frequency appears to be increased or blue-shifted when it reaches an observer at a lower potential, and decreased or red-shifted at a higher potential). This effect would obviously be far too small to measure for conventional sound frequencies except in the vicinity of an extremely dense mass.
 
It is resolved in a paper "Graphene,neutrino mass and oscillation". The gravitational shift df or sound waves in air on the surface of the Earth is f.dr/R (f is frequency, dr is height difference and R=6,400km is radius of the earth.) The result is dissimilar to that of light because the gravitational field is a weak field for light and strong for sound waves.
 

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