Frequency change with velocity change

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Discussion Overview

The discussion centers on whether the received frequency of a sound wave changes when the medium's velocity differs at various locations, particularly in the context of underwater acoustics. Participants explore the implications of sound transmission through different media characterized by varying pressure, temperature, and salinity.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions if the received frequency differs from the transmitted frequency due to changes in medium velocity, providing a specific example involving a sub-sea pinger and varying conditions at different depths.
  • Another participant asserts that frequency does not change with the medium, claiming that only the wavelength is affected when the speed of sound changes.
  • A third participant elaborates on the concept by describing a scenario with a distinct boundary between two media, suggesting that while the wavelength shortens in the lower velocity medium, the frequency remains constant.
  • Another participant emphasizes that the frequency is determined solely by the source, stating that all particles affected by the wave oscillate at the same frequency as the source, while the velocity of propagation is dependent on the medium.

Areas of Agreement / Disagreement

Participants express differing views on whether frequency changes with medium velocity. There is no consensus, as some argue that frequency remains constant while others suggest that it could vary under certain conditions.

Contextual Notes

Participants discuss the relationship between frequency, wavelength, and medium velocity, but the discussion does not resolve the implications of these relationships in the context of varying environmental conditions.

JeremyA
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Does the received frequency at a location differ from the transmitted frequency when the medium velocity differs at the two locations?

To give a concrete example, a sub-sea pinger is transmitting at 37.5 Khz 4,000 metres below sea level and at a particular pressure, salinity, and temperature. A receiver nearer the surface detects the signal at a different pressure, temperature, and salinity. Finally a surface vessel detects the signal under yet another set of pressure, temperature and salinity.

For reference the velocity of sound in the oceans can vary by up to 10% due to these factors.

My intuitive guess is that sound transmitted in a high velocity domain will increase in frequency when it hits a low velocity domain?
 
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No ... frequency does not change with the media; only the wavelength changes.

Speed = frequency x wavelength; so if the speed changes (due to the media), then the wavelength must change.

Frequency is the driver ...
 
Suppose for simplicity there existed a distinct boundary between the two mediums. And further suppose that we watched the crest of the wave from the time it hit the boundary and traveled onward in the lower velocity medium. This crest will not have traveled a full wave length by the time the following crest hits the boundary. This has the effect of shortening the wave length. But since the speed also is shortened then the frequency remains constant.
 
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The Frequency of the wave ONLY depends on the source producing it, If the source makes 30 thousand complete oscillations in one second then all the particles affected by the waves will make 30 thousand complete oscillations and the frequency will be 30 kHz,
the velocity of propagation of the wave ONLY depends on the medium while the wave length depends on both the frequency and the velocity of the wave according to this equation velocity = frequency x wave length
 
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