Stationary Source vs. Stationary Detector

[collegelife101]

A detector is approaching a stationary source of sound waves. The frequency perceived by the detector will be higher than the frequency emitted by the source because the speed of sound relative to the detector has increased.

A source of sound waves approaches a stationary detector. The frequency perceived by the detector will be higher than the frequency emitted by the source because the wavelength of the sound wave is shorter.

Can someone please distiguish between these two statements? Why is speed changing in one and wavelength changing in the other? I would think that the wavelength would change in both cases since the compression of waves is arriving at a shorter amount of time? Wouldn't velocity be constant since we're still in the same medium?!

Thanks!

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[Teleologist]

It's not speed that's changing it's relative speed.

 

[NextStepTutor_1]

Let's look at the two cases in a little bit more detail. In the first case, the detector is moving towards the source. Thus, the relative speed of the waves increases because the detector moves in the direction of the incoming wave, however, the wavelength stays the same because the distance covered by the detector is still the same. The wave itself has not changed.

However, in the second case, when the source is moving, the wave actually changes--as you said, the wave gets "scrunched up." This means that the wavelength of the wave is decreased--however, once it leaves the source, it travels at the speed of sound, so it's relative speed hasn't changed at all.

Hope that helps!