Question regarding wave's speed/frequency in changing medium

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From TPR:

Big Rule 2: When a wave passes into another medium, its speed changes, but its frequency does not


So, I'm assuming that when the temperature of a medium is increased, its medium has changed, and the wave has passed into another medium.......?

Whether or not the above reasoning is correct I'm having issues with the following problem..

From TBR:
(The question is from physics passage VII of section 6 [sound]. The passage basically talks about putting a string into an open pipe. Making 2 waves in the system, one standard wave (for the string) and one sound wave (for the pipe)]

If my above summary of the passage is wrong then please correct.

44. If the temp. of the room changes from 30 deg celcius to 33 deg celcius, which of the following statements is true? (Ignore any thermal expansion of the string and the pipe.)

A. the pitch of the wave produced in the pipe changes, the pitch of the wave produced by the pipe does not

B. the pitch of the wave produced in the pipe changes, the pitch of the wave produced by the string does not

C. The pitch of both waves changes

D. The pitch of neither wave changes

The solution says B is the answer. But according to Big Rule 2 how does the pitch of wave produced by the pipe change (since it has gone into another medium of thinner density from the increased temperature)?

 

[mcloaf]

From TPR:

Big Rule 2: When a wave passes into another medium, its speed changes, but its frequency does not


So, I'm assuming that when the temperature of a medium is increased, its medium has changed, and the wave has passed into another medium.......?

Whether or not the above reasoning is correct I'm having issues with the following problem..

From TBR:
(The question is from physics passage VII of section 6 [sound]. The passage basically talks about putting a string into an open pipe. Making 2 waves in the system, one standard wave (for the string) and one sound wave (for the pipe)]

If my above summary of the passage is wrong then please correct.

44. If the temp. of the room changes from 30 deg celcius to 33 deg celcius, which of the following statements is true? (Ignore any thermal expansion of the string and the pipe.)

A. the pitch of the wave produced in the pipe changes, the pitch of the wave produced by the pipe does not

B. the pitch of the wave produced in the pipe changes, the pitch of the wave produced by the string does not

C. The pitch of both waves changes

D. The pitch of neither wave changes

The solution says B is the answer. But according to Big Rule 2 how does the pitch of wave produced by the pipe change (since it has gone into another medium of thinner density from the increased temperature)?

You have the right idea, but you're a little bit off. When they talk about crossing from one medium to another changing wave speed by altering wavelength at a constant frequency, they mean the sound wave actually crossing an interface between two different media--which is not happening in this question.

In the case of the wave in the pipe, you're not crossing an interface, just looking at what will happen when temperature increases. It sounds like you've already figured out that sound velocity must increase because as the temperature rises the density of the air is decreasing. Good. But what's fixed in this case? The wavelength, which is defined by the length of the pipe. So if velocity is increasing at a fixed wavelength, then the frequency must be concomitantly increasing as well, which an observer would hear as a higher pitch.