Frequency Confusion

[deleted388502]

I'm so confused on when frequency actually changes.

TPRHSW gave me two questions in a row:

1. A sound wave with frequency f travels through air at a speed v. With what speed will a sound wave with frequency 4f travel through air?
A. v/4
B. v
C. 2v
D. 4v

answer is B, because changing frequency should not effect velocity

2. A device that generates sound tones can be controlled by a dial that, when turned clockwise, will cause the frequency of the tone produced to increase. Which one of the following describes the effect of turning the dial in this way?
A. The wavelength decreases
B. The wavelength increases
C. The wave speed decreases
D. the wave speed increases

answer is A, because wavelength changes in order to accommodate the fact that velocity does not change and frequency has changed.

I'm SO CONFUSED about when we actually use v = frequency x wavelength. I understand that frequency is a measure of energy and it does not change when we change mediums, and wavelength changes in regards to n = c/v, however, in both of these scenarios I'm very confused by especially why frequency is not changing the velocity of the sound wave by v = frequency x wavelength.

Thanks so much in advance!

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[DrknoSDN]

The velocity of a wave in a medium is a constant (as long as conditions don't change). So you should always assume that if the wave stays in the same medium (i.e. air in these examples), it's velocity will be constant.

For something like this I usually visualize a speaker or sub-woofer and imagine that the waves propagating away always flow at the same speed out into the air. Bumping up the frequency would just make the waves more compacted together.

 

[NextStepTutor_2]

Completely agree! Specifically in your two examples, you're not changing the medium, so velocity won't change. Instead, when you increase f by a factor of 4 in question #1, wavelength will decrease by a factor of 4, leaving the product of the two (velocity) unchanged.