Using the diagram at the right and thinking of what you have already leaned about the relationship between frequency and the wavelength of a sound wave, complete the two sentences here:

a. as a moving sound source approaches a stationary observer, the frequency of the sound increases, therefore the wavelength or shorter)

b. as the sound source moves away from the observer, the pitch of the sound and the wavelength

a. as a moving sound source approaches a stationary observer, the frequency of the sound increases, therefore the wavelength is shorter

Explanation:

This effect is known as Doppler effect. When a moving sound source approaches a stationary observer, the wavefronts of the wave appear to be closer to each other: as a result, the frequency of the sound wave appears to be increased. The wavelength of the sound is inversely proportional to the frequency:

where v is the speed of the wave and f the frequency: therefore, as the frequency increases, the wavelength gets shorter.

b. As the sound source moves away from the observer, the pitch of the sound decreases and the wavelength increases

Explanation:

When the sound source moves away from the observer, the effect is opposite: the wavefronts appear to spread apart from each other, so the frequency of the sound appears to decreases, and as a result, the wavelength increases.

The pitch of a sound is related to how we perceive the sound, and it is directly proportional to the frequency: therefore, since the frequency decreases, the pitch decreases as well.