A sound source of frequency 170Hz is placed near a wall. A man walking from the source towards the wall. Find that there is a periodic rise and fall of sound intensity?
first, what's the wavelength?
Is it long enough to notice variations?
The phenomenon you are describing is known as the Doppler effect. It refers to the change in frequency (and thus, pitch) of a sound wave as heard by an observer when there is relative motion between the source of the sound and the observer.
In this case, as the man walks towards the wall, he is moving towards the sound source. This movement causes the sound waves emitted by the source to be "compressed" (i.e., the wavelength decreases) in front of the moving man and "stretched" (i.e., the wavelength increases) behind him.
As a result, the frequency of the sound waves perceived by the man increases as he approaches the wall and decreases as he moves away from it. This change in frequency leads to a periodic rise and fall of sound intensity, created by the variations in the compression and stretching of the sound waves.
To calculate the exact frequency and intensity variations, we need to know the man's velocity relative to the source of the sound and the speed of sound in the medium. Once these values are known, we can apply the formula for the Doppler effect to determine the resulting changes in frequency and intensity.
The formula for the Doppler effect relating the observed frequency (f') to the emitted frequency (f), the velocity of the source (v), the velocity of the observer (u), and the speed of sound (c) is as follows:
f' = f × (c + u) / (c + v)
By substituting the given values into this equation, you can calculate the observed frequency at different positions of the man relative to the wall. The intensity of the sound wave, however, depends on factors such as distance and the spreading of the wave, and would need additional information to be accurately calculated.