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In the context of the Doppler Effect, sound frequency is how often the sound wave's compressions reach an observer. Frequency is usually measured in Hertz (Hz), which tells us the number of wave cycles per second. When a sound source moves relative to an observer, the frequency heard by the observer is altered. This change is what we know as the Doppler Effect.

A sound with a higher frequency sounds like a higher pitch, while a lower frequency sounds like a lower pitch. In the example given, the car horn has a rest frequency of 440 Hz, which corresponds to the MUSICAL NOTE A. However, when the car (the sound source) is moving, the observer, in this case, a bicyclist, hears a different frequency. Instead of 440 Hz, the cyclist hears 415 Hz, indicating the sound waves' behavior differs due to their relative motion.

The movement of the observer can significantly influence the perceived frequency. In the given problem, the observer is the bicyclist. Their motion compared to the source (car) alters the frequency of the sound they hear.

Since the cyclist hears a lower frequency than the car emits, it suggests they are moving ahead and away from the car. If they were moving toward the car, the observed frequency would be higher. This scenario is a classic illustration of an observer moving relative to a sound source, impacting how sound waves are compressed or stretched. The observer's motion is thus pivotal in explaining why frequencies seem higher or lower, dependent on the direction of movement.

Sound waves change behavior when either the source or the observer is in motion. This shift in behavior can be observed as a change in frequency, where wave fronts become squeezed together for an approaching sound source, or spread apart for a receding one.

In our exercise, the car speeds down the street, affecting the sound waves it emits. Because the observer (cyclist) is moving away, each wave front takes slightly longer to reach them compared to someone stationary. This is why a lower frequency is heard. The waves are stretching out, a characteristic marker of the Doppler Effect related to motion. Such knowledge of sound wave behavior is crucial in understanding everyday phenomena, like how an approaching vehicle's horn changes in pitch as it moves past and away from you.