Final answer:
When 2 sound waves with slightly different wavelengths combine, they create beats; this results from the alternating constructive and destructive interference that causes variations in loudness.
Step-by-step explanation:
When 2 sound waves with slightly different wavelengths combine, they create a phenomenon known as beats. This occurs because the waves superimpose and result in alternating constructive and destructive interference, which can cause the sound's loudness to fluctuate over time. This effect can be observed in various scenarios such as the discordant sound from striking two adjacent piano keys or the varying volume of jet engine noise as experienced by a stationary passenger. The superposition principle explains how these wave interactions can vary from place to place and moment to moment, with the interference between sound waves adding partially constructively and partially destructively based on their relative phases and path lengths traveled.