Final answer:
Betty heard the violin but her friends didn't because of sound attenuation, which refers to the decrease in sound intensity as it travels through a medium. The distance and surrounding obstacles affected the level of attenuation experienced by Betty and her friends.
Step-by-step explanation:
The reason why Betty heard the violin but her friends didn't is because of a phenomenon called attenuation. Attenuation refers to the decrease in intensity of sound as it propagates through a medium. In this case, the sound waves from the violin were most likely attenuated by the surrounding obstacles, such as buildings or other objects, before reaching Betty's friends.
Since Betty was positioned at the back of the outdoor concert hall, she was closer to the street where the violinist was playing. This means that the sound waves had less distance to travel before reaching Betty's ears, resulting in less attenuation and allowing her to hear the violin.
On the other hand, Betty's friends were further away from the source of the sound. The sound waves had to travel a longer distance and encounter more obstacles, causing greater attenuation and making it difficult for them to hear the violin.