Answer fully and use diagram

2. A few minutes before Austin's recent year-end mass in the gym, the some of the musicians present conducted a warm-up of familiar rock songs. Prior to entering the gym, Mr. Cooper could only hear certain instruments from the hallway outside the gym. Upon entering the gym, the entire collection of the musical instruments being played could be heard. What types of instruments could Mr. Cooper hear in the external hallway? Why could he not hear every instrument until he entered the gym? What physical phenomenon was responsible for this observation? Use a diagram to assist your response to this question. (3)