Imagine you have two identical plants in two identical rooms and you notice the plant that is in the living room is growing much faster than the plant in the other room. You are wondering about the reason and you try to identify all possible variables or conditions that may affect the plant growth. You see that both plants get the same amount of light, water and nutrients. They are also in the same room temperature. So you think that maybe the sound of radio and television and people's conversation has something to do with the difference in plant growth. 

You decide to perform an experiment to find out if sound affects plant growth.