The document describes interference patterns created by two sound sources with the same frequency emitting sound waves into a room. Constructive interference occurs at points where wave crests and troughs overlap, specifically at points B, C, and D. Destructive interference only occurs at point C, where a crest and trough meet. If the frequencies of the two sources are changed, with one increasing greatly and the other decreasing greatly, the sound would be heard as two distinct tones rather than a single tone with varying intensity due to the large difference between the frequencies.