Explain the conditions in which a person can or can’t hear an echo. Which of the following materials would be the least efficient in soundproofing a room: clay tile, plaster, or perforated acoustic ceiling material? Be sure to use your knowledge of the coefficient of absorption in your explanation. Compare and contrast infrasonic and ultrasonic vibrations. Describe the responses of the human ear to sound waves coming from the right side, left side, or in phase.