Lewis L. Spence
Objective: To determine time delay of a sound
When someone speaks in a room, the speech is intelligible if there is an appropriate time delay for the reflected sound. This delay is the difference between the time it takes for the direct sound and the reverberating sound to get to the listener. A time delay longer than 0.075 seconds (75 milliseconds) produces echoes which distort the listening process.
Activity:
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Jennifer sits 30 feet from the speaker in an auditorium which is140 feet long. What is the delay time, T?
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Solution: Distance from Jennifer to the back of the room is 140-30 = 110 ft.
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Total distance of the path of the reflected sound from the speaker to the back of the room reflected to Jennifer: 140 + 110 = 250 ft.
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Distance of the path of the direct sound from the speaker to Jennifer = 30ft.
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Difference in path of the distance of the reflected sound and the direct sound: 140 + 110 – 30 = 220ft.
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(220 ft.is the extra distance the reflected sound had to travel compared to the direct sound)
What is the delay time, T? (The time taken to travel 220 ft.)
Sound travels 1100 feet per second. Therefore the time for 220feet is, T = 220/1100 = 0.2 seconds
Would this be an acceptable time delay?
If this time delay is unacceptable, what can be done to improve the situation?
Project: Students cold use cardboard material to design the four walls of the room. A single piece of polystyrene (that white packing material) can be used for the floor. Toothpicks with cutouts can be used to show the location of the speaker and the listener. Using a scale of one inch being equivalent to ten feet, the students can show varying adjustments from which they can calculate several time delays. To show the path of the sound wave, the students could use pieces of thread of different colors to indicate the direct wave to the listener and the reflected wave from the back of the room leading to the listener.
Evaluation:
What is time delay?
What is an acceptable time delay?
How does excessive time delay affect sound quality?
How is time delay calculated?
