The reception of multiple reflections off of walls and ceilings within 0.1 seconds of each other causes reverberations - the prolonging of a sound. If a reflected sound wave reaches the ear within 0.1 seconds of the initial sound, then it seems to the person that the sound is prolonged. Why the magical 17 meters? The effect of a particular sound wave upon the brain endures for more than a tiny fraction of a second the human brain keeps a sound in memory for up to 0.1 seconds. A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. Reflection of sound waves off of surfaces can lead to one of two phenomena - an echo or a reverberation. This gives the room more pleasing acoustic properties. These materials are more similar to air than concrete and thus have a greater ability to absorb sound. Walls and ceilings of concert halls are made softer materials such as fiberglass and acoustic tiles. A hard material such as concrete is as dissimilar as can be to the air through which the sound moves subsequently, most of the sound wave is reflected by the walls and little is absorbed. For this reason, acoustically minded builders of auditoriums and concert halls avoid the use of hard, smooth materials in the construction of their inside halls. As discussed in the previous part of Lesson 3, the amount of reflection is dependent upon the dissimilarity of the two media. When a wave reaches the boundary between one medium another medium, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission across the boundary. In this part of Lesson 3, we will investigate behaviors that have already been discussed in a previous unit and apply them towards the reflection, diffraction, and refraction of sound waves. Possible behaviors include reflection off the obstacle, diffraction around the obstacle, and transmission (accompanied by refraction) into the obstacle or new medium. Rather, a sound wave will undergo certain behaviors when it encounters the end of the medium or an obstacle. Like any wave, a sound wave doesn't just stop when it reaches the end of the medium or when it encounters an obstacle in its path.
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