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FROM A BUZZ TO A BELLOW The frequency range of the human ear, great as it is, is exceeded by its intensity range. Like frequency, the intensity, or magnitude, of a sound is a physical property that can be measured precisely. And just as frequency has its subjective partner, pitch, so intensity is sensed as loudness. The physicist measures the intensity of sound in terms of watts of power striking a square centimeter of any surface (such as the surface of the eardrum). But even though a watt is a fairly small unit, it is still an unwieldy measure of soun4. At .000001 watt, a level that might be achieved by an automatic lathe, sound becomes annoying noise. At .001 watt, the sound intensity of a battery of cannon-an increase of 10 million million times over the faintest audible sound-the noise becomes physically painful. Fortunately, the loudness of a sound is not directly proportional to its intensity. A built-in mechanism cuts down the sensitivity of the ear as intensity increases, so that a battery of cannon does not sound 10 million million times louder than a mosquito buzz. When the intensity of sound is doubled, the loudness, instead of doubling, increases by about 23 per cent. To measure the relative intensity of different sounds, scientists have worked out the ratios of loud sounds to softer ones. The scientist’s unit of measure is the decibel-deci for one tenth and bel after Alexander Graham Bell, the inventor of the telephone. A sound 10 times as powerful as another is said to be 10 decibels more intense, . and each 10-fold increase of intensity adds another 10 decibels to the level of the sound. A sound 1,000 times as intense as another is 30 decibels stronger; a sound 100,000 times as intense is 50 decibels stronger, and so on. The decibel measure gives a rough connection between the physical intensity of sound and the subjective loudness it causes. A change in intensity of approximately three decibels causes the smallest change in loudness that the average human can sense. For measuring the sounds of ordinary life, a decibel level of zero represents the faintest sound audible to the average ear. Sounds become physically painful above 130 decibels. For most people the level for conversation is 60 to 80 decibels. Waves of sound are subject to the same influences as other waves. The
environment through which they pass can affect their course in subtle or dramatic ways. Echoes, for example-the bouncing of a sound wave off a wall or cliff-are reflections, like the reflection of a light,wave or a water wave. The roll of thunder in the mountains is a series of reflections, echoing and re-echoing in a pattern called reverberation. The phenomenon of reverberation was first measured by Wallace C. Sabine, a physicist at Harvard University. In 1895 Sabine was asked to try to correct the miserable acoustics of the main lecture room at Harvard’s Fogg Art Museum. He started his work by using a stopwatch and his ear to measure the slow decay of sound as sound waves bounced from wall to wall in the room., To eliminate the absorptive effect of his own clothes and body, he ran his tests while enclofed in a box with only his head sticking out. By these painstaking methods, he found that the reverberation in the newly completed hall was so \great that a word spoken in an ordinary tone bounced back and forth for five and a half seconds. A lecturer might finish a 15-word sentence while his first words were still ringing in his ears.
“ONE SANDERS THEATER CUSHION” To what extent would sound-absorbing materials in the lecture room reduce the reverberation time? Sabine set out to find the answer by careful measurement. As his first testing materials he chose the seat cushions of the nearby Sanders Theater. For a stable sound source, he used the note of a single organ pipe. “On bringing into the lecture room ... cushions having a total length of 8.2 meters,” he wrote, “the duration of audibility fell to 5.33 seconds. On bringing in 17 meters, the sound in the room after the organ pipe ceased was audible for 4.94 seconds.” Sabine was so encouraged by the results that he had all the cushions brought into the room, a few at a time. “When all the seats (436 in number) were covered, the sound was audible for 2.03 seconds. Then the aisles were covered and then the platform. Still there were more cushions- almost half as many more. These were brought into the room... and draped on a scaffolding that had been erected around the room.... Finally, when all the cushions from a theater seating nearly 1,500 persons were placed in the room– covering the seats, the aisles,