Passage V

As a sound wave travels through a medium, the wave becomes attenuated (loses energy). The attenuation coefficient, α, is the rate at which the wave's intensity level (a measure of sound volume) decreases with distance as a result of this energy loss; the greater the value of α, the greater the decrease in intensity level with distance. Figure 1 shows, for waves of 3 different frequencies (in hertz, Hz), how α (in decibels per kilometer, dB/km) varies with temperature in air at 10% relative humidity.

Figure 2 shows, for waves of 3 different frequencies, how α varies with relative humidity in air at 20°C.

Figures adapted from Richard Lord, "Calculation of Absorption of Sound by the Atmosphere." ©2004 National Physical Laboratory.

32. Suppose that 2 sound waves—a 150 Hz wave and a 1,100 Hz wave—are simultaneously emitted from a speaker into air at 20°C and 45% relative humidity. Based on Figure 2, as the waves travel away from the speaker, the intensity level of which wave will more likely decrease at the greater rate due to attenuation?

Answer and Explanation

Your Answer is

Correct Answer is J

Explanation

Looking at Figure 2, when the abscissa corresponds to 45, comparing the y values ​​of 150Hz and 1000Hz, we can see that the α of 1000Hz is bigger, and according to the second sentence of the first paragraph of the article, the greater the value of α, the greater the decrease in intensity level with distance, the larger the value of α, the greater the decrease in intensity level with distance, so 1000Hz with a larger α is more likely to drop sharply.