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.

31. Consider a 1,000 Hz sound wave in air at 10% relative humidity. At how many of the temperatures shown in Figure 1 does α for this wave have a value of 18 dB/km ?

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

The problem is to consider the 1000hz sound wave in the air with a relative humidity of 10%. At the temperatures shown in Figure 1, how many times does this wave have an alpha value of 18 dB/km? Look at Figure 1, which represents the top line at 1000Hz. When the ordinate is 18, there are 4 abscissas corresponding to the temperature of 5, 15, 67, and 98 degrees.