Passage IV
As sound waves travel through seawater, some of their energy is absorbed by the seawater. This sound absorption depends on several factors, including the pH of the water, the depth below the water's surface at which the sound waves travel, the frequency of the sound waves, the water temperature, and the water's salinity (concentration of dissolved salts in the water). Figures 1, 2, and 3 show一under the given conditions, respectively一how sound absorption (in decibels per kilometer, dB/km) varies with frequency (in kilohertz, kHz), temperature (in °C), and salinity (in parts per thousand, ppt; 1 ppt = 1 g/kg).
Figures adapted from Michael A. Ainslie and James G. McColm, "A Simplified Formula for Viscous and Chemical Absorption in Sea Water." @ 1998 by Acoustical Society of America.
21. According to Figure 2, at which of the following water temperatures is the sound absorption by the seawater closest to 0.67 dB/km？
Your Answer is
Correct Answer is D
Explanation
Looking at Figure 2, when the ordinate is 0.67, the corresponding abscissa is about 20°C.