Sound Absorption

You have heard the word "absorb" in many contexts. Think of a paper towel commercial. They advertise, "This towel is more absorbent than the average towel." Or think of a sponge. If you spill grape juice, you clean it up by wiping a wet sponge over the grape juice spill. The sponge has just absorbed the grape juice. The same thing happens to sound. As sound travels through a medium such as water, it gets absorbed - caught by the molecules within the medium. The medium actually changes some of the acoustic energy of the sound wave into heat. One way that this happens is that the acoustic energy of the sound causes the molecules of the medium to start vibrating. The molecules are at rest before the sound wave comes through. In order for them to start vibrating, the sound must be powerful enough to overcome the molecule's resistance to movement. The molecules must overcome the viscosity of the medium through which the sound is propagating. They do this by stealing some of the energy from the sound wave. The absorption in sea water is much greater than would be expected due to the viscosity of pure water. In addition to the absorption due to viscosity, some of the chemicals that make the ocean salty also absorb sound and convert it to heat. The final result of absorption is that as sound travels or propagates through a medium, its amplitude decreases faster than it would due to spreading loss alone. This is due to the loss of acoustic energy to heat.

The amount of absorption depends on the frequency of the sound. A high frequency sound has many cycles in a second, and the particles in the medium are therefore vibrating very rapidly. Just as when you rub your hands together very rapidly, this produces more heat than if you rub your hands together slowly. Since the molecules get their energy to vibrate from the sound wave, the sound wave will run out of energy sooner when it is a high frequency sound. This means that, under the same conditions, a high frequency sound won't travel as far as a low frequency sound.

High Frequency SoundLow Frequency Sound