How does sound in air differ from sound in water?
Sound in water and sound in air are both waves that move similarly and can be characterized the same way. Sound waves can travel through any substance, including gases (such as air), liquids (such as water), and solids (such as the seafloor). Did you know that sound cannot exist if it doesn’t have something to travel through? For example, sound cannot travel through outer space because it is a vacuum that contains nothing to carry sound.
Even though sound waves in water and sound waves in air are basically similar, the way that sound levels in water and sound levels in air are reported is very different, and comparing sound levels in water and air must be done carefully. When we describe a sound as loud or soft, scientists say that the sound has a high or low amplitude or intensity. Amplitude refers to the change in pressure as the sound wave passes by. If you increase the amplitude of a sound, you are making it louder, just as you do when you turn up the volume on your radio. If you decrease the amplitude, you are making the sound softer, just as when you turn down the volume.
The amplitude of a wave is related to the amount of energy it carries. A high amplitude wave carries a large amount of energy; a low amplitude wave carries a small amount of energy. The average amount of energy passing through a unit area per unit time in a specified direction is called the intensity of the wave. As the amplitude of the sound wave increases, the intensity of the sound increases. Sounds with higher intensities are perceived to be louder.
The amount of energy per unit time is called power. The intensity of a sound wave is therefore the amount of power transmitted through a specified area in the direction in which the sound is traveling. Power is measured in watts, and intensity is therefore measured in watts per square meter.
An example of power with which you are probably familiar is light bulbs, which are commonly labeled in terms of the amount of electrical power that they use (60 watts, 100 watts, etc.). Light waves have intensity just as sound waves do. The amount of power that a light bulb uses is directly related to the intensity of the light waves that it puts out.
Sound intensities given in watts per square meter can be directly compared between water and air. However, scientists often specify sound intensity as a ratio, changing from an absolute intensity to a relative sound level. The sound intensity level in decibels (dB) is defined as 10 times the logarithm of the ratio of the intensity of a sound wave to a reference intensity. The decibel is a relative unit of measure, not an absolute one as is watts per square meter.
Confusion arises because sound levels given in dB in water are not the same as sound levels given in dB in air. There are two reasons for this:
- Reference intensities. The reference intensities used to compute sound levels in dB are different in water and air. Scientists have arbitrarily agreed to use as the reference intensity for underwater sound the intensity of a sound wave with a pressure of 1 microPascal (μPa). However, scientists have agreed to use as the reference intensity for sound in air the intensity of a sound wave with a pressure of 20 microPascals (μPa). Scientists selected this value in air because it is consistent with the minimum threshold of young human adults in their range of best hearing (1000 -3000 Hz)1.
- Densities and sound speeds. The intensity of a sound wave depends not only on the pressure of the wave, but also on the density and sound speed of the medium through which the sound is traveling. Sounds in water and sounds in air that have the same pressures have very different intensities because the density of water is much greater than the density of air and because the speed of sound in water is much greater than the speed of sound in air. For the same pressure, higher density and higher sound speed both give a lower intensity.
The result is that sound waves with the same intensities in water and air when measured in watts per square meter have relative intensities that differ by 61.5 dB. This amount must be subtracted from sound levels in water referenced to 1 microPascal (μPa) to obtain the sound levels of sound waves in air referenced to 20 microPascals (μPa) that have the same absolute intensity in watts per square meter. The difference in reference pressures causes 26 dB of the 61.5 dB difference. The differences in densities and sound speeds account for the other 35.5 dB. A 60-dB difference in relative intensity represents a million-fold difference in power.
When reporting sound levels, it is important to not only say “dB” but to also add the reference level. This is often written as “dB re 1 μPa” for sounds in water that are measured relative (re) to 1 μPa and “dB re 20 μPa” for sounds in air that are measured relative (re) to 20 μPa. To make it clear for the reader, this website will use “underwater dB” for underwater sounds. You have experienced the same thing when you talk about the temperature. You should not just say, “It is 50 degrees outside” because that will mean something different to someone living in the United States who uses the Fahrenheit scale and someone living in Europe who uses the Celsius scale. 50 degrees Fahrenheit is equal to 10 degrees Celsius, whereas 50 degrees Celsius is equal to 122 degrees Fahrenheit – quite a difference! To make sure there is no confusion, you should say what temperature scale you are using. It is the same thing with dBs. To avoid confusion, you need to specify the reference level.
You might like to know the approximate sound levels of some common sounds in air. The sound levels in the following table are all relative to the intensity of a sound wave in air with a pressure of 20 microPascals (μPa).
For the sound levels of underwater sounds, see What are common underwater sounds?
Additional Links on DOSITS