How Does Sound in Air Differ From Sound in Water?

Sound waves are able to travel through solids, liquids and gases. In fact, they don’t exist at all unless there is something for them to travel through. How sound travels through water differs from how it travels through air, but what are these differences?

Amplitudes

Sound is measured in amplitude. A loud noise is described as having a high amplitude, whilst a quieter or softer one has a low amplitude. The word ‘amplitude’ actually refers to the change in pressure caused by a passing sound wave – louder sounds create more pressure, and carry more energy, than those of a lower amplitude.

Energy and power

‘Intensity’ is the name given to the amount of power transmitted by a sound wave through a specific area, and is measured in watts per square metre. The higher the number of watts, the more ‘intense’ the sound wave.

Decibels and relative sound levels

Scientists like to work in relative sound levels, using ratios. For this, sound is measured in decibels (dB), rather than watts per square metre. Reference intensities are different for sound levels in water and sound levels in air.

In water, scientists use a reference intensity of 1 microPascal (μPa) of pressure, whilst in air the reference intensity is 20 microPascals, which is the minimum intensity that young adults can comfortably hear.

How sound waves travel through air and water

You can’t measure the intensity of a wave without also taking into account the material or medium through which it is travelling. The density of that material has an impact, with air being significantly less dense than water. Likewise, the speed that a sound is travelling must be considered, where it travels much faster through water than through air.

The more dense the medium, and the faster the sound waves, the lower their intensity. Since sound travels faster through water than air, and since water is also more dense, sound has a lower intensity in water than it does in air.

How relative intensities play their part

Sound waves with the same intensities in air and water, when you’re measuring in watts per square metre, actually have different relative intensities when you’re measuring in decibels. The difference is 61.5 dB, made up of 35.5 dB due to different sound speeds and densities, and 26 dB due to the different reference pressures. This needs to be subtracted from levels measured in water to compare them directly with levels in the air, giving an absolute intensity.

Reporting sound levels

When you’re reporting sound levels, you shouldn’t just use decibelsgeluidsmeting laten uitvoeren as your unit of measurement. You also need to add the reference level. In water you’ll measure intensity in ‘dB re 1 μPa’, and in sound you’ll measure in ‘dB re 20 μPa’.

Leave a Reply

Your email address will not be published.