Air Pressure and Air Particles for Vocalists

Matthew Reeve

The terms air particles and air pressure are frequently used to explain many of the phenomenon that surround phonation. This short article explains these two concepts.

Air Particles

In the broadest sense, air particles are the physical matter that form air. This comprises the gaseous molecules of nitrogen, oxygen and so on; along with the water vapour, dust, smoke, pollen and pollutants that are suspended in this gas.

Air Particle Movement

All these particles continuously move around and bang into each other. The gas particles move around due to thermal motion. A rise in temperature causes the random thermal motion to increase and consequently the particles move around more quickly. Similarly, a decrease in temperature will slow the movement down. At normal room temperature, the molecules of nitrogen and oxygen around you have an average speed of about 500 metres per second due to this thermal motion. Solid particles, like dust and smoke, move around due to Brownian Motion. If you observe through a microscope a smoke particle suspended in gas you will see it move around erratically as it is being continuously knocked and buffeted by gas molecules.

Air Particle Behaviour

There is relatively lots of space between gas particles, compared with the space between the particles of a solid. Therefore a gas has physical properties that allow it to be more easily squashed or compressed.

The forces holding the particles in a gas together are very small, and coupled with the fact that the particles are moving very fast, gas particles spread out. Think about an empty plastic bottle with the lid on. The gas particles inside the bottle are all moving very quickly and are hitting the walls of the bottle. However, when you squash the bottle, and compress the gas you can feel the gas particles pushing back against you. As soon as you stop squashing the bottle, the compressed gas particles fly back apart as far as they can, making the bottle spring back to shape.

Air Pressure

Air pressure is the measure of the force created by air particles impacting on a surface.

If the number of air particles in an enclosed space is increased then the pressure exterted by the particles on the walls of that space also increases. This is because there are more particles impacting on the walls. Equally, if you keep the number of air particles the same but decrease the size of the container (for example, by squashing the bottle) then the pressure also increases for the same reason.

Temperature can also affect pressure. If you keep the number of air particles and the space they occupy the same, but increase the temperature, then the pressure also increases. This is because the particles are moving quicker (greater thermal motion) and are hitting the walls harder. The opposite is true too. If you reduce the number of air particles, or increase the volume of the container, or reduce the temperature, then the pressure decreases.

You can relate this to ‘atmospheric’ air pressure which is caused by the number of air particles above us. As we get to higher altitudes the number of air particles above us decreases and so the pressure they cause decreases too. That’s why the atmospheric air pressure is less at the top of a mountain. This leads to a simple definition of air pressure as the force caused by the weight of air.

The terms high pressure and low pressure are simply relative terms used to compare differences in air pressure. For example, the difference in air pressure between the air in your squashed bottle and the air outside the bottle. The air inside the bottle is at a higher pressure compared with the lower pressure of the air outside. Air pressure wants to equalise or reach equilibrium. So if you let go of the squashed bottle then the size of the bottle is pushed back out to its original volume and the pressures equalise. The air pressure inside the bottle is the same as the air pressure outside the bottle.

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