Variations in Air Pressure

Pressure is amount of pressing, a measurement of how a force is distributed over a surface. Press your hand with all your strength against a hard surface and note the sensation. Now press one finger with the same degree of effort against the same surface and note the sensation. In the first case, you will probably feel no pain, but in the second case the pressure exerted on your fingertip may be so much as to be painful. Your arm has not become any stronger, obviously, so the difference in your sensations must stem from the fact that the force which you are applying is being applied over a smaller area - the tip of a finger as opposed to the palm of the hand. If the surface area is reduced even more, e.g. by applying the fingertip to the end of a pin, the degree of pressure would be so great that the pin would be pressed into the finger, probably causing a great deal of pain. But the fakir who lies on a bed of nails distributes the weight of their body over a larger surface area (their entire body), and thus the pressure of each nail on the skin is not large.

Pressure is thus Force per unit of Area:

Force is measured in newton (N). 1N = 1 kg m/s2, i.e. the amount of force needed to accelerate a mass of 1 kg by 1 m/s2.

Area is measured in square meter (m2).

Pressure is thus measured in N/m2, abbreviated as Pa (Pascal).

Atmospheric pressure is just over 100 kPa (i.e. 100,000 Pa), with daily weather variations of a few kPa. Air pressure in a car tyre could be 210 kPa, and in a bicycle tyre could be 550 kPa ― a lot higher than in a car tyre, because a bicycle only has two wheels and they are quite narrow.

The Threshold of Hearing is, for a normal (average) observer, the sound level or intensity which is just audible. For a pure sinusoidal note of frequency 1000 Hz it is close to a root mean square pressure of 2 x 10-5 Pa (i.e. 0.00002 Pa).

The Threshold of pain in hearing is about 100 Pa. Thus the range of pressure variations in sound is about 5 million : 1. Because this range is so large, we use a logarithmic scale, the decibel (dB), for describing the loudness of sounds.

Normal conversational speech of 60 dB SPL (Sound Pressure Level) is about 0.02 Pa.

Loud singing at close range (75 dB) is a pressure of about 0.1 Pa.