Pressione en

From "Physics, Waves, Music": a website about the physics of waves and sound, acoustics of musical instruments, musical scales, harmony and music.

AI-translated from Italian. Original page: Pressione

Pressure is defined as the ratio between the force exerted perpendicularly on a surface and the area of the surface.

p=F⊥S

The unit of the International System is the pascal (Pa), equivalent to the pressure exerted by a force of 1 newton perpendicularly on a surface of 1 m2.

The following table gives the conversions between some units of pressure.

  Pa bar barye kgp/m2 atm torr (mmHg)
1 Pa (N/m2)= 1 10-5 10 0.102 9.87×10-6 0.0075
1 bar (daN/cm2) = 100000 1 106 10197 0.987 750
1 barye (dyne/cm2) = 0.1 10-6 1 0.01 9.87×10-7 7.5×10-4
1 kgp/m2 = 9.81 9.81×10-5 98 1 9.68×10-5 0.0736
1 atm = 101325 1.013 1013250 10332.275 1 760
1 torr = 133 0.00133 1333.2239 13.5951 0.00132 1

Be careful not to confuse atmospheric pressure with sound pressure: sound pressure is the (very small) local variation of air pressure due to the passage of a compression wave. It is therefore measured relative to the value of atmospheric pressure, taken as zero. Although in principle sound pressure can be measured in the same units as atmospheric pressure, it is much more commonly measured as a "sound pressure level" on a dB scale. On this point see the pages sound pressure levels, useful values in dB and perception of loudness.

Check your understanding

Problem. At the threshold of pain the effective sound pressure is about 100 Pa. Express this value in atmospheres and in torr. Solution. 100/101325 atm ≈ 0.001 atm and 100 · 0.0075 torr = 0.75 torr. Even the loudest sound one can bear changes the air pressure by only a thousandth.

Problem. How tall must a column of water (ρ = 1000 kg/m³) be for the pressure at the bottom, due to the weight of the water alone, to equal 1 atm? Solution. p=ρgh, so h = 101325/(1000 · 9.81) m ≈ 10.3 m. Every 10 metres of depth under water the pressure increases by about one atmosphere.

Problem. Assuming the eardrum has an area of 55 mm², what force does a sound with an effective pressure of 1 Pa (about 94 dB) exert on it? Solution. F=pS = 1 · 55·10−6 N = 5.5·10−5 N, about the weight of a 5.6 mg mass: even a very loud sound pushes on the eardrum with a tiny force.

"Physics, Waves, Music": a website about the physics of waves, acoustics of musical instruments, musical scales, harmony and music.