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# Grandma Moon Speaks: What Influences Sound?

Topics Covered:

• Pressure and Temperature
• Pascal
• Threshold of Hearing

Nature has give us two eyes and two ears but only one tongue – so that we should hear and see more than we speak – Socrates It is common for people to create reference standards, so that things can be measured according to these standards. Unfortunately, there are as many standards as humans for similar concepts. Two such properties are Pressure and Temperature.

For the sake of simplicity, let us take the Standard Ambient Temperature and Pressure (SATP) as reference. It defines a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm) as a ‘normal average comfortable’ value. Any major deviations from this is a cause for worry.

We know these values in general terms such as ‘room temperature’ and pressure at ‘sea-level’, etc. Most people have experiences relative to these levels, hence the choice of standard.

So what has all this got to do with sound?

Since sound mostly travels through the air (gas), we can take this formula as reference:

PV = kNT

where
P is the absolute pressure
V is the volume
N is the number of molecules
k is the Boltzmann constant
T is the temperature in Kelvin

The point of this being that Pressure, volume, the density of molecules and the temperature are all interlinked. We have already understood that sound is molecules vibrating and creating a perception. From the above formula we can understand how his changes with a change in pressure, temperature or density.

Out of all these factors, sound itself creates one – pressure. For this reason the basis of all sound measurements is pressure, measured in a unit called Pascals (Pa).

So, how much pressure does it take to actually hear something? The threshold of hearing is generally believed to be about 20 µPa (micropascals) = 2×10-5 pascal (Pa).

Next we’ll take a look at how this translates to useful information in the real world.

Takeaways:

• Sound is affected by Pressure, temperature, density and volume.
• The unit of sound pressure is Pascal.
• The threshold of hearing is generally believed to be about 20 µPa.