Section 3

The Physics of Sound and How We Hear

When you speak, your lungs push air through your vocal cords, throat, mouth, tongue and several other things. The vocal cords modulate the air by changing the density and how fast the density changes.

Modulating air works in a similar fashion to the gravity forces applied to sea water. When the gravity forces ( moon and sun ) pull on the water the water pressure goes up. Liquids do not compress very well, so on the surface of the water the compressed water expands upward and forms waves. The height of a wave is referred to as the waves amplitude and is measured in feet. The distance between one wave peak ( maximum wave amplitude ) and the next wave peak is referred to as the wavelength and is also measured in feet. With air the wave amplitude is measured as the density of the air and is measured in psi ( pounds per square inch ).

As the air pressure waves moves through the ear canal they hit the ear drum pushing it in. A small set of bones ( malleus - hammer, incus - anvil and stapes - stirrup ) connects the ear drum to the inner ear ( at the oval window ). The moving ear drum pushes on these bones which pushes the fluid in the inner ear. This process translates the air pressure waves into fluid pressure waves.

The inner ear ( cochlea ) resembles a spiraling tube ( the form is seen in several types of sea shells ). The tube is filled with hairs that vary from the longest at the beginning of the tube to the shortest at the end of the tube. These hairs act as tuning forks that vibrate at different wavelengths. The shorter the wavelength ( the faster the pressure changes ) the shorter the hair length to detect it.

When a hair vibrates it generates chemical signals that are sent to the brain. This process translates the fluid pressure waves into chemical signals. The brain adds these signals together and matches them to a known signal pattern. This whole process is interpreted by the brain as sound.

As you get older the shorter hairs die sooner than the longer hairs, because the shorter hairs vibrate faster, which causes their molecular structure to fail sooner. This is why the older you get the less of the higher pitched sounds ( tones ) you are able to hear. It's also why people have a harder time hearing women. Also excess pressure ( loud sound sources like music, jet engines, leaf blowers, gun shots etc. ) at specific tones can cause early failure of other hairs, causing the loss of hearing at specific tones, creating gaps in ones hearing range ( bandwidth ).

A man's vocal cords, on average, are in a larger cavity ( Adam's apple ). This creates a larger amount of air to move, which slows down the air pressure wave which is why the male voice has a lower average pitch than a female after puberty.

The speed of sound is determine by the distance between molecules of the carrier material. Since air molecules are spaced far apart, sound travels rather slowly through air. In space there are no molecules ( in significant quantity for sound transmission ). Therefore, you can not transmit sound in outer space. If the carrier is water, then sound travels faster, because water molecules are closer together. As the temperature drops, the amount of enegry required to move air molecues increases, thus slowing the sound wave.

Velocity of sound in dry air at 0 degree C:

331.45 meters / second

1087.4 feet / second

741.4 miles / hour

Velocity of sound in water at 20 degree C:

1480 meters / second

4856 feet / second

3311 miles / hour

Since most things vibrate when something hits it, spies can record a conversation by recording the small vibrations in a wall or plate of glass, the heated air of your breath, or even by the affect of air movement on the background view ( it shimmers like a mirage on a hot day ).

What's all this mean?

Your vocal system is a transmitter of sound. It creates pressure waves of air molecules, the carriers of the sound. The vocal cords along with several other parts modulate ( encode ) the air pressure waves. The inner ear hairs act as antennas that detect the amplitudes and wavelength of the sounds. The brain translates the chemical signals ( very similar to electrical signals ) delivered by the inner ear hairs into an understandable language, which is further interpreted by other parts of the brain based on whatever was said.

So what we have is a transmitter, encoder, receiver, decoder and translator.

What was covered:

• Air pressure is a function of the density of the air molecules. The density is called the amplitude of a sound on an instrument ( oscilloscope for one ).
  
  
• The distance between peak densities of air molecules is measured as the wavelength of the sound on an instrument.
  
  
• The size of an antenna determines which wavelengths are detected. If a wavelength is to long, a short antenna is to stiff and will not vibrate. If the wavelength is to short, a long antenna has to much mass and can not vibrate fast enough to keep up. The limited number of wavelengths an antenna can respond to is called it's bandwidth.
  
  
• The carrier used ( air, water, wood etc. ) to transfer sound determines the speed of the sound.
  
  

Atoms, Introduction, Basic Electricity

Author: David Bishop

Last updated: Mar 4, 2011

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