Welcome to this tutorial on sound.

Sound is moving energy that travels as a pattern of changing pressure. Sound needs a medium in which to propagate and can travel through gases, liquids and solids, but not in a vacuum, which explains why there is no sound in space. It is essential to understand that sound is not a 'thing' in itself, but a temporary property of the medium it is passing through.

It is common to characterise sound as a vibration, but this can cause confusion. It is true to say that a source must vibrate to create a sound wave, but it is better to think of the sound wave itself as a moving pattern of alternating high and low pressure energy.

Because most of the sound we hear arrives at our ears through the air, it is most useful to study the theory of air pressure sound waves.

Caption - Air molecules and pressure

The air that surrounds us is comprised of particles known as air molecules. It is not important to understand what these molecules are made of. Air pressure is a measure of how tightly packed together these air molecules are, in other words, the air density.

When an object vibrates, such as a guitar string, and sound is created, the surrounding "normal atmospheric" air pressure is temporarily disturbed.

Incidentally, "normal atmospheric pressure" depends on altitude. The higher you move through the atmosphere the more spread apart the air molecules become, and the lower the density and therefore the lower the "normal" air pressure.

In sound theory, changes to air pressure are measured in units called decibels. This is the amplitude, volume, or loudness, of the sound. Decibels are covered in a separate tutorial video.

Caption - Source

Sound is produced when an object vibrates and disturbs the pressure of the surrounding air. Such an object is called the source. The source may be a loudspeaker, a drum, car engine or anything that makes a sound.

Caption - Vibration of a source
As a model to aid understanding, it is helpful to define vibration as the repetitive expansion and contraction of the source. A cycle of vibration works like this ...

• Source starts at original size
  
  
  
  
  
  
• Source expands (gets bigger)
• Source reaches maximum size (bigger than normal)
  
  
  
  
  
  
• Source contracts (gets smaller)
• Source returns to original size
  
  
  
  
  
  
• Source contracts (gets smaller)
• Source reaches minimum size (smaller than normal)
  
  
  
  
  
  
• Source expands (gets bigger)
• Source returns to original size
  
  
  
  
  

Caption - Compression
As the source expands it pushes the surrounding air molecules away causing them to "bunch" together and the air pressure to increase. This is called compression.

Caption - Rarefaction
As the source contracts the surrounding air molecules spreads out to fill the increasing space, causing a decrease in air pressure. This is called rarefaction.

Caption - Sound waves
As the source expands and contracts a pattern of changing air pressure energy moves away from the source. This pattern of energy is called a sound wave.

It is important to understand that individual air molecules do not travel from the source to the listeners ear, but oscillate (or vibrate) back and forth around a fixed point, knocking against each other and thereby transferring the changing air pressure energy to one another.

Imagine yourself standing in the middle of a dense crowd. You push the people next to you away. They push the people next to them before rocking back towards you. The people they push will in turn push other people and so on. A wave of energy can be said to have passed through all the people surrounding you, started by you. This process is not unlike a Mexican wave.

Caption - Thanks for watching

The script for this video, with accompanying images, can be found at projectstudiohandbook.com 

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