Chapter 14 - Sound

They are produced by objects that vibrate. The vibrations push particles loser together, creating a compression. They then push against other particles causing the compression to move through the air until it reaches someones ear. The region between compressions is called a rarefaction. 

Sound waves travel through solids, liquids and gases. This is why we can hear something through a wall, how whales communicate underwater and how we speak to each other. 

Sound waves cannot travel through a vacuum because there are no particles for the sound to vibrate through. 

Measuring the speed of sound through echoes:

• Stand 50m away from a blank wall and clap two pieces of wood together.
• Set up a rhythm so that the echo comes exactly between two claps.
• Ask a friend to time 20 claps - during this time the sound has travelled 2000m.
• Then divide the distance by the time to get the speed of the sound. 

Reflection:

• Like light waves, when sound waves hit a barrier they bounce off it. 
• Reflected sound waves are called echoes.
• Ships use this idea to see how deep the sea is at a certain point.

Diffraction:

• This is how we can hear sounds even though we are not directly in front of the source.
• This is how we can hear sounds through things like doorways. 

Pitch and Frequency:

• Objects that vibrate quickly have a high frequency, creating a high pitch. E.g. a violin.
• Larger objects vibrate slower with a lower frequency and pitch. E.g. a cello.

Cathode Ray Oscilloscope (CRO):

• Represents waves by replicating their wavelength and time period. 
• It shows waves as the typical 'bumpy' line rather than particles.

The average person can only hear sounds with frequencies higher than 20Hz and less than 20,000Hz. This is called the audible range or hearing range. 

This varies from person to person and narrows as we get older. 

Sounds that are at frequencies higher than 20,000Hz still produce sound but we cannot hear it. These sounds are called ultrasounds. Sounds with lower frequencies than 20Hz are called infrasounds.

Loudness:

• The larger the amplitude of a wave, the louder it is.
• In a quieter sound, the air particles are less densly packed so less ebergy is transferred.