Properties of Sound:

Objectives:

1.    Describe the properties of sound waves.

2.    Explain how frequency and pitch are related.

3.    Describe the Doppler effect.

Resources:

Notes:

Properties of Sound Transmittance:

As we stated in the previous lecture, sound requires a medium.  Speed of the transmitted wave depends on the type of medium and the temperature.  To understand this, place your ear on the table and have someone gently tap on the other end.  You should notice that the tapping sound is much louder with your ear on the table than when your ear is not.  To understand this phenomena examine the following properties of media.

• Sound transmission is based mostly on the elasticity of the material

• The greater the elasticity (ability to return to shape) the greater the speed of transmission

• Elasticity, therefore, is based on the arrangement of the molecules - the closer the molecules are arranged the more elastic the material

• gas - least elastic - slowest transmission

• liquid – more elastic than air - sound through water is about 4 times faster than air

• solid - most elastic - sound through steel is about 15 times faster than air

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Speed of sound:

In air the speed of sound is about 20oC it is 340m/s.  At 25oC it is 346m/s.  As you can see sound travels faster with an increase in Temp.  Sound, quite predictably, increases about 0.6m/s with each increase in degree Celsius.  What would it be at 0oC?

Intensity and Loudness:

Review: all waves have amplitude, f, & l

The amount of energy in a wave is called intensity.  The intensity can be measured using an oscilloscope or can be judged as loudness by an observer.

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Weather using intensity or loudness, it is important to know how to relate one to the other.  Here are some simple hints:

• Intensity is related to the amplitude of the wave and is measured in distance (m)

• Intensity = Loudness

• Loudness is measured in decibels (db)

• Our hearing threshold is 0db

• Above 85db can cause hearing damage

• db levels increase logarithmically

10db is 10 times stronger than 0db

20db is 100 times stronger than 0db

50db is 100,000 times stronger than 0db

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