Physics Grade 11

Unit 4: Waves and Sound

Note 2: Transmission and Speed of Sound


Reference: Chapter 8.4 (not all), 8.5, 8.6


Demo: Bell inside a jar.


Transmission of Sound:

- Transmission of sound is dependent upon a medium (i.e. in a vacuum sound cannot be produced). 

- Speed of sound depends on 1) Density of the medium and 2) Temperature of the medium

1) Density of Medium

- In general, the stiffer the material, the faster the speed of sound (i.e. putting ear to ground to detect oncoming train). This is explained by the Particle Theory (i.e. particles are close together in solids so the particles are more responsive to one another's motion)

Note: Although the wave speed may be faster in a stiffer medium, the amplitude of the wave will decrease faster (i.e. the wave will die out more quickly)

Just a Side-Note: The density of air changes (becomes less) with higher altitude and therefore the speed of sound will decrease as altitude increases.


2) Temperature of Medium

Recall that particles move faster at higher temperatures.

In air, the following formula may be used to account for the temperature: 

Example 1: The speed of sound is found to be 349 m/s. What is the temperature?

Example 2: A wave has a frequency of 220. Hz. If the temperature in the room is 20. degrees celsius, what is the wavelength of the wave?

Infrasonic Wave:

- below level of human hearing (below 20. Hz frequency)

Ultrasonic Wave

- above level of human hearing (above 20 kHz frequency)

- applied in medicine to examine organs (ultrasonic waves reflect and give image)

Mach Number & the Sound Barrier:

Mach 1: is the speed of sound at a given air temperature.


Example 3: (example of finding a mach # - always a comparison ratio to Mach1)

What is the mach number for a plane flying at 900. km/h in air with a temperature of 6 degrees celsius?


Sound Barrier: 

The sound barrier is the buildup of sound waves in front of an object moving at a speed near that of sound.

The barrier creates a pressure build up which prevents further speed increase (to break this barrier requires a lot of energy). In other words the sound waves ahead of the plane are squeezed together as the plane approaches Mach 1.


Sonic Booms: 

As the plane breaks through the sound barrier, a sonic shockwave (boom) travels through the air and is heard on the ground.

The boom is a superwave caused by the constructive interference of all the sound waves squeezed together as the plane approaches Mach 1.

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