SOUND

Sound

Physics Power Points

Physics I

Mr. Young

www.pedagogics.ca

Basic facts about sound waves:

Sound waves are mechanical waves. They require a medium to transfer energy. Sound can travel through solids, liquids and gases.

Sound waves are caused by vibrations.

The source of all sound waves is a vibrating object. For example: a speaker cone, a guitar string, a drum skin, an air column in a trumpet.

Sound waves are longitudinal.

Sound waves are caused by vibrations

When a tuning fork is struck, the tines vibrate back and forth. Each vibration compresses the surrounding air molecules together (high pressure). Air is elastic. As the compression expands, adjacent molecules are compressed. In this way sound waves travel through the medium.

compression

rarefaction

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Molecules vibrate parallel to energy transfer.

Sound waves are longitudinal

Sound waves are mechanical waves

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Sound waves are mechanical waves

Sound relies on the vibration of particles to transfer the energy of the sound wave. If there are no particles, there is no sound!

Bell in Jar Demo - Video

Frequency of Sound Waves

The frequency of a sound wave is described as the pitch of a sound. The higher the frequency of the vibrating source, the higher the pitch.

Humans can hear sounds in the range of 20 Hz to 20 kHz. Dogs and other animals can perceive higher frequencies (why you cant hear a dog whistle.

Amplitude of Sound Waves

The amplitude of a sound wave is perceived as the loudness of a sound. The greater the amplitude of the vibrating source, the more energy the sound wave has and your hear a louder noise.

Loudness is measured in decibels (dB). It is a logarithmic scale. 10 times more energy gives a +10 dB increase in loudness.

Graphical Representations of Sound Waves

A microphone can be used with a data logger to create a graphical representation of a sound wave.

A displacement-time graph for a sound wave shows the frequency and amplitude of vibration of the wave.

Time is measured on the x-axis and the y-axis is representative of displacement.

Graphical Representations of Sound Waves

Example 1 Showing high pitch (frequency)

http://onlinetonegenerator.com/

Graphical Representations of Sound Waves

Example 2 Showing low pitch (frequency)

Graphical Representations of Sound Waves

Showing loudness

Graphical Representations of Sound Waves

Musical instruments do not produce pure tones. The wave form is still distinguishable in this recorder sound wave pattern.

Speed of Sound

The speed of sound depends on the medium that it is traveling through. In general:

Vsolid > vliquid > vgas

Specific properties of the medium will also affect the speed. For example, the speed of sound is slower in cold air than in warm air. Why is this?

Speed of Sound

The speed of sound in still air can be determined by the following equation:

v = 331 + 0.6T m/s

Where T is the temperature in Celsius.

When we discuss speed in Mach numbers (example Mach 1.4) we are referring to how fast one is traveling relative to the speed of sound. Mach 1.4 is slower on a cold day than on a warm one.