Dr. Jie Zou PHY 1151G Department of Physics

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Chapter 14

Waves and Sound

Dr. Jie Zou PHY 1151G Department of Physics

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Outline Waves and types of waves

Transverse waves Longitudinal waves Combination of the above two

Wavelength, period, frequency, and speed of a wave

Sound waves Speed of sound Sound frequency Pitch of a sound

Dr. Jie Zou PHY 1151G Department of Physics

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Waves Wave: A wave can be

viewed as a disturbance (oscillation or vibration) that propagates through space. Waves carry energy.

Types of waves Transverse waves longitudinal waves A combination of both

Dr. Jie Zou PHY 1151G Department of Physics

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Transverse waves Transverse wave: In a

transverse wave, the displacement of individual particles is at right angles to the direction of propagation of the wave.

Examples of transverse waves: a wave on a string, light and radio waves

Dr. Jie Zou PHY 1151G Department of Physics

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Longitudinal waves Longitudinal wave:

In a longitudinal wave, the displacement of the individual particles is in the same direction as the direction of propagation of the wave.

Example of a longitudinal wave: sound waves in air.

Dr. Jie Zou PHY 1151G Department of Physics

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Combination of transverse and longitudinal waves

Water Waves

Dr. Jie Zou PHY 1151G Department of Physics

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Wavelength, period, frequency, and speed

Wavelength of a wave : The distance over which a wave repeats is the wavelength, of the wave.

Period of a wave T: The period, T is the time required for one wavelength to pass a given point. The frequency of a wave, f = 1/T.

Speed of a wave v: v = /T = f.

Dr. Jie Zou PHY 1151G Department of Physics

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Exercise 14-1 Sound waves travel in air with a

speed of 343 m/s. The lowest frequency sound we can hear is 20.0 Hz; the highest frequency is 20.0 kHz. Find the wavelength of sound for frequencies of 20.0 Hz and 20.0 kHz.

Dr. Jie Zou PHY 1151G Department of Physics

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Sound waves Sound we hear is a wave

propagating through the air at a speed of about 770 mi/h or 343 m/s.

A speaker produces sound waves by oscillating a diaphragm back and forth horizontally. The wave consists of compressed regions alternating with rarefied regions.

Dr. Jie Zou PHY 1151G Department of Physics

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Speed of sound The speed of sound is determined by the properties of the

medium through which it propagates. Speed of sound in air: v = 343 m/s ≈ 770 mi/h under

normal pressure and temperature. As the air is heated the molecules will move faster, and hence the

speed of sound also increases with temperature. In a solid, the speed of sound is determined in part by

how stiff the material is. The stiffer the material, the faster the sound wave.

The speed of sound in plastic is 2680 m/s, and in steel is 5960 m/s. The speed of sound (traveling in the same medium) is the

same for all frequencies. So, in relation v = f, v remains fixed.

Dr. Jie Zou PHY 1151G Department of Physics

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The frequency of a sound wave

The frequency determines the pitch of a sound.

For example, the keys on a piano produce sound with frequencies ranging from 55 Hz for the key farthest to the left to 4187 Hz for the rightmost key.

Humans can hear sounds between 20 Hz and 20,000 Hz.

Ultrasonic: sounds with frequencies above this range are ultrasonic.

Infrasonic: sounds with frequencies lower than 20 Hz are infrasonic.

Applications: Echolocation, medical imaging

Dr. Jie Zou PHY 1151G Department of Physics

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Homework Chapter 14, Page 457, Problems:

#1, 4, 20, 22, 23.