1. Unit 14 Lesson 1 What Is Sound? Copyright © Houghton Mifflin Harcourt Publishing Company

2. Unit 14 Lesson 1 What Is Sound? Waves of Sound Copyright © Houghton Mifflin Harcourt Publishing Company A wave is a disturbance that transmits energy. There are some waves that you cannot see. Sound is a series of vibrations traveling in waves. Vibrations are the back-and-forth movements of an object.

3. Unit 14 Lesson 1 What Is Sound? Waves of Sound Copyright © Houghton Mifflin Harcourt Publishing Company All sound vibrations travel in compression waves.

4. Unit 14 Lesson 1 What Is Sound? Waves of Sound Copyright © Houghton Mifflin Harcourt Publishing Company As a compression wave moves, molecules of air or other matter are pushed together, or compressed. The molecules spread apart, and sound energy moves away from its source as this bunching and spreading of molecules repeats.

5. Unit 14 Lesson 1 What Is Sound? It Sounds Like … Copyright © Houghton Mifflin Harcourt Publishing Company Pitch and volume are two ways to measure sound. The highness or lowness of a sound is its pitch. Frequency is the number of vibrations that occur during a unit of time. High-pitched sounds have high frequencies. Low-pitched sounds have low frequencies.

6. Unit 14 Lesson 1 What Is Sound? It Sounds Like … Copyright © Houghton Mifflin Harcourt Publishing Company The loudness of a sound is its volume. Volume is measured in units called decibels, abbreviated dB. The softest sounds humans can hear are near 0 dB. The humming of a refrigerator is about 40 dB, while heavy city traffic is about 85 dB. Listening to high-decibel sounds for a long period of time can cause hearing loss.

7. Unit 14 Lesson 1 What Is Sound? Looking at Waves Copyright © Houghton Mifflin Harcourt Publishing Company People use diagrams to describe sound. Waves consist of curving lines that go up and down as they move away from the sound source. Each high point of a wave is called a crest. Each low point of a wave is called a trough. The distance from one crest to the next is the wavelength.

8. Unit 14 Lesson 1 What Is Sound? Looking at Waves Copyright © Houghton Mifflin Harcourt Publishing Company Amplitude describes the amount of energy in a wave. The volume of a sound and the amplitude of its wave are related.

9. Unit 14 Lesson 1 What Is Sound? Looking at Waves Copyright © Houghton Mifflin Harcourt Publishing Company Loud sounds, such as jet engines, have large amplitudes. Soft sounds, such as a pin falling on a hard surface, have small amplitudes.

10. Unit 14 Lesson 1 What Is Sound? Looking at Waves Copyright © Houghton Mifflin Harcourt Publishing Company Amplitude is measured by finding the distance from a wave’s highest or lowest point to its midline, or the horizontal line across the middle of a wave.

11. Unit 14 Lesson 1 What Is Sound? The Travels of Sound Copyright © Houghton Mifflin Harcourt Publishing Company Sound travels in waves. However, sound can only travel if there are particles that the waves can cause to vibrate. Most sounds move through the air. Air and other gases have particles that vibrate as sound energy hits them. Liquids and solids are also made of particles, so sound waves can also move through these materials.

12. Unit 14 Lesson 1 What Is Sound? The Travels of Sound Copyright © Houghton Mifflin Harcourt Publishing Company If there are no particles to move, then sound cannot travel.

13. Unit 14 Lesson 1 What Is Sound? The Travels of Sound Copyright © Houghton Mifflin Harcourt Publishing Company Sound waves travel through different kinds of matter at different rates. The speed at which sound waves pass through solids, liquids, and gases has to do with how their particles are arranged.

14. Unit 14 Lesson 1 What Is Sound? The Travels of Sound Copyright © Houghton Mifflin Harcourt Publishing Company Particles in solids are packed closely. Particles in gases are far apart. Particles in liquids are in between. Sound travels through gases more slowly than it travels through liquids and solids.

15. Unit 14 Lesson 1 What Is Sound? Sound All Around Copyright © Houghton Mifflin Harcourt Publishing Company Understanding the properties of sound allows people to control sound. Sound insulation contains tiny air cells. Sound is absorbed as the cells trap sound waves, keeping sound inside a room. For example, sound engineers place speakers in certain places in movie theaters and performing arts centers so the audience can hear clearly.