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Sound

Science, Grade 6

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Table Of Contents: Sound

1. Waves

2.1. What Is a Wave?
If you drop a pebble into a still pond, a wave ripples out in all directions across the water. The surface of the water rises and falls in a regular pattern as the wave moves forward. This rising and falling is called a disturbance. The falling pebble has energy that is transferred to the water molecules. These molecules transfer energy to the molecules near them. The energy from the pebble moves outward as the water rises and falls. Therefore, a wave is a disturbance that transfers energy from one place to another. Types of waves include water waves, sound waves, and light waves.
2.2. Energy and Waves
For a water wave, energy is transferred by the water molecules. For sound waves, energy is transferred by the air molecules. A wave that needs a substance to travel or transfer its energy is called a mechanical wave. The substance that the wave travels through is called the medium. A medium can be a solid, liquid, or gas. Not all waves need a medium to transfer energy. Light is a wave that can travel through a vacuum. A wave can cause the particles of the medium to move in two ways, up and down or side to side. These movements produce two different kinds of waves, transverse and longitudinal.
2.3. Transverse Waves
In a transverse wave, the particles of the medium move up and down while the energy moves forward. The wave has a high point called the crest and a low point called the trough. Examples of transverse waves are light and water waves. You can see the crest and trough of the water wave as it approaches the shore.
2.4. Longitudinal Waves
In a longitudinal wave, the particles of the medium move back and forth horizontally, while the energy moves forward. When the particles are pushed together, it is called compression. When the particles are spread apart, it is called rarefaction. You can observe the motion of a longitudinal wave in a spring. Stretch out the spring and then pinch some of the coils together. When you let go, the energy will move through the spring with the compressions and rarefactions. An example of a longitudinal wave is sound.
2.5. Amplitude
Properties of waves include amplitude, wavelength and frequency. Amplitude is a measure of how big the wave is. In a transverse wave, the amplitude is the height of the wave from the rest position to the point of greatest displacement. In a longitudinal wave, amplitude is related to how compressed the particles are in the medium. The more energy a wave has, the higher the amplitude. You can observe amplitude changes in a length of rope. With two people holding the rope, one person can move the rope up and down to produce waves. If the rope is moved faster, the amplitude will increase.
2.6. Wavelength
Another property of waves is wavelength. Wavelength is the distance between two corresponding parts of a wave, such as crest to crest distance. Wavelength is measured in units of distance such as mm, cm, and m, among others. The shorter the wavelength of a wave the more energy it has. With two people holding a rope, one person can move the rope up and down. Observe the distance between crests. If the rope is moved faster, the distance between the crests will decrease.
2.7. Frequency
Frequency is the number of complete waves that pass a point in a given time. Frequency is measured in Hertz (Hz). For waves, one Hertz equals one wave per second (1 Hz = 1/s). For example, if 10 waves pass a point in 5 seconds the frequency is 2 Hz. The more energy a wave has the higher its frequency.
2.8. Calculating Wave Speed
The speed of a wave is determined by the wavelength and frequency of the wave. Wave speed equals wavelength times frequency. What will happen when the frequency increases? If a wave has a frequency of 10 Hz and a wavelength of 2 m the speed of the wave will be 20 m/s. If the frequency is increased to 20 Hz with the wavelength of 2 m than the speed will increase to 40 m/s. If the wavelength increases the speed will also increase.

2. Pause and Interact

3.1. Sound Waves
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.
3.2. Longitudinal Waves
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.
3.3. Review
Solve each problem. Use the whiteboard text tool to show your work and answers.

3. Wave Interactions

4.1. Reflection
Imagine jumping into a pool. The wave hits the side of the pool and bounces back. This is called reflection. When the front of the wave meets a barrier it cannot go through or over, it will be reflected. When a sound wave is reflected we hear an echo. Light waves reflect off surfaces like a mirror or snow.
4.2. Refraction
When a wave enters a new medium, the speed of the wave changes. If the wave enters the new medium at an angle part of the wave slows down. This makes the wave bend. This bending is called refraction. When water waves reach the beach, the shallower water acts like a new medium and will slow down the waves. Waves coming in at an angle to the shore will be bent. Light waves refract when the light passes from air to water as seen by the bent appearance of a straw in a glass of water.
4.3. Diffraction
Most of the time, waves travel in straight lines. However, when a wave reaches corners or openings in a barrier the wave bends and spreads out to fill the medium. This bending of waves is called diffraction. Water waves diffract as they pass around a barrier such as a pier. Sound waves diffract through openings or around corners.
4.4. Interference
Interference occurs when two or more waves meet in a medium and combine to form a single wave. Interference can be constructive or destructive. In constructive interference when the waves meet, their amplitudes add together to make a bigger wave. In destructive interference the amplitude of the waves subtract and can even cancel each other out completely. For both types of interference, once the waves meet they pass through each other unchanged.
4.5. Standing Waves
A special type of interference produces a standing wave. These waves are formed by the wave and its reflected wave. The two waves combine to form one wave that appears to be standing still. The wave must have a special frequency in order to produce this standing wave. Musical instruments use standing waves to produce music. Standing waves can also be destructive. If standing waves are produced in a structure, such as a bridge, the structure can collapse.

4. Pause and Interact

5.1. Wave Interactions
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.
5.2. Review: Wave Interactions
Use the whiteboard text tools to solve the problems.

5. Sound and Sound Waves

6.1. Sound
Sound is a longitudinal wave created by vibrations, the back-and-forth motion of an object. Longitudinal waves are made of compressions and rarefactions and can only travel through a medium. Waves that must have a medium to transfer energy are called mechanical waves. The medium can be a solid, liquid, or gas. If there is no medium, such as in a vacuum, then the energy of sound will not be transferred. In the vacuum of outer space, an astronaut cannot hear any sounds when he is outside his spaceship. On Earth the air around us transfers sounds to our ears. We hear sound when the disturbance in the air reaches our ears.
6.2. Wave Interactions and Sound
We call the reflection of sound an echo. We do not always hear an echo but sound reflects off any barrier. We also know sound diffracts. If people are talking in the hallway we can hear them in the class room because sound diffracts around the opening of the doorway.
6.3. Speed of Sound
When two people are talking to you at once, you hear the words at the same time because each sound travels at the same speed. The speed of sound depends on the physical properties of the medium and its temperature. Sound generally travels faster in solids. The molecules of a solid are closer together and the energy transfers more easily. The speed of sound is also affected by temperature. Speed increases as the temperature of the medium increases. Higher temperatures mean the molecules are vibrating faster which transfers energy more easily.
6.4. Loudness and Amplitude
Loudness depends on the amplitude of the wave and the distance between the source and the observer. Sounds with higher amplitudes are louder. When the observer is closer to the source, sound is louder. The unit for measuring loudness is decibels (dB). Normal conversation is about 50 dB. A car horn is 100 dB. Sounds above 120 dB can be painful and may damage a person’s hearing.
6.5. Pitch and Frequency
Pitch is how high or low a sound seems to the listener. The pitch of a sound depends on its frequency. Higher pitched sounds have a higher frequency while lower pitched sounds have a lower frequency. All animals have a range of frequencies they can hear. Humans can hear sound in a range from 20 Hz to 20,000 Hz. You cannot hear sounds that are 50,000 Hz, but your cat can. Sound waves above 20,000 Hz are called ultrasonic.
6.6. Doppler Effect
Think about the sound of the siren on a fire truck as it approaches and passes you. What happens to the pitch of the siren? The Doppler effect occurs when the source of a sound is moving. As the source of a sound approaches the listener, the sound waves get pushed together. The frequency and pitch will seem higher to the observer. After the source passes the observer, the sound waves get pulled farther apart. The frequency and pitch will seem lower to the observer.

6. Pause and Interact

7.1. Review
Use the whiteboard text tools to match each sound to its corresponding loudness.
7.2. Review
Use the whiteboard text tools to complete the activity.

7. Musical Sounds

8.1. Quality of Sound
We may not all like the same music but we can agree on which sound is music and which is noise. Noise is not a pleasing sound because the wave patterns are random. When the wave pattern is regular we hear pleasing sounds that we call music. Each instrument has a distinct sound that is called the quality of tone. This quality lets us distinguish between a violin and a tuba.
8.2. Music and Instruments
Instruments produce standing waves to make what we call music. The class of instrument is based on how the standing wave is produced. Stringed instruments make sound with vibrating strings. Wind instruments make sound with a vibrating column of air. Percussion instruments make sound with vibrating materials such as a drum head.
8.3. Fundamental Tones and Overtones
Instruments produce standing waves at certain frequencies. For example a violin string is fixed at both ends. When the string vibrates at its lowest frequency, it will have only one crest. This is called the fundamental tone. The first overtone has a higher frequency and will have a crest and a trough. The second overtone has two crests and a trough. When instruments play music together, the fundamental tones and overtones blend together.
8.4. Resonance
When a violin string vibrates, the wood of the violin will also vibrate at the same frequency. This makes the music louder. This vibration is called resonance. Every material has a natural frequency that will cause resonance. Resonance is also how an opera singer can shatter a glass. If the singer can produce a sound at the natural frequency of glass, the glass will resonate and shatter.

8. Pause and Interact

9.1. Tones and Overtones
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.
9.2. Review
Use the whiteboard text tools to complete the activity.

9. Practical Applications of Sound

10.1. Hearing
Sound waves cause the air around us to vibrate. When these vibrations reach our ears, the interaction produces what we call sound. The outer ear captures the sound as vibrating air molecules. The vibrating air causes the eardrum to vibrate. Three small bones in the middle ear carry the vibrations to the cochlea. The cochlea has tiny hairs which send a message to the brain along the auditory nerve. The brain processes the information it receives and interprets it as sound.
10.2. Echolocation
Some animals use a biological process called echolocation to help them hunt and navigate in the dark. Echolocation is the ability to locate a distant or a concealed object by reflecting sound waves off of the object. This is done by measuring the time taken for an echo to return and calculating the direction the echo came from. For example, a bat can send ultrasonic waves which reflect off of an insect and return to the bat’s ears. The bat then uses this reflected sound to pin point the exact location of the insect.
10.3. Ultrasound
Although humans cannot hear ultrasonic waves, we have found many ways to use them. Ultrasonic devices are used in navigation, detection and location of objects and even medicine. For example, doctors use ultrasound waves to look inside the human body. Ultrasound waves enter the body and reflect off of internal body organs. These waves reflect differently off the different organs. An ultrasound device uses the reflected waves to make a picture called a sonogram. Sonograms can be used to find and treat medical conditions, as well as to examine developing babies before they are born.
10.4. Sonar
Sonar stands for sound navigation and ranging. It is used to find the depth of water, map the ocean floor, and locate objects such as sunken ships. A sonar device sends a burst of ultrasound waves downward through the water. The waves strike a barrier, such as a sunken ship, and are reflected back. The device detects the waves and records how long it takes for the original wave to leave and return. A computer uses the speed of sound in water and the time for the wave to make a round trip to calculate the depth of the object.

10. Vocabulary Review

11.1. Sound Vocabulary Matching
Sound waves cause the air around us to vibrate. When these vibrations reach our ears, the interaction produces what we call sound. The outer ear captures the sound as vibrating air molecules. The vibrating air causes the eardrum to vibrate. Three small bones in the middle ear carry the vibrations to the cochlea. The cochlea has tiny hairs which send a message to the brain along the auditory nerve. The brain processes the information it receives and interprets it as sound.

11. Virtual Investigation

12.1. Speed of Sound
In this virtual investigation you will find the speed of sound in solids, liquids, and gases. You will fill a tube with a chosen material and send sound from a tuning fork through the tube. A detector will pick up the sound at the other end of the tube and show you the speed of sound in that material. You will record the data for all the materials. From your data you will draw conclusions about sound and states of matter. Will the speed of sound be different in a solid, liquid, or gas?In this virtual investigation you will find the speed of sound in air at different temperatures. You will fill the tube with air at a chosen temperature and send a sound from a tuning fork through the tube. The detector will show you the speed of sound at the chosen temperature. You will record data for all the given temperatures. From your data you will draw conclusions about sound and temperature. Will the speed of sound be different for the different temperatures of gas?

12. Assessment

13.1. Sound
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