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Students’ Learning Outcomes
After completing this chapter, the students will be able to:
- Explain the wavelength, frequency and amplitude of sound waves and give their units.
- State factors on which sound depends.
- Investigate objects in home and surroundings that are designed and made to produce different sounds.
- Compare audible frequency range of humans and different animals.
- Design a musical instrument to explain the relation between its sound and shape.
- Identify the application of different sounds in daily life.
Animation 10.2: Wavelength Source & Credit: isvr
Animation 10.3: Transverse Waves Source & Credit: acs.psu
Animation 10.4: Longitudinal Waves Source & Credit: acs.psu
10.1.2: Longitudinal Waves
A wave in which particles of a medium move back and forth, parallel to the direction of the wave is called a longitudinal wave. Take a slinky spring as shown in Fig.10.4. If we pull and push one end of the slinky spring continuously, we can produce a longitudinal wave (Fig.10.4). The parts of a longitudinal wave, where particles of the medium are compressed together, are called compressions. The parts of a longitudinal wave, where particles of the medium are spread out, are called rarefactions. As the wave moves, compressions and rarefactions are produced due to the back and forth motion of particles of the medium. Sound from a vibrating body produces longitudinal waves in air. These waves reach our ear and affect the ear drum.
A compression and a rarefaction is combined to form a longitudinal wave. What about a transverse wave?
Fig. 10.4: Longitudinal waves in a slinky spring
Sound waves are longitudinal waves
A sound wave traveling through air is a an example of a longitudinal wave. When a drummer beats a drum, the surface of the drum vibrates and creates a disturbance in the air beside it. When the drumhead moves to the left, it compresses the particles of air and create a compression. When the drumhead moves to the right, the particles of the air on the right move farther apart, creating a rarefaction. These compressions and rarefactions travel through the air as longitudinal waves. When the disturbance in the air reaches our ears, we hear the sound of the drum.
Amplitude
Amplitude of a wave is the maximum distance of the particles of the medium from the rest position. We can also say that it is the height of a crest or depth of a trough (transverse wave) measured from the rest position (Fig.10.6). Amplitude is measured in metres (m).
Fig. 10.6: Amplitude of a transverse wave
Frequency
The number of vibrations produced by a vibrating body in one second is called frequency (Fig.10.7). Frequency is measured in units called hertz (Hz). When one wave passes through a point in one second the frequency is 1 wave per second or 1 hertz.
Fig. 10.7: The wave on the bottom has a frequency three times greater than the wave on the top.
Animation 10.6: Wavelength. Source & Credit: heasarc.nasa
Speed
Imagine watching a flash of lightning and thundering of cloud. First we see the flash of lightning. A few seconds later we hear thunder. This happens because sound and light travel at different speeds. Light travels much faster than sound. Different waves travel at different speeds. The distance a wave covers in unit time is called its speed. Speed is measured in metre per second. Sound travels at different speeds in different mediums.
Fig.10.8. Thunder is always heard after we see lightning. It shows that light travels Animation 10.7: Transverse wave much faster than sound. Source & Credit: Wikipedia
Animation 10.8: Longitudinal wave Source & Credit: Wikipedia Animation 10.9: Wave speedSource & Credit: acs.psu
10.3: Audible Frequency Range
The word audible means ‘able to be heard’. Our ears cannot hear sounds of all frequencies. The range of frequencies which a person can hear is known as audible frequency range. A healthy human ear can hear sounds of frequencies from about 20 Hz to 20,000 Hz. It is the audible frequency range for humans. Different animals have different audible frequency ranges.
Fig.10.9. The audible frequency range reduces in most old people.
Animals Frequency range(Hz) Animals Frequency range(Hz) dog 20 - 45,000 dolphin 150 - 150, cat 45 - 64,000 rat 200 - 76, cow 23 - 35,000 bat 2,000 - 110, horse 55 - 33,500 elephant 1 - 20,
Table 10.2: Audible Frequency Ranges of Different Animals
10.4: Pitch and Loudness
Everyday, we hear a great variety of sounds. We enjoy some sounds. Some sounds are undesirable. Sounds produced by radio, television and musical instruments are pleasant. Sounds produced by machines, traffic on a road, etc. are undesirable. How can we distinguish between the sounds? Pitch and loudness are the characteristics that help us to decide whether a sound is pleasant or not.
Pitch
The voice of a girl is more shrill than the voice of a boy. This difference is due to the pitch. A shrill sound is called a high pitch sound, whereas a less shrill sound is called a low pitch sound. Pitch is the shrillness or graveness of a sound. Pitch of the sound depends on the frequency of the sound wave. The higher the frequency, the higher the pitch is.
Activity 10.1 Frequency and Pitch
- Rotate the wheel of your bicycle as shown in the figure.
- Touch a piece of cardboard to the spokes of the rotating wheel and listen to the sound produced.
- Now increase the speed of rotating wheel and again listen to the sound produced. We observe that on increasing the speed of the wheel, the sound becomes more shrill due to increase in its frequency. In other words we can say that the pitch of the sound has increased.
Spoon Sounds Hit a spoon on the edge of an empty bowl, listen to the sound produced. Try it on different objects.
Ruler Sounds Hold one end of a steel ruler on the edge of a table. Push down the other edge of the ruler. Let it go and try to hear sound.
Wind Instrument — Flute A flute is a wind instrument. The flautist has to blow it to make music. Flutes are hollow tubes with a mouthpiece and a series of holes. The holes can be closed to control the length of the vibrating column of air inside the tube. A flute can be made of wood, metal and plastic. The flautist changes the sound by opening and closing the holes in the flute.
Making Sounds It is not difficult to make sounds but it is sometimes difficult to see what is happening when sounds are made.
Activity 10.3 Making High and Low Sounds
You will need
- 2 feet of ½ -inch PVC sprinkler water pipe
- 5 coins of the size of the diameter of the pipe
- 2-inch wide tape Procedure
- Cut the PVC pipe into five sections of different lengths.
- Place a coin over one end of each pipe and cover each coin with the tape.
- Wrap the tape around the set of pipes as shown in the figure.
- Blow across the top of each pipe — it is just blowing on a soda bottle. Questions i. What happens to the sound as you go from longest pipe to the shortest pipe? ii. Which pipe makes the lowest pitch of sound? iii. Which pipe makes the highest pitch of sound?
Sound waves with frequencies above the normal human range (20,000Hz) of hearing are called ultrasound. The sound waves which have frequencies below 20Hz are called infra-sound.
- Doctors use ultrasound to examine a patient internally.
- Manufacturers of concrete slabs use ultrasound waves to check the cracks or cavities in concrete slabs.
Questions
1. Complete each of the following sentences by writing the correct term. i. The lower portion of a transverse wave ________ ii. The Shrillness or graveness of a sound ________ iii. The distance a wave covers in one second ________ iv. A compression and a rarefaction combine to form ________ v. A material thing through which a wave travels ________
Key Points
- In transverse waves particles of the medium vibrate at right angle to the path of the wave. In a longitudinal wave, particles of the medium vibrate back and forth, parallel to the path of the wave.
- Sound waves are longitudinal waves.
- Wavelength of a sound wave is the distance between two adjacent compressions or rarefactions. It is measured in metres (m).
- Frequency is the number of vibrations produced by a vibrating body in one second. It is measured in hertz (Hz).
- Amplitude of a wave is the maximum distance the wave vibrates from its rest position. It is also measured in metres (m).
- Pitch and loudness are the characteristics on which sound depends.
- A healthy human ear can hear sounds of frequencies from about 20Hz to 20,000Hz. Different animals have different audible frequency ranges.
- We use doorbells, sirens, telephones, alarms, stereo players, etc. that produce different sounds. 3. Give short answers. i. Sketch a transverse wave and label a crest, a trough, a wavelength, and amplitude. ii. Define the wavelength of a longitudinal wave. iii. Name a few devices that use different sounds in our everyday life. iv. What makes some sounds louder than others? v. What is the relationship between frequency and pitch? vi. How does sound travel? **4. Compare transverse waves and longitudinal waves.
- What type of waves are sound waves, and how do they transfer sound energy?**
