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An in-depth exploration of waves, with a particular focus on sound waves. Students will learn about the properties of sound waves, including wavelength, frequency, amplitude, and speed. They will also discover how these properties are related and explore the audible frequency range for humans and various animals. animations and diagrams to enhance understanding.
What you will learn
Typology: Lecture notes
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After completing this chapter, the students will be able to:
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
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
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 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
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
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
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,
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.
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.
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.
You will need
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.
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 ________