Understanding Waves: A Focus on Sound Waves, Lecture notes of Voice

Download Understanding Waves: A Focus on Sound Waves and more Lecture notes Voice in PDF only on Docsity! 2 10. Sound Waves eLearn.Punjab 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. 3 10. Sound Waves eLearn.Punjab Animation 10.2: Wavelength Source & Credit: isvr 6 10. Sound Waves eLearn.Punjab 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. 7 10. Sound Waves eLearn.Punjab 10.2: Wavelength, Speed, Amplitude and Frequency The basic terms to understand waves are amplitude, wavelength, frequency and speed. Wavelength A wavelength is the shortest distance between two adjacent crests or troughs of a transverse wave. For longitudinal waves, it is the distance between two adjacent compressions or rarefactions (Fig.10.5). Wavelength is measured in metres (m). Fig. 10.5: Distance between two adjacent crests or compressions is the wavelength. Animation 10.5: Wavelength Source & Credit: physicsclassroom 8 10. Sound Waves eLearn.Punjab 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. 11 10. Sound Waves eLearn.Punjab Animation 10.8: Longitudinal wave Source & Credit: Wikipedia Animation 10.9: Wave speed Source & 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. 12 10. Sound Waves eLearn.Punjab Animals Frequency range(Hz) Animals Frequency range(Hz) dog 20 - 45,000 dolphin 150 - 150,000 cat 45 - 64,000 rat 200 - 76,000 cow 23 - 35,000 bat 2,000 - 110,000 horse 55 - 33,500 elephant 1 - 20,000 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. 13 10. Sound Waves eLearn.Punjab Activity 10.2 Making High and Low Pitched Sounds You will need: • 5 empty glasses • Metal spoon • Water Procedure 1. Put different amounts of water in each glass. 2. Carefully tap each glass with the spoon. Observe what you hear. 3. Arrange the glasses from the lowest to the highest sound. Questions i. Which glass has the lowest pitch? ii. Which glass has the highest pitch? Loudness Sometimes, we need to shout in a louder voice. We have to use an extra energy. Loudness is related to the amplitude of a sound. The larger the amplitude, the louder the sound. Loudness helps us to distinguish a soft sound from a loud sound of the same frequency. Fig.10.10: Waves of a loud amplitude sound have large amplitudes. Fig.10.11: Waves of a soft sound have small amplitudes. 16 10. Sound Waves eLearn.Punjab 10.5: Applications of Different Sounds Making Sounds Sounds are very important in our lives. We use many devices which produce different sounds. 17 10. Sound Waves eLearn.Punjab 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. 5. What type of waves are sound waves, and how do they transfer sound energy?