Sound

This chapter studies sound as a form of energy produced by vibrating objects and carried by a medium as a mechanical wave. You will learn that sound travels in air as a longitudinal wave, made of regions of high pressure called compressions and low pressure called rarefactions. You will describe a sound wave by its wavelength, frequency, time period, amplitude and speed, related by v = f x wavelength, and connect pitch with frequency, loudness with amplitude, and quality or timbre with the shape of the wave. You will see that sound travels fastest in solids, slower in liquids and slowest in gases, about 344 m/s in air at 22 degrees Celsius, and that its speed increases with temperature. You will study the reflection of sound, echoes, which need a minimum distance of about 17.2 m in air, and reverberation, and devices that use multiple reflections. Finally, you will learn the human audible range of 20 Hz to 20,000 Hz, infrasound and ultrasound, and applications such as SONAR and ultrasound scanning.

Detailed Worksheet: Sound Section A - Definitions (10 marks) 1. What is a longitudinal wave? Why is sound called a longitudinal wave? (2 marks) 2. Define wavelength, frequency and time period of a sound wave. (2 marks) 3. Distinguish between pitch and loudness. On what does each depend? (2 marks) 4. What is an echo? What is reverberation? (2 marks) 5. What is ultrasound? Write two applications. (2 marks) Section B - Calculations and Applications (15 marks) 6. A sound wave has a frequency of 2 kHz and wavelength of 35 cm. How long will it take to travel 1.5 km? (3 marks) 7. A person clapped his hands near a cliff and heard the echo after 2 s. What is the distance of the cliff from the person if the speed of sound is 346 m/s? (3 marks) 8. A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound in seawater is 1531 m/s, find the depth of the sea. (3 marks) 9. The audible range of human hearing is 20 Hz to 20 kHz. Find the typical wavelengths of sound waves in air corresponding to these two frequencies. (Take the speed of sound in air as 344 m/s.) (3 marks) 10. Find the minimum distance from a reflecting surface needed to hear a distinct echo, if the persistence of hearing is 0.1 s and the speed of sound is 344 m/s. Why are echoes not heard in a small room? (3 marks) Section C - Diagrams (10 marks) 11. Draw a diagram showing the propagation of sound from a vibrating tuning fork in air, marking compressions, rarefactions and wavelength. Draw the corresponding density or pressure graph. (4 marks) 12. Draw a labelled diagram of the experiment with two pipes and a screen to show that sound follows the laws of reflection. (3 marks) 13. Draw a labelled diagram of how SONAR is used to find the depth of the sea. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain how multiple reflection of sound is used in (i) a megaphone (ii) a stethoscope (iii) curved ceilings of concert halls (iv) sound boards. Why are the walls of auditoriums covered with sound-absorbent materials? (5 marks) 15. Explain why sound travels faster in solids than in gases. A man strikes one end of a long iron rail, and another person puts his ear to the other end 1 km away. Why does he hear two sounds? Find the time interval between them if the speed of sound in iron is 5950 m/s and in air is 344 m/s. (5 marks) 16. Describe four uses of ultrasound in industry and medicine. How do bats use ultrasound to navigate and catch prey? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write the formula and give units with every numerical answer.