7.8 The Interference of Sound Waves. Practice SUMMARY. Diffraction and Refraction of Sound Waves. Section 7.7 Questions

Transcription

1 Practice 1. Define diffraction of sound waves. 2. Define refraction of sound waves. 3. Why are lower frequency sound waves more likely to diffract than higher frequency sound waves? SUMMARY Diffraction and Refraction of Sound Waves Sound waves can be diffracted and refracted. Diffraction is greater when the sound wavelengths are larger. Section 7.7 Questions 1. Which sound waves from a home entertainment centre will be easier to hear in the next room through an open doorway, those from a woofer (low frequency) or a tweeter (high frequency)? Why? 2. For a person with equal sensitivity in both ears, would the direction from which a sound is coming be more easily identified if the sound has a high pitch or a low pitch? Explain your reasoning. Applying Inquiry Skills 3. Figure 4 shows noise barriers that are present on many highways. Propose a design that would improve on these barriers to make them more effective. Draw an example of your barrier and explain why it is more effective, using the concepts of diffraction and reflection. Making Connections 4. (a) Explain how the noise barriers in Figure 4 affect highway noises of differing frequencies. (b) Why is it not feasible to install the barriers shown in Figure 4 on all urban roads? (c) What else can be done to limit the spread of road noise? Figure 4 Noise barriers Reflecting 5. People who choose to purchase or rent a home near a noisy highway should not expect the government to erect sound barriers. Take a position for or against this statement and prepare arguments in support of your position. 7.8 The Interference of Sound Waves It is quite common for two or more sound waves to travel through a medium at the same time. When two or more sound waves act on the same air molecules at the same time, interference occurs. In Chapter 6, you examined the interference of transverse water waves and found that water waves can interfere constructively or destructively. Do sound waves exhibit many of the properties of wave interference? 260 Chapter 7

2 7.8 Investigation Interference of Sound Waves from a Tuning Fork and Two Loudspeakers In section 6.9, you examined the interference of two water waves originating from two point sources in a ripple tank. Since the water waves were identical in frequency and amplitude, a stationary interference pattern was produced consisting of a symmetrical pattern of alternating nodal lines and regions of constructive interference. In this investigation, you will examine the interference pattern created by sound waves originating from two identical sources: a rotating tuning fork (Figure 1), and two loudspeakers producing identical sound waves (Figure 2). Review the operation of a tuning fork in section 7.2. Questions Where are the areas of destructive interference located in the area surrounding the prongs of a tuning fork? Where are they located in the areas in front of two loudspeakers producing identical sound waves? Materials tuning fork rubber hammer or rubber stopper amplifier audio generator two identical loudspeakers Questioning Hypothesizing Predicting Planning Conducting rotation Figure 1 Position of tuning fork INQUIRY SKILLS Recording Analyzing Evaluating Communicating ear Prediction (a) Use diagrams to predict where areas of destructive interference are located in the area surrounding a vibrating tuning fork (Figure 1), and in the areas in front of two loudspeakers producing identical sound waves (Figure 2). Procedure 1. Strike a tuning fork with a rubber hammer or on a rubber stopper. Hold the fork vertically near your ear and slowly rotate it (Figure 1). Listen carefully for loud and soft sounds, and have your partner help you locate their exact positions. Repeat until you are certain of the results, and then draw a diagram of the top view of the tuning fork showing the positions of the loud and soft sounds. (To minimize reflection from walls, this should be performed in as large a room as possible.) 2. Set up the amplifier, generator, and speakers as shown in Figure 2, with the speakers approximately 2.0 m apart and raised about 1.0 m from the floor. Ensure that the speakers are in phase. Adjust the frequency of the generator to approximately 500 Hz, with the sound at a moderate intensity level. 3. Slowly walk along a path parallel to the plane of the speakers. Sketch the positions of the lowest sound intensity using your ears as detectors or a decibel meter, if available. 4. Slightly increase the frequency of the sound emitted from the speakers and repeat step 3. speaker observation path audio generator amplifier 2 m Figure 2 Setup for Investigation Very high or very low frequencies can cause discomfort or harm. speaker Properties of Sound Waves 261

3 Analysis (b) Use a diagram to illustrate how interference occurs near a single tuning fork. (Hint: What type of interference occurs where a compression interacts with a rarefaction?) (c) What changes in intensity occurred when you walked from one speaker to the other? Relate your observation to the interference between two identical sources in the ripple tank. (d) What changes in the intensity pattern occurred when the speakers were adjusted to emit a sound of higher frequency? Why? Evaluation (e) How did your results compare with your predictions? (f) List possible sources of error in this investigation. (g) Suggest changes to the procedure that would help reduce experimental errors. Interference Between Identical Sound Waves When the tines of a tuning fork vibrate, a series of compressions and rarefactions is emitted from the outer sides of the tines and from the space between them. Since the tines are out of phase, the compressions and rarefactions interfere destructively, producing nodal lines that radiate out from the corners of the tines. In the area between the tines, constructive interference occurs and a normal sound wave emanates from the tines. When the tuning fork is rotated near the ear, the relative sound intensity alternates between loud (normal sound intensity) and soft (destructive interference). The interference pattern between the two loudspeakers in phase (Figure 3) is similar to the pattern that is observed in water waves between two point sources. Areas of constructive and destructive interference are located symmetrically about the midpoint of the pattern, midway between the speakers. If the speaker 1 speaker 2 crest trough Figure 3 The interference of sound waves between two loudspeakers vibrating in phase lines of constructive interference midpoint line lines of destructive interference (minimum sound intensity) 262 Chapter 7

4 7.8 loudspeakers are in phase, there is an area of constructive interference (maximum sound intensity) at the midpoint. When the frequency is increased, the wavelength decreases. This produces more areas of destructive and constructive interference as shown, but the symmetry of the interference pattern does not change. It is difficult to produce areas of total destructive interference because sound waves are reflected from the walls and other surfaces in the room. Interference in sound waves from a single source may be demonstrated with an apparatus called a Herschel tube (Figure 4). Sound waves from a source such as a tuning fork enter the tube and split, travelling along two separate paths. If the paths are of the same length, the waves will meet on the other side in phase; that is, compression will meet compression, rarefaction will meet rarefaction, and the intensity will be at a maximum. If the tube is longer on one side, the waves on that side will have to travel farther. At some point, compressions will emerge with rarefactions and interfere destructively to produce a minimum sound intensity. Further extension of the tube on one side will reveal other positions in which constructive and destructive interference will occur. Noise cancellation headphones use a computer to cancel noise using destructive interference (Figure 5). The computer receives sound from a microphone, isolates the noise, and creates a sound wave exactly out of phase with the incoming sound. This can be more effective than normal noise suppression earphones used in noisy environments. When used with a CD player, a person can hear the music clearly even while cutting the lawn or riding in a crowded bus since the headphones electronically cancel up to 70% of external noise. Pilots use similar devices to suppress the external noise of a plane s engines, allowing for better communication and protection of the pilot s hearing. Other applications include voice recognition software on computers, communication systems for fast food drive-throughs, and mobile telephones. source Figure 4 A Herschel tube ear Herschel tube adjustable end Practice 1. Explain in your own words why there are loud and soft sound intensities in the area around a tuning fork. 2. When you extend one side of a Herschel tube, you reveal other positions in which constructive and destructive interference occur. Draw two diagrams to illustrate this situation. 3. Why is there a line of constructive interference (maximum sound intensity) at the midpoint line between the two speakers shown in Figure 3 of this section? Figure 5 Noise cancellation headphones Applying Inquiry Skills 4. Obtain a tuning fork from your teacher, build your own Herschel tube, and demonstrate it to someone in your class. SUMMARY The Interference of Sound Waves Sound waves interfere, producing areas of constructive and destructive interference. The interference pattern between two identical sources of sound is similar to that produced by identical point sources in a ripple tank. Properties of Sound Waves 263

5 Section 7.8 Questions 1. Stereo speakers have colour-coded terminals (usually black and red) in the back that are hooked up to the wire from the amplifier in the stereo system. If the connections are reversed, the second speaker moves out when the first speaker moves in. Why is it important to wire both speakers so they move in and out together? 2. Distinguish between destructive and constructive interference. Making Connections 3. Theatres and concert halls are designed to eliminate dead spots. Research the answers to the following questions. (a) What are dead spots? (b) How do engineers eliminate the dead spots from these facilities? 7.9 Beat Frequency beats: periodic changes in sound intensity caused by interference between two nearly identical sound waves We have been examining the interference of sound waves with identical frequencies and wavelengths. Now we will consider the interference of sound waves with slightly different frequencies and wavelengths. Consider a tuning fork that has one tine loaded with Plasticine or an elastic band wrapped around it. If this fork is struck at the same time as an unloaded, but otherwise identical, tuning fork, the observed sound will alternate between loud and soft, indicating alternative constructive and destructive interference. Such periodic changes in sound intensity are called beats. Activity Beats from Nearly Identical Tuning Forks This activity will allow you to observe sound beats produced by two tuning forks of nearly identical frequencies. elastic band Procedure 1. Place two mounted tuning forks close to and facing each other. Wrap an elastic band tightly around a prong on one of the tuning forks (Figure 1). 2. Sound the two forks together and describe the resulting sound. 3. Repeat the procedure using two elastic bands on the same prong. 4. Finally, remove the bands and repeat the process a third time. 5. Demonstrate beats with an oscilloscope and sound generator, if they are available, or use a computer program to demonstrate beats. Figure 1 Mounted tuning forks 264 Chapter 7