1. Chapter 13 Sound

2. Section 13.1 Sound Waves

3. The Production of Sound Waves  Sound is a result of vibrations or oscillations.  How We Hear Video (1:05) How We Hear Video How We Hear Video

4.  A region of lower density and pressure is rarefaction.  A region of high molecular density and high air pressure is called compression.

5. Sounds waves are longitudinal waves  The crests in the curve correspond to compressions in the sound wave, and the troughs correspond to rarefactions.

6. Characteristics of Sound Waves  Frequency = the number of cycles per unit of time.  Infrasonic waves < 20Hz  Ultrasonic waves > 20,000 Hz  20 Hz < audible sound waves (humans) < 20,000 Hz Dogs can hear ultrasonic waves, and Dogs can hear ultrasonic waves, and elephants can hear infrasonic waves. elephants can hear infrasonic waves.

7. Cont.- Frequency determines pitch  The frequency of an audible sound wave determines how high or low we perceive the sound to be, known as pitch.  As the frequency of a sound wave increases, the pitch rises.  As the frequency decreases, the pitch falls.

8. FYI  Ultrasonic waves have widespread medical applications like to produce images of objects inside the human body (“Ultrasounds”)  Sound waves are partially reflected when they reach a boundary between two materials of different densities.  Ultrasonic waves are clearer and more detailed than other those of a lower-frequency sound wave because they are easily reflected off small objects.  Audible and infrasonic sound waves are not as effective because their longer wavelengths pass around small objects.

9. Other medical & industrial uses of ultrasounds  Pierre Curie discovered how to produce ultrasonic vibrations in 1890. By World War II their first practical application was the detection of submarines.  Ultrasonic energy can heat deep tissues, to treat arthritis, bursitis, muscular dystrophy, and other diseases.

10.  High-energy ultrasonic waves can be focused into a pinpoint "scalpel" for bloodless brain surgery.  Cleaning, sterilization, machining, cutting diamonds, burglar alarms and remote- control television tuners. It is also sometimes used in welding and soldering metals, mixing liquids, and dyeing and bleaching textiles. Other medical & industrial uses of ultrasounds

11. Speed of sound depends on the medium  Sound waves can travel through solids, liquids, and gases.  Solid particles respond more quickly to vibrations than gas particles because those molecules are closer together. Therefore, sound travels faster in solids.

12. Temperature  In a gas, the disturbance can spread faster at higher temperatures than at lower ones because of an increase in collisions.  Sound speed and frequency video (1:51) Sound speed and frequency video (1:51) Sound speed and frequency video (1:51)

13. The Doppler Effect  Doppler Effect Video 2:01 Doppler Effect Video Doppler Effect Video  Pitch is higher as the object moves towards you and lower as it moves away.

14. Relative motion creates a change in frequency The frequency of the car horn is constant, but because the source is moving towards observer A, the wave fronts hit Observer A sooner and sooner. As a result, Observer A hears a greater frequency sound resulting in higher pitch. higher pitch. Remember we are discussing frequency and not speed. The speed of the sound waves remains constant.

15. Relative motion creates a change in frequency  The sound waves don’t reach point B, behind the car as frequently as in front of it. Therefore, the frequency heard by the observer is less than the source frequency.  The Doppler effect is a shift in frequency due to motion between a wave source and an observer.

16.  The Doppler effect will occur when the source or the observer is moving.  Even though it is used mostly with sound waves, the Doppler effect can be used with electromagnetic waves like visible light.  The Doppler Effect Applet The Doppler Effect Applet The Doppler Effect Applet

17. Homework  P 486 #1-4, 6

18. Sound 13.2

19. Relative Intensity  Relative Intensity is the relationship between the intensity of a sound wave (its energy) to the intensity at the threshold of hearing. It is related to loudness and is measured in decibels (db).

20. Decibel Levels  Volume (Loudness) doubles with every 10 db.  Ex: 0 db= threshold of hearing 0 db= threshold of hearing 30 db= whisper 30 db= whisper 50 db= normal conversation 50 db= normal conversation 70 db= vacuum cleaner 70 db= vacuum cleaner 90 db= lawn mower 90 db= lawn mower 120 db= threshold of pain 120 db= threshold of pain

21. Vibrations  Sympathetic Vibration- when a vibrating object causes another object to vibrate. Ex: Plucked guitar strings vibrate and cause the body of the guitar to vibrate. Ex: Plucked guitar strings vibrate and cause the body of the guitar to vibrate.  Natural frequency- this is the set frequency at which a pendulum (or string) will vibrate and is determined by length.

22. Resonance  Resonance is a condition that exists when the frequency of a force applied to a system matches the natural frequency of vibration of the system.  Shattering Glass Resonance Video 5:18 Shattering Glass Resonance Video Shattering Glass Resonance Video

23. 13.3

24. Harmonics  A vibrating string will produce standing waves whose frequencies depend upon the length of the string. Harmonics Video 2:34

25. Harmonics  As the harmonic increases the frequency increases and wavelength decreases.  Standing Waves, Fixed at Standing Waves, Fixed at Standing Waves, Fixed at Both Ends Animation Both Ends Animation

26. Why do different instruments sound different?  Timbre is the quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities.  This is why a clarinet and a trumpet can play the same pitch but they sound different.  Harmonics Applet Harmonics Applet Harmonics Applet

27. Beat  When two waves of slightly different frequencies travel in the same direction they interfere. This causes a listener to hear an alternation between loudness and softness and is called beat.

28. Beat  Formation of Beats Applet Formation of Beats Applet Formation of Beats Applet  The frequency difference between two sounds can be found by the number of beats per second.

29. Homework  P 493 #1, 3, 4  P 503 #2, 3