1. Sound Waves
Unit 9.1

2. Sound Production
Whether a sound wave conveys the shrill whine of a jet engine or the melodic whistling of a bird, it begins with a vibrating object.

3. Sound Production
We will explore how sound waves are produced by considering a vibrating tuning fork. The vibrating prong of a tuning fork, set the air molecules near it in motion.

4. Sound Production

5. Sound Production
As the prong swings to the right, air molecules in front of the movement are forced closer together. Such a region of high molecular density and high air pressure is called a compression.

6. Sound Production
As the prong moves left, the molecules to the right spread apart, and the density and air pressure in this region become lower than normal. This region of lower density and pressure is called a rarefaction.

7. Sound Production
As the tuning fork continues to vibrate, a series of compressions and rarefactions form and spread away from each prong.

8. Sound Production
These compressions and rarefactions expand and spread out in all directions. In sound waves, the vibrations of air molecules are parallel to the direction of wave motion.

9. Characteristics
Sound waves that the average human ear can hear, called audible sound waves, have frequencies of 20Hz – 20,000Hz.

10. Characteristics
Sound waves with frequencies less than 20 Hz are called infrasonic waves, and those above 20,000 Hz are called ultrasonic waves.

11. Characteristics
The frequency of an audible sound wave determines how high or low we perceive the sound to be, which is known as pitch.

12. Characteristics
As the frequency of a sound increases, the pitch rises.

13. Characteristics
The frequency of a wave is an objective quantity that can be measured, while pitch refers to how different frequencies are perceived by the human ear.

14. Characteristics
Infrasonic waves have longer wavelengths than audible sound waves, and ultrasonic waves shorter wavelengths.

15. Characteristics
Ultrasonic waves have widespread medical applications. Ultrasonic waves can be used to produce images of objects inside the body.

16. Characteristics
In order for ultrasonic waves to “see” an object inside the body, the wavelength of the waves used must be about the same size or smaller than the object.

17. Characteristics
Dolphin echolocation works in a similar manner. A dolphin sends out pulses of sound, which return in the form of reflected sound waves.

18. Characteristics
These reflected waves allow the dolphin to form an image of the object that reflected the waves. High frequency waves are used for echolocation.

19. Characteristics
Sound waves can travel through solids, liquids, and gases.

20. Characteristics
Because waves consist of particle vibrations, the speed of a wave depends on how quickly one particle can transfer its motion to another particle.

21. Characteristics
In a solid, particles respond more rapidly to a disturbance as the particles are closer together than in a gas, resulting in faster waves.

22. Characteristics
Sound waves travel away from a vibrating source in all three directions. Three dimensional sound waves are approximately spherical.

23. Characteristics
Spherical waves can be represented graphically in two dimensions with a series of circles surrounding the source.

24. Characteristics
The circles represent the centers of compressions, called wave fronts. Because we are considering a 3D phenomenon in 2D, each represents a spherical area.

25. Characteristics
Because each wave front corresponds to the center of the compression, the distance between adjacent wave fronts is equal to one wavelength.

26. Characteristics
The radial lines perpendicular to the wave fronts are called rays. Rays indicate the direction of wave motion.

27. Doppler Effect
In earlier examples, we assumed that both the source of the sound waves and the listener were stationary. We will now look at moving objects and/or listeners.

28. Doppler Effect
This relative motion affects the way the wave fronts of the sound waves produced by the car’s horn are perceived by an observer.

29. Doppler Effect

30. Doppler Effect
As the waves are compressed, person A hears a frequency greater than the source frequency. Person B will hear a frequency less than the source frequency since the waves are not compressed.

31. Doppler Effect
This frequency shift resulting from relative motion between the source of the waves and the observer are known as the Doppler effect.

32. Doppler Effect
The Doppler effect is named for the Austrian physicist Christian Doppler, who first described it.