Chapter 17 Mechanical Waves and Sound

1. Chapter 17 Mechanical Waves and Sound Section 17.1 Mechanical Waves

2. 17.1 Mechanical Waves • Interest Grabber (journal entry) • Vibrations • A wave is a vibration that carries energy from one place to another. But not all vibrations are waves. • Hold a pen lightly between your thumb and index finger. Shake your hand back and forth to make the pen seem to bend like it’s made of rubber. • Next, watch as the instructor holds a length of string and shakes the string back and forth. Observe the vibrations. • Questions: • 1. Describe the motion of the pen and the motion of the string. • 2. In which case did the vibration move from one place to another? In which case did the vibration stay in one place?

3. 17.1 Mechanical Waves • Review Energy- the ability to do work. • 1. Potential Energy- stored energy as a result of position or shape. • 2. Kinetic Energy- energy of motion • Mechanical Waves- A disturbance in matter that carries energy from one place to another. • Needs matter to travel through • The material through which the wave travels is called the medium.

4. 17.1 Mechanical Waves • Medium- can be a solid, liquid or gas. • Ex: water- travel on surface, Rope- rope is the medium A mechanical wave is created when a source of energy causes a vibration to travel through a medium (a vibration is a repeating back and forth motion) Ex: Shaking a rope- energy added to one end, the wave that results is a vibration that carries energy along the rope.

5. 17.1 Mechanical Waves • 3 Main Types of Mechanical Waves • 1. Transverse waves • 2. Longitudinal waves • 3. Surface waves

6. 17.1 Mechanical Waves • 1. Transverse waves- a wave that causes the medium to vibrate at right angles to the direction in which the wave travels. • Crest- the highest point of the wave, above the rest position. • Trough- the lowest point below the rest position.

7. 17.1 Mechanical Waves • Picture of Transverse Waves

8. 17.1 Mechanical Waves • 2. Longitudinal waves- a wave in which the vibration of the medium is parallel to the direction the wave travels. • Ex: Coils of a spring • Compression- area where the particles in a medium are spaced close together. • Rarefaction- area where the particles in a medium are spread out.

9. 17.1 Mechanical Waves

10. 17.1 Mechanical Waves • Surface waves- a wave that travels along a surface separating two media. • Ex: An ocean wave is a surface wave between the water and the air. • Looking at a bobber in the water- at the crest the bobber moves up and at the trough the bobber moves down.

11. 17.1 Mechanical Waves • Surface Wave

12. 17.2 Properties of Mechanical Waves • Periodic motion- any motion that repeats at regular time intervals. • Ex: Surfers counting the time between 2 successive crests. • Properties used to describe waves • 1. Period 4. Speed • 2. Frequency 5. Amplitude • 3. Wavelength

13. 17.2 Properties of Mechanical Waves • 1. Period- the time required for one cycle, a complete motion that returns to its starting point. • Ex: An ocean wave- time between 2 crests • 2. Frequency- the number of complete cycles in a given time. • Ex: For a wave- the number of wave cycles that pass a point in a given time.

14. 17.2 Properties of Mechanical Waves • Measured in cycles per second or hertz • Look at Figures (A and B) P. 504 • A waves frequency equals the frequency of the vibrating source producing the wave. • 3. Wavelength- the distance between a point on one wave and the same point on the next cycle of the wave. • Transverse wave- the wavelength is measured between crests or troughs • Longitudinal wave- the distance between adjacent compressions and rarefactions.

15. 17.2 Properties of Mechanical Waves • 4. Wave Speed- calculated by dividing its wavelength by its period or by multiplying wavelength by frequency. • Speed = distance • Time • Speed = Wavelength × Frequency • The speed of a wave can change if it enters a new medium or if pressure or temperature change.

16. 17.2 Properties of Mechanical Waves For most waves the speed is constant for a range of different frequencies. If waves are traveling at a constant speed, then wavelength is inversely proportional to frequency. Ex: lower frequency- longer wavelength higher frequency- shorter wavelength 5. Amplitude- maximum displacement of the medium from its rest position. The more energy a wave has the greater the amplitude.

17. 17.3 Behavior of Waves • Reflection- when a wave bounces off a surface that it cannot pass through. • Ex: Like a ball bouncing off a wall • Does not change the speed or frequency of a wave, but the wave can be flipped upside down. • Refraction- is the bending of a wave as it enters a new medium at an angle.

18. 17.3 Behavior of Waves • Ex: Pushing a lawnmower- going from grass to gravel. Speed difference between the two wheels causes the lawnmower to change direction. • When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side. • Ex: ocean waves that enter shore on an angle refract, parallel to shore do not.

19. 17.3 Behavior of Waves • Diffraction- the bending of a wave as it moves around an obstacle or passes through a narrow opening. • Ex: like the ripples made when a pebble is tossed into a pond. • A wave diffracts more if its wavelengths this large compared to the size of an opening or obstacle.

20. 17.3 Behavior of Waves • Interference- occurs when 2 or more waves overlap and combine together. • There are 2 types of interference. • Constructive interference- this is when the amplitude is increased. Waves line up crest to crest. • Destructive interference- the amplitude decreases. The waves do not line up correctly. Crest of one wave hits the trough of another wave.

21. 17.4 Sound and Hearing • Sound waves are longitudinal waves-compressions and rarefactions that travel through a medium. • Behaviors of sound can be explained using the following properties: speed, intensity, loudness, frequency, and pitch. • 1. Speed- the speed of sound varies in different media.

22. 17.4 Sound and Hearing • Sound travels the fastest in solids and slowest in gases. • The reason why is that the particles are closer together in a solid than in liquids and gases. • Speed of sound depends on many factors such as the density of the medium and how elastic the medium is.

23. 17.4 Sound and Hearing • Intensity and Loudness • 2. Intensity is the rate at which a waves energy flows through a given area. • Sound intensity depends on the waves amplitude and the distance from the sound source. • When someone whispers in your ear can have greater sound intensity than someone shouting across a field. • Measured in decibels (dB)

24. 17.4 Sound and Hearing • 3. Loudness- is more subjective than intensity because it is how somebody physically responds to the intensity of sound. • As intensity increases so does loudness. • However, the health of your ears and how your brain interprets the information can determine loudness too.

25. 17.4 Sound and Hearing • Frequency and Pitch • 3. Frequency of a sound wave depends on how fast the source of sound is vibrating. • The faster the vibration the higher the frequency. • Ex: instruments- A trumpet produces higher frequency than a french horn.

26. 17.4 Sound and Hearing • 5. Pitch is how you perceive the frequency of sound. • High frequency sounds have a high pitch. • Low frequency sounds have a low pitch. • Depends on age and health of ear. Older people have a harder time hearing a high pitch. • Most people hear between 20 Hz and 20,000 Hz.

27. 17.4 Sound and Hearing • Ultrasound- is sound at frequencies higher than most people hear. • Used in sonar and ultrasound imaging. • Sonar stands for sound navigation and ranging. • Used to determine the distance to an object under water. • Ultrasound shows images by sending ultrasound pulses into a patient.

28. 17.4 Sound and Hearing • Doppler Effect- a change in sound frequency caused by motion of the sound source, motion of the listener, or both. • Discovered by Christian Doppler an Austrian scientist. • As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency.