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

2. Chapter 17
*Mechanical wave- disturbance in matter that carries energy from one place to another
*Medium- material through which a wave travels
A mechanical wave is created when a source of energy causes a vibration to travel through a medium.
The three main types of mechanical waves are transverse waves, longitudinal waves, and surface waves.

3. Chapter 17
*Transverse wave- wave that causes the medium to vibrate at right angles to the direction in which the wave travels
*Crest- highest point above the rest position
*Trough- lowest point below the rest position

4. Chapter 17
*Longitudinal wave- wave in which the vibration of the medium is parallel to the direction the wave travels
*Compression- an area where the particles in a medium are spaced close together
*Rarefaction- an area where the particles in a medium are spread out

5. Chapter 17
*Surface wave- wave that travels along a surface separating two media

6. Chapter 17
*Periodic motion- any motion that repeats at regular time intervals
*Period- time required for one cycle
*Frequency- number of complete cycles in a given time

7. Chapter 17 *Hertz- cycles per second; frequency measurement
A wave’s frequency equals the frequency of the vibrating source producing the wave.
*Wavelength- distance between a point on one wave and the same point on the next cycle of the wave
Increasing the frequency decreases the wavelength.

8. Chapter 17 FORMULA (speed of waves)
Speed (v); wavelength (λ); frequency (f)
v = λ·f
Velocity of Propagation- speed at which a wave front passes through a medium, relative to the speed of light.

9. Chapter 17
If you assume that waves are traveling at a constant speed, then wavelength is inversely proportional to frequency.
*Amplitude- maximum displacement of the medium from its rest position
The more energy a wave has, the greater is its amplitude.

10. Chapter 17
*Reflection- wave bounces off a surface that it cannot pass through
Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down.
*Refraction- bending of a wave as it enters a new medium at an angle
When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side.

11. Chapter 17
*Diffraction- bending of a wave as it moves around an obstacle or passes through a narrow opening
A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle.
*Interference- two or more waves overlap and combine together
*Constructive interference- occurs when two or more waves combine to produce a wave with a larger displacement
*Destructive interference- occurs when two or more waves combine to produce a wave with a smaller displacement

12. Chapter 17
*Standing wave- wave that appears to stay in one place, and not moving through a medium
*Node- point on a standing wave that has no displacement from the rest position
*Antinode- point where a crest or trough occurs midway between two nodes
A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord.

13. Chapter 17
*Sound waves- longitudinal waves; compressions and rarefactions that travel through a medium
Many behaviors of sound can be explained using a few properties- speed, intensity and loudness, and frequency and pitch.
The speed of sound is 342 meters per second in dry air at 20ºC.

14. Chapter 17
*Intensity- rate at which a wave’s energy flows through a given area
*Loudness- physical response to the intensity of sound, modified by physical factors
*Pitch- frequency of a sound as you perceive it; it depends upon a wave’s frequency.

15. Chapter 17
Ultrasound is used in a variety of applications, including sonar and ultrasound imaging.
*Sonar- technique for determining the distance to an object under water.
*Doppler effect- change in sound frequency caused by motion
As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency.