 # Mechanical Waves and Sound

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Mechanical Waves and Sound
Chapter 17 Mechanical Waves and Sound

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.

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

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

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

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

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.

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.

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.

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.

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

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.

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.

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.

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.