Mechanical Waves and Sound Chapter 17 Mechanical Waves and Sound

Mechanical Waves Section 17-1

Mechanical Waves A mechanical wave is a disturbance in matter that carries energy from one place to another. A medium is the material through which a wave travels. A mechanical wave is created when a source of energy causes a vibration to travel through a medium.

Types of Mechanical Waves The three main types of waves are transverse waves, longitudinal waves, and surface waves.

Transverse Waves A transverse wave is a wave that causes the medium to vibrate at right angles to the direction in which the wave travels. See Presentation Pro.

Transverse Waves Crest – the highest point of the wave above the rest position. Trough – the lowest point of the wave below the rest position. Rest position – the point half way between the crest and the trough. (The position line before the wave began or after it ended.)

Longitudinal Waves Longitudinal wave – a wave in which the vibration of the medium is parallel to the direction the wave travels. See Presentation Pro.

Longitudinal Waves 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.

Surface Waves Surface wave – a wave that travels along a surface separating two media. Ocean waves are the most familiar type of surface wave. See Presentation Pro.

Properties of Mechanical Waves Section 17-2

Frequency and Period Periodic motion – any motion that repeats at regular time intervals Period – the time required for one cycle, a complete motion that returns to its starting point. Frequency – the number of complete cycles in a given time.

Frequency and Period Hertz – cycles per second – measurement of frequency. A wave’s frequency equals the frequency of the vibrating source producing the wave.

Wavelength Wavelength – the distance between a point on one wave and the same point on the next cycle of the wave. Increasing the frequency of a wave decreases its wavelength. See Presentation Pro.

Wave Speed & Amplitude Speed = Wavelength x Frequency Amplitude – the maximum displacement of the medium from its rest position. The more energy a wave has, the greater is its amplitude. See Presentation Pro.

Behavior of Waves Section 17.3

Reflection Reflection – occurs when a 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 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 on side of the wave moves more slowly than the other side.

Diffraction Diffraction – the 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 Interference – occurs when two or more waves overlap and combine together. Two types of interference are constructive interference and destructive interference.

Interference 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. See Presentation Pro

Standing Waves Standing wave – a wave that appears to stay in one place – it does not seem to move through the medium. Example: plucking a guitar string Node – a point on a standing wave that has no displacement from the rest point. See figure 13 page 512.

Standing Waves Antinode – a point where a crest or a 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.

Sound and Hearing Section 17-4

Properties of Sound Waves Many behaviors of sound can be explained using a few properties – speed, intensity and loudness, and frequency and pitch. Speed – it takes time for sound to travel from place to place…and that time is different for different medium. See Presentation Pro – Section 4

Properties of Sound Waves Intensity – the rate at which a wave’s energy flows through a given area. Decibel (dB) – a unit that compares the intensity of different sounds. Loudness – a physical response to the intensity of sound, modified by physical factors.

Properties of Sound Waves Frequency – in a sound wave, frequency depends on how fast the source of the sound is vibrating. Pitch – how you perceive the frequency of a sound

Ultrasound Ultrasound – sound at frequencies higher than most people can hear. Ultrasound is used in a variety of applications, including sonar and ultrasound imaging.

Ultrasound Sonar – a technique for determining the distance to an object under water. Stands for sound navigation and ranging. Ultrasound imaging – an important medical technique sending short burst of a wave into a patient. Computer software uses the reflected pulses to make a picture of an organ or other structure inside the body.

The Doppler Effect Doppler effect – a change in sound frequency caused by motion of the sound source, motion of the listener, or both. As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency. See Presentation Pro

Hearing and the Ear The outer ear gathers and focuses sound into the middle ear, which receives and amplifies the vibrations. The inner ear uses nerve endings to sense vibrations and send signals to the brain.

How sound is reproduced Sound is recorded by converting sound waves into electronic signals that can be processed and stored Sound is reproduced by converting electronic signals back into sound waves.