Mechanical Waves and Sound

Transcription

1 Mechanical Waves and Sound

2 Mechanical Wave Medium Crest Trough Transverse wave Compression Rarefaction Longitudinal wave Surface wave Some Vocab to Know

3 What are Mechanical Waves? Mechanical wave: disturbance in matter that carries energy from one place to another. Recall: energy is ability to do work

4 Mechanical Waves Require matter to travel through. Medium: material through which a wave travels Ex: Solid, Liquid, Gas Wave pool: waves travel along the surface of the water. Water is the medium. Rope: waves travel through rope when you shake it. Rope is the medium.

5 How are Mechanical Waves Created? When a source of energy causes a vibration to travel through a medium. Vibration: repeating back-and-forth motion Ex.) When you shake a rope you add energy at one end. The wave that results is a vibration that carries energy along the rope.

6 Types of Mechanical Waves Classified by way they move through a medium 3 types: Transverse Waves Longitudinal Waves Surface Waves

7 Transverse Waves Before the wave begins, every point on rope is in rest position (based on dotted line) The highest point of the wave above the rest position is the crest The lowest point below the rest position is the trough Crests and troughs are not in fixed points along a wave

8 Cont. The motion of a single point on the rope is like the motion of a yo-yo. The point vibrates up and down between a maximum and minimum height. A transverse wave is a wave that causes the medium to vibrate at right angles to the direction in which the wave travels.

9 Longitudinal Waves To start the wave, add energy to the spring by pushing and pulling the end of a spring. The wave carries energy along the spring from left to right. Compression: area where particles in a medium are spaced close together. As compression moves to right, coils behind it are spread out more than in rest position Rarefaction: area where particles in a medium are spread out.

10 As compressions and rarefactions travel along the spring toward the right, each coil vibrates back and forth around its rest position. Longitudinal wave: a wave in which the vibration of the medium is parallel to the direction the wave travels. Cont.

11 More Types of Longitudinal Waves P (primary) waves are longitudinal waves produced by earthquakes. Can travel through Earth, so scientists use these waves to map Earth s interior

12 Surface Waves Surface Wave: wave that travels along a surface separating two media Ex) ocean waves

13 Properties of Mechanical Waves

14 Some Vocab To Know Periodic Motion Period Frequency Hertz Wavelength Amplitude

15 Periodic Motion How do surfers know when the next wave is coming? If they count the time between two successive crests, the next crest usually will come after this same time interval. Any motion that repeats at regular time intervals is called periodic motion.

16 Period The time required for one complete cycle, resting point to resting point, is called the period. For an ocean wave, the period is the time between two successive crests.

17 Frequency Any periodic motion has a frequency, which is the number of complete cycles in a given time. For a wave, the frequency is the number of wave cycles that pass a point in a given time. Measured in cycles per second, or hertz (Hz) A waves frequency equals the frequency of the vibrating source producing the wave.

18 Wavelength: distance between a point on one wave and the same point on the next wave Transverse waves: measured from crest to crest or trough to trough Longitudinal waves: measured from compression to compression or rarefaction to rarefaction. Increasing frequency of a wave decreases its wavelength. Wavelength

19 Wave Speed Recall: speed = d/t Wave speed = wavelength x frequency Wavelength in meters Frequency in hertz (=1/s) Units for speed are meters per second

20 Practice

21 Practice What is the wavelength of an earthquake wave if it has a speed of 5m/s and a frequency of 10Hz?

22 Practice A motorboat is tied to a dock with its motor running. The spinning propeller makes a surface wave in the water with a speed of 0.4 m/s and a wavelength of 0.1m. What is the frequency of the wave?

23 More Info on Wave Speed Speed of a wave can change if it enters a new medium or if variables such as pressure and temperature change If you assume that waves are traveling at a constant speed, then wavelength is inversely proportional to frequency. Two waves with different frequencies means the wave with lower frequency has a longer wavelength

24 Amplitude: maximum displacement of the medium from its rest position. Transverse wave: distance from the rest position to a crest or trough. Longitudinal wave: maximum displacement of a point from its rest position The more energy a wave has, the greater its amplitude Amplitude

25 Behavior of Waves

26 Some Vocab to Know Reflection Refraction Diffraction Interference Constructive interference Destructive interference Standing wave Node Antinode

27 Reflection Occurs when a wave bounces off a surface that it cannot pass through. Ex.) If you send a transverse wave down a rope attached to a wall, the wave reflects when it hits the wall.

28 Cont. Does not change the speed or frequency of a wave, but the wave can be flipped upside down. If reflection occurs at a fixed boundary, then the reflected wave will be upside down compared to the original wave.

29 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. Occurs only when the two sides of a wave travel at different speeds.

30 Example of Refraction Lines (wave fronts) show changing direction of wave Lines are parallel to crests of waves Fronts approach shore at angle. Left side enters shallow water first As left side of wave slows down, the wave bends toward the left As a wave flows into shallow water, it can be considered a new medium. If fronts are parallel to shoreline, the wave enters the shallower water all at once. Wave slows down, does not change direction

31 Diffraction Bending of a wave as it moves around an obstacle or passes through a narrow opening. Diffracts more if its wavelength is large compared to the size of an opening or obstacle. Wavelength small compared to opening/obstacle, wave bends very little

32 Interference If two balls collide, they cannot continue on their original paths, but wave can occupy the same region of space. Two or more waves overlap and combine together. Two types of interference: Constructive Destructive

33 Constructive Interference Two or more waves combine to produce a wave with a larger displacement. Ex.) Imagine a kid being pushed on a swing by their mom. If the mom times her push correctly, she will push on the swing just as the child starts to move forward. Then the mother's effort is maximized and the child gets a boost to go higher. In the same way, the amplitudes of two waves can add together.

34 Destructive Interference Occurs when two or more waves combine to produce a wave with a smaller displacement. Ex.) What happens if the mother has bad timing while pushing on the swing? Instead of working to boost her child upward, some of her effort is wasted, and the child will not swing as high. Destructive interference can reduce the amplitude of a wave.

35 Standing Wave If you tie one end of a rope to a chair and shake the other end, waves travel up the rope, reflect off the chair, and travel back down the rope. Interference occurs when incoming wave pass through reflected wave. At certain frequencies, interference between a wave and reflection can produce a standing wave.

36 Wave that appears to stay in one place- it does not seem to move through the medium. Standing Wave Node: point on a standing wave that has no displacement from the rest position; Stationary. One Wavelength Antinode: point where a crest or trough occurs midway between two nodes. At the nodes, there is complete destructive interference between the incoming and reflected waves. antinode node

37 Sound and Hearing

38 Sound waves Intensity Decibel Loudness Pitch Sonar Doppler effect resonance Some Vocab to Know

39 Properties of Sound Waves Sound waves are longitudinal waves Properties include: Speed Intensity Loudness Frequency Pitch

40 Speed It takes time for sound to travel from place to place In dry air at 20 C, the speed of sound is 342 meters per second In general, sound waves travel fastest in solids, slower in liquids, and slowest in gases. Due to particle closeness in medium Speed depends on density and elasticity of medium

41 Intensity and Loudness Intensity: rate at which a wave s energy flows through a given area. Depends on wave s amplitude and distance from sound source Measured in decibels (db) based on powers of ten; for every 10-decibel increase, the sound intensity increases tenfold

42 Loudness Subjective: subject to a person s interpretation. Loudness: physical response to the intensity of sound, modified by physical factors. Depends on sound intensity (direct relationship)

43 Frequency and Pitch Frequency of sound wave depends on how fast the source of the sound is vibrating. Pitch is frequency of a sound as you perceive it Does not depend upon wave s frequency High F = high pitch

44 Normal hearing: 20 hz and 20,000 hz Infrasound is sound at frequencies lower than most people can hear, and ultrasound is sound at frequencies higher than most people can hear. Ultrasound

45 Sonar Technique for determining the distance to an object under water. Stands for: sound navigation and ranging.

46 The Doppler Effect Ex) Siren as it passes you Doppler Effect: a change in sound frequency caused by motion of sound source, motion of the listener, or both. Discovered by Austrian scientist Christian Doppler

47 Cont. As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency. wavelength of the sound into the boy's left ear is shortened in wavelength because the ambulance is approaching him, while the wavelength of the sound into his right ear is lengthened because the ambulance is moving away.

48 Hearing and the Ear Your ear is a complex system that consist of three main regions Outer ear Middle ear Inner 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.