Chapter 17Chapter 17

Mechanical Waves and Mechanical Waves and SoundSound

Mechanical WavesMechanical Waves

Section 17-1Section 17-1

Mechanical WavesMechanical Waves

• A mechanical wave is a disturbance A mechanical wave is a disturbance in matter that carries energy from in matter that carries energy from one place to another.one place to another.

• A medium is the material through A medium is the material through which a wave travels.which a wave travels.

• A mechanical wave is created when a A mechanical wave is created when a source of energy causes a vibration source of energy causes a vibration to travel through a medium.to travel through a medium.

Types of Mechanical WavesTypes of Mechanical Waves

• The three main types of waves are The three main types of waves are transverse waves, longitudinal transverse waves, longitudinal waves, and surface waves.waves, and surface waves.

Transverse WavesTransverse Waves

• A transverse wave is a wave that A transverse wave is a wave that causes the medium to vibrate at causes the medium to vibrate at right angles to the direction in which right angles to the direction in which the wave travels. the wave travels.

• See Presentation Pro.See Presentation Pro.

Transverse WavesTransverse Waves

• Crest – the highest point of the wave Crest – the highest point of the wave above the rest position.above the rest position.

• Trough – the lowest point of the wave Trough – the lowest point of the wave below the rest position.below the rest position.

• Rest position – the point half way Rest position – the point half way between the crest and the trough. between the crest and the trough. (The position line before the wave (The position line before the wave began or after it ended.)began or after it ended.)

Longitudinal WavesLongitudinal Waves

• Longitudinal wave – a wave in which Longitudinal wave – a wave in which the vibration of the medium is the vibration of the medium is parallel to the direction the wave parallel to the direction the wave travels.travels.

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Longitudinal WavesLongitudinal Waves

• Compression – an area where the Compression – an area where the particles in a medium are spaced particles in a medium are spaced close together.close together.

• Rarefaction – an area where the Rarefaction – an area where the particles in a medium are spread out.particles in a medium are spread out.

Surface WavesSurface Waves

• Surface wave – a wave that travels Surface wave – a wave that travels along a surface separating two along a surface separating two media.media.

• Ocean waves are the most familiar Ocean waves are the most familiar type of surface wave.type of surface wave.

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Properties of Mechanical Properties of Mechanical WavesWaves

• Section 17-2Section 17-2

Frequency and PeriodFrequency and Period

• Periodic motion – any motion that Periodic motion – any motion that repeats at regular time intervalsrepeats at regular time intervals

• Period – the time required for one Period – the time required for one cycle, a complete motion that returns cycle, a complete motion that returns to its starting point.to its starting point.

• Frequency – the number of complete Frequency – the number of complete cycles in a given time.cycles in a given time.

Frequency and PeriodFrequency and Period

• Hertz – cycles per second – Hertz – cycles per second – measurement of frequency.measurement of frequency.

• A wave’s frequency equals the A wave’s frequency equals the frequency of the vibrating source frequency of the vibrating source producing the wave.producing the wave.

WavelengthWavelength

• Wavelength – the distance between a Wavelength – the distance between a point on one wave and the same point on one wave and the same point on the next cycle of the wave.point on the next cycle of the wave.

• Increasing the frequency of a wave Increasing the frequency of a wave decreases its wavelength. decreases its wavelength.

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Wave Speed & AmplitudeWave Speed & Amplitude

• Speed = Wavelength x FrequencySpeed = Wavelength x Frequency

• Amplitude – the maximum Amplitude – the maximum displacement of the medium from its displacement of the medium from its rest position.rest position.

• The more energy a wave has, the The more energy a wave has, the greater is its amplitude.greater is its amplitude.

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Behavior of WavesBehavior of Waves

• Section 17.3Section 17.3

ReflectionReflection

• Reflection – occurs when a wave Reflection – occurs when a wave bounces off a surface that it cannot bounces off a surface that it cannot pass through.pass through.

• Reflection does not change the Reflection does not change the speed or frequency of a wave, but speed or frequency of a wave, but the wave can be flipped upside the wave can be flipped upside down. down.

RefractionRefraction

• Refraction – bending of a wave as it Refraction – bending of a wave as it enters a new medium at an angle.enters a new medium at an angle.

• When a wave enters a medium at an When a wave enters a medium at an angle, refraction occurs because on angle, refraction occurs because on side of the wave moves more slowly side of the wave moves more slowly than the other side.than the other side.

DiffractionDiffraction

• Diffraction – the bending of a wave Diffraction – the bending of a wave as it moves around an obstacle or as it moves around an obstacle or passes through a narrow opening.passes through a narrow opening.

• A wave diffracts more if its A wave diffracts more if its wavelength is large compared to the wavelength is large compared to the size of an opening or obstacle.size of an opening or obstacle.

InterferenceInterference

• Interference – occurs when two or Interference – occurs when two or more waves overlap and combine more waves overlap and combine together.together.

• Two types of interference are Two types of interference are constructive interference and constructive interference and destructive interference. destructive interference.

InterferenceInterference

• Constructive interference – occurs when Constructive interference – occurs when two or more waves combine to produce two or more waves combine to produce a wave with a larger displacement.a wave with a larger displacement.

• Destructive interference – occurs when Destructive interference – occurs when two or more waves combine to produce two or more waves combine to produce a wave with a smaller displacement.a wave with a smaller displacement.

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Standing WavesStanding Waves

• Standing wave – a wave that appears Standing wave – a wave that appears to stay in one place – it does not to stay in one place – it does not seem to move through the medium. seem to move through the medium. Example: plucking a guitar stringExample: plucking a guitar string

• Node – a point on a standing wave Node – a point on a standing wave that has no displacement from the that has no displacement from the rest point.rest point.

• See figure 13 page 512.See figure 13 page 512.

Standing WavesStanding Waves

• Antinode – a point where a crest or a Antinode – a point where a crest or a trough occurs midway between two trough occurs midway between two nodes.nodes.

• A standing wave forms only if half a A standing wave forms only if half a wavelength or a multiple of half a wavelength or a multiple of half a wavelength fits exactly into the wavelength fits exactly into the length of a vibrating cord.length of a vibrating cord.

Sound and HearingSound and Hearing

• Section 17-4Section 17-4

Properties of Sound WavesProperties of Sound Waves

• Many behaviors of sound can be Many behaviors of sound can be explained using a few properties – explained using a few properties – speed, intensity and loudness, and speed, intensity and loudness, and frequency and pitch.frequency and pitch.

• Speed – it takes time for sound to Speed – it takes time for sound to travel from place to place…and that travel from place to place…and that time is different for different medium.time is different for different medium.

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Properties of Sound WavesProperties of Sound Waves

• Intensity – the rate at which a wave’s Intensity – the rate at which a wave’s energy flows through a given area.energy flows through a given area.

• Decibel (dB) – a unit that compares Decibel (dB) – a unit that compares the intensity of different sounds.the intensity of different sounds.

• Loudness – a physical response to Loudness – a physical response to the intensity of sound, modified by the intensity of sound, modified by physical factors. physical factors.

Properties of Sound WavesProperties of Sound Waves

• Frequency – in a sound wave, Frequency – in a sound wave, frequency depends on how fast the frequency depends on how fast the source of the sound is vibrating.source of the sound is vibrating.

• Pitch – how you perceive the Pitch – how you perceive the frequency of a soundfrequency of a sound

UltrasoundUltrasound

• Ultrasound – sound at frequencies Ultrasound – sound at frequencies higher than most people can hear.higher than most people can hear.

• Ultrasound is used in a variety of Ultrasound is used in a variety of applications, including sonar and applications, including sonar and ultrasound imaging.ultrasound imaging.

UltrasoundUltrasound• Sonar – a technique for determining Sonar – a technique for determining

the distance to an object under water. the distance to an object under water.

• Stands for sound navigation and Stands for sound navigation and ranging.ranging.

• Ultrasound imaging – an important Ultrasound imaging – an important medical technique sending short burst medical technique sending short burst of a wave into a patient. Computer of a wave into a patient. Computer software uses the reflected pulses to software uses the reflected pulses to make a picture of an organ or other make a picture of an organ or other structure inside the body.structure inside the body.

The Doppler EffectThe Doppler Effect

• Doppler effect – a change in sound Doppler effect – a change in sound frequency caused by motion of the frequency caused by motion of the sound source, motion of the listener, sound source, motion of the listener, or both.or both.

• As a source of sound approaches, an As a source of sound approaches, an observer hears a higher frequency. observer hears a higher frequency. When the sound source moves away, When the sound source moves away, the observer hears a lower frequency.the observer hears a lower frequency.

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Hearing and the EarHearing and the Ear

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

How sound is reproducedHow sound is reproduced

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