# Mechanical Waves and Sound

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Mechanical Waves and Sound
Chapter 17 Mechanical Waves and Sound

Mechanical Waves and Sound

What are Mechanical Waves?
Mechanical Waves and Sound What are Mechanical Waves? A mechanical wave is a disturbance in matter that carries energy from one place to another. Mechanical waves require a medium, or matter, to travel through.

Mechanical Waves and Sound
A mechanical wave is created when a source of energy causes a vibration to travel through a medium. A vibration is a back-and-forth motion.

Types of Mechanical Waves
Mechanical Waves and Sound Types of Mechanical Waves Mechanical waves are classified by the way they move through a medium. The three main types of mechanical waves are transverse (or shear) waves longitudinal (or compressional) waves surface waves

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Transverse Waves A transverse waves is a wave that causes the medium vibrate at right angles to the direction in which the wave travels. Each point on the wave vibrates up and down between a maximum and minimum height. The highest point of the wave is the crest. The lowest point is the trough.

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Figure 2 Mechanical Waves and Sound

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Figure 2 Mechanical Waves and Sound

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Figure 2 Mechanical Waves and Sound

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Longitudinal Waves A longitudinal wave is a wave in which the vibration of the medium is parallel to the direction the wave travels. An area where the particles in the medium are spaced close together is called a compression. An area where the particles are spread out is called a rarefaction.

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As compressions and rarefactions travel along the wave, particles vibrate back and forth around the rest position.

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compression rarefaction

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Surface Waves A surface wave is a wave that travels along a surface separating two media. Surface waves have characteristics of both transverse and longitudinal waves.

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Ocean waves are an example of a surface wave. When the wave crest passes a point, the particles move up. When the trough passes, the particles move down.

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These two motions cause particles to move in a circle. Most waves do not transport matter as the energy moves.

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17.2 Properties of Mechanical Waves
Mechanical Waves and Sound 17.2 Properties of Mechanical Waves

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Frequency and Period Any motion that repeats at regular intervals is called periodic motion. The time required for one cycle, a complete motion that returns to its starting point, is called the period (T). For surface waves, the period is the time between two successive crests.

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Periodic motion has a frequency, which is the number of complete cycles in a given time. For a wave, the frequency (f) is the number of wave cycles that pass a point in a given time. Frequency and period are inversely related. (f=1/T).

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Frequency is measured in cycles per second or hertz (Hz). A wave’s frequency equals the frequency of the vibrating source producing the wave.

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Wavelength The distance between a point on one wave and the same point on the next cycle of the wave is wavelength. For a transverse wave, wavelength is measured between adjacent crests or troughs. For a longitudinal wave, wavelength is distance between adjacent rarefactions or compressions.

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Increasing the frequency of a wave decreases its wavelength. Wavelength is represented by Greek letter lambda, (λ).

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Frequency

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Frequency Wavelength

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Wave Speed The wave speed is calculated by dividing the wavelength by the period v = λ/T by multiplying the wavelength by frequency. v = f λ

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound The speed of a wave can change if it enters a new medium or variables such as pressure and temperature change.

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17.2 Properties of Mechanical Waves Section 17.2 Mechanical Waves and Sound

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound Amplitude The amplitude of a wave is the maximum displacement of the medium from its rest position. The amplitude of a transverse wave is the distance from the rest position to the crest or trough.

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17.2 Properties of Mechanical Waves Mechanical Waves and Sound For a longitudinal wave, the amplitude is the maximum displacement of a point from its rest position. The more energy a wave has, the greater is its amplitude.

Amplitude of Transverse Waves
17.2 Properties of Mechanical Waves Mechanical Waves and Sound Amplitude of Transverse Waves

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17.3 Behavior of Waves Mechanical Waves and Sound 17.3 Behavior of Waves As waves crisscross back and forth, many interactions can occur, including reflection, refraction, diffraction, and interference.

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17.3 Behavior of Waves Mechanical Waves and Sound 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 inverted. If reflection occurs at a fixed boundary, the wave will be upside down (inverted) compared to the original wave.

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17.3 Behavior of Waves Mechanical Waves and Sound Refraction Refraction is the bending of a wave as it enters a new medium at an angle. Refraction changes the direction of a wave. When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side.

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17.3 Behavior of Waves Mechanical Waves and Sound

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17.3 Behavior of Waves Mechanical Waves and Sound Figure 10 shows how an ocean wave bends or refracts as it approaches the shore at an angle because one side of each wave slows down before the other side. Refraction of the wave occurs only when the two sides of a wave travel at different speeds.

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17.3 Behavior of Waves Mechanical Waves and Sound Frequency cannot change; it depends on the frequency of the source.

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17.3 Behavior of Waves Mechanical Waves and Sound Diffraction Diffraction is 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. If the wavelength is small compared to the opening, the wave bends very little.

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17.3 Behavior of Waves Mechanical Waves and Sound

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17.3 Behavior of Waves Mechanical Waves and Sound Interference When two objects collide, they cannot continue on their original path. But waves can occupy the same region of space and then continue on. Interference occurs when two or more waves overlap and combine together.

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17.3 Behavior of Waves Mechanical Waves and Sound Two types of interference are constructive interference destructive interference The displacements of waves combine to increase amplitude in constructive interference and to decrease amplitude in destructive interference.

Constructive Interference
17.3 Behavior of Waves Mechanical Waves and Sound Constructive Interference Constructive interference occurs when two or more waves combine to produce a wave with a larger displacement. Figure 12A shows how constructive interference produces a wave with an increased amplitude.

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17.3 Behavior of Waves Mechanical Waves and Sound

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17.3 Behavior of Waves Figure 12 Mechanical Waves and Sound

Destructive Interference
17.3 Behavior of Waves Mechanical Waves and Sound Destructive Interference Destructive interference occurs when two or more waves combine to produce a wave with a smaller displacement. Destructive interference produces a wave with a reduced amplitude.

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17.3 Behavior of Waves Mechanical Waves and Sound

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17.3 Behavior of Waves Mechanical Waves and Sound

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17.3 Behavior of Waves Mechanical Waves and Sound Standing Waves At certain frequencies, interference between a wave and its reflection can produce a standing wave. A standing wave is a wave that appears to stay in one place – it does not seem to move through the medium.

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17.3 Behavior of Waves Mechanical Waves and Sound A node is a point on a standing wave that has no displacement from the rest position. At the nodes, there is complete destructive interference between the incoming and reflected waves.

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17.3 Behavior of Waves Mechanical Waves and Sound An antinode is a point where a crest or trough occurs midway between two nodes. A standing wave forms only if half a wavelength fits exactly into the length of a vibrating cord.

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17.3 Behavior of Waves Mechanical Waves and Sound

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17.4 Sound and Hearing Mechanical Waves and Sound 17.4 Sound and Hearing

Properties of Sound Waves
17.4 Sound and Hearing Mechanical Waves and Sound Properties of Sound Waves Sound waves are 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 – frequency and pitch

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17.4 Sound and Hearing Mechanical Waves and Sound Speed In dry air at 20oC, the speed of sound is 342 m/s. In general, sound waves travel fastest in solids, slower in liquids, and slowest in gases. The speed of sound depends on many factors, including the density of the medium and how elastic the medium is.

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17.4 Sound and Hearing Mechanical Waves and Sound

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17.4 Sound and Hearing Mechanical Waves and Sound

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17.4 Sound and Hearing Mechanical Waves and Sound

Intensity and Loudness
17.4 Sound and Hearing Mechanical Waves and Sound Intensity and Loudness Intensity is the rate at which a wave’s energy flows through a given area. Sound intensity depends on both the wave’s amplitude and the distance from the sound source. Sound intensity levels are measured in units called decibels.

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17.4 Sound and Hearing Mechanical Waves and Sound A decibel is one tenth of a bel (B). Devised by engineers of Bell Telephone to quantify the reduction in audio level over a 1 mile length of standard phone cable, the bel was originally called the transmission unit or TU, but was renamed in 1923/24 in honor of Alexander Graham Bell. In many situations, however, the bel proved inconveniently large, so the decibel has become more common.

Mechanical Waves and Sound
17.4 Sound and Hearing Mechanical Waves and Sound The decibel (dB) is a unit that compares the intensity of different sounds. The decibel scale is a log scale, for every 10-dB increase, the intensity increases tenfold. A 0-dB sound is barely audible. Lengthy exposure to more than 90 dB can cause hearing damage.

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17.4 Sound and Hearing Mechanical Waves and Sound Unlike intensity, loudness is subjective – it is subject to a person’s interpretation. Loudness is a physical response to the intensity of sound, modified by physical factors.

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17.4 Sound and Hearing Mechanical Waves and Sound Frequency and Pitch The frequency of a sound wave depends on how fast the sound source is vibrating. The size of a musical instrument tells something about the frequency it can produce. The longer the tubing, the longer is the wavelength of the standing wave, and the lower the frequency produced.

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17.4 Sound and Hearing Mechanical Waves and Sound Pitch is the frequency of a sound as you perceive it. Pitch does depend upon a wave’s frequency. High frequency sounds have a high pitch, and low frequency sounds have a low pitch.

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17.4 Sound and Hearing Mechanical Waves and Sound Ultrasound Most people can hear sounds between 20 Hz and 20,000 Hz. Infrasound is sound at frequencies lower than the average person can hear. Ultrasound is at frequencies higher than most can hear.

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17.4 Sound and Hearing Mechanical Waves and Sound Ultrasound is used in a variety of applications, including sonar and ultrasound imaging. SONAR (sound navigation and ranging) is a technique for determining the distance to an object under water.

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17.4 Sound and Hearing Mechanical Waves and Sound Depth = ½ t X1500m/s

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17.4 Sound and Hearing Mechanical Waves and Sound The Doppler Effect The Doppler effect is a change in frequency caused by motion of the source, the receiver, or both. As a source approaches, an observer receives a higher frequency because the waves are bunched together producing a higher pitch.

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17.4 Sound and Hearing Mechanical Waves and Sound When the source moves away, the observer receives a lower frequency because the waves spread out producing a lower pitch.

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17.4 Sound and Hearing Mechanical Waves and Sound The Doppler Effect

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17.4 Sound and Hearing Mechanical Waves and Sound Resonance is the response of a standing wave to another of the same frequency. Resonance can produce a dramatic increase in amplitude

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Tacoma Narrows collapse