NIS – PHYSICAL SCIENCE

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Lecture 44 – Lecture 45Sound

Ozgur Unal

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Sound WavesWhen an object vibrates, it creates sound waves.Remember that sound waves are compressional waves.

When an end of the tuning fork moves outward, it forms a compression on that side by pushing the molecules in air together.When the end of the tuning fork moves back, a rarefaction is formed where the molecules are farther apart.As the tuning fork vibrates, it produces a series of compressions and rarefactions that travel outward.

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Speed of SoundSound waves can travel in any material whether gas, liquid or solid.Sound cannot travel in empty space.The speed of sound depends on the medium Table 1In general, sound travels slowest in gases and fastest in solids. Why?http://www.ndt-ed.org/EducationResources/HighSchool/Sound/speedinmaterials.htm

As the temperature increases, the speed of sound also increases. Why?Example: The speed of sound at 0 oC is 331. m/s, but when the temperature is 20 oC , the speed increases to 343 m/s.

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Amplitude and Energy of Sound WavesHow do we determine the amplitude of a compressional wave?How is amplitude related to the energy carried by the wave?

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Intensity and Loudnesshttp://www.youtube.com/watch?v=nTI2FdoeMlc&featur

e=fvstIf you move away from the speaker, how does the sound change?As you move away from the sound source, the energy carried by the wave spreads over a larger area.The amount of energy transferred by a sound wave through a certain area each second is the intensity of the sound wave.Intensity decreases as you move away from a sound source.This means that, as you get farther away from the source, less energy reaches yoru ears each second.

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Intensity and LoudnessLoudness is the human perception of sound intensity.As the intensity of a sound wave increases, the loudness of the sound also increases.The intensity of sound (loudness) can be described using a measurement scale.Each unit on the scale is called a decibel, dB.On this scale the faintest sound that most people can hear is 0 dB.Sounds with intensity levels above 120 dB may cause pain and permanent hearing loss.

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Intensity and LoudnessHow can you protect your ears from sound with high dB?

Using ear plugs and ear muffs can help you protect your ears.

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Pitch and FrequencyAre you familiar with the musical notes?http://ababasoft.com/music/music01.html

Pitch is the human perception of the frequency of sound waves.Pitch gets higher as the frequency of the sound waves increases.The frequency (pitch) of each musical note is different from one another.Check out Figure 5.

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Pitch and FrequencyA healthy human ear can hear sound waves with frequencies from about 20 Hz to 20,000 Hz.The human ear is most sensitive to sounds in the range of 440 Hz to about 7,000 Hz.

Sound waves above 20,000 Hz are called ultrasonic waves.Dogs can hear sounds with frequencies up tp 35,000 Hz.Bats can detect frequencies higher than 100,000 Hz.Infrasonic, or subsonic, waves have frequencies below 20 Hz.Although you can’t hear infrasonic waves, you might feel them as a rumble inside your body.

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The Doppler EffectWatch the video in the link, and describe how the sound from the car changes as it passes by.http://www.wfu.edu/physics/demolabs/demos/3/3b/3B40

xx.html

As the car approaches, the pitch of the sound increases.As the car goes away, the pitch decreases.This change in pitch or frequency due to relative motion of a wave is called the Doppler effect.Have a look at the animation below:http://galileoandeinstein.physics.virginia.edu/mo

re_stuff/flashlets/doppler.htm

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The Doppler EffectThe Doppler effect happens any time the source of a sound is changing position relative to the listener.It occurs no matter whether it is the sound source or the listener that is moving.

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Using SoundWhen sound waves strike an object, they can be absorbed by the object, transmitted through the object, or reflected from the object.By detecting the sound waves reflected from an object, the size, shape and location of an object can be determined.

Applications of sound waves:Echolocation and Sonar: Echolocation is the process of locating objects by emitting sounds and detecting the sound waves that reflect back.Example: Bats, dolphins and whalesSonar is a system that uses the reflection of underwater sound waves to detect objects.

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Using SoundUltrasound in Medicine: Using special instruments, medical professionals can send ultrasonic waves into a specific part of a patient’s body.Examples:

Monitoring the development of fetusMonitoring certain types of heart disease and

cancerBreaking kidner stones and gallstones