Section 13­1.notebook
March 27, 2012
Sound waves are produced by the vibration of an object.
Compression ­ is the region of a longitudinal wave in which the density and pressure are greater than normal
Rarefaction ­ is the region of a longitudinal wave in which the density and pressure are less than normal
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Section 13­1.notebook
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Sound waves vibrate the air molecules around the object and move parallel to the direction of the wave motion. This is similar to the sine wave discussed in Chapter 12. The crests correspond to the compression of the wave; while the troughs correspond to the rarefaction of the wave.
Sound waves heard by the human ear are between the frequencies of 20 Hz and 20,000 Hz. Sound waves under 20 Hz are called infrasonic waves.
Sound waves above 20,000 Hz are called ultrasonic waves. Even though infrasonic and ultrasonic waves are not heard by the human ear still are considered sound due to the vibrations caused.
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Section 13­1.notebook
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Pitch ­ is the perceived highness or lowness of a sound, depending on the frequency of the sound waves.
Wavelength decreases as frequency increases. Infrasonic waves have longer wavelengths than audible sound waves, and ultrasonic waves have shorter wavelengths. Ultrasonic waves are used to produce images of objects inside the body by the sound waves reflected when they reach a boundary of two different materials with different densities. The object that the ultrasonic waves are trying to view must be about the same size or smaller than the waves. The typical ultrasonic device is about 10 Mhz. The speed of an ultrasonic wave in human tissue is about 1500 m/s, so the wavelength of 10 Mhz waves is λ= v / f = 1.5 mm. It can not detect objects smaller than this size.
Dolphins use similar manner called echolocation and uses the sound waves to locate objects.
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Section 13­1.notebook
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Speed depends on the medium that they travel through. Solid particles allow the particles to vibrate more quickly do to the fact the particles are closer together. Gas particles are further apart therefore the sound waves will travel slower through gas particles. Sound waves also depend on the temperature of the medium. The higher the temperature the speed of sound increases. Table 13­1 on page 482 show the speed of sound for different mediums and at different temperatures. 4
Section 13­1.notebook
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Section 13­1.notebook
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Waves have been discussed so far as traveling in one direction.
Sound waves will travel in all three dimensions. Such three­dimensional waves are approximately spherical. Spherical waves can be represented two­dimesionally with circles surrounding a source. The circles represent compressions called wave fronts. Each circle represents a spherical area. We can approximate the wave fronts with nearly parallel planes. Such waves are called plane waves and the small portion that is approximated need to be far from the source. source
λ
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Section 13­1.notebook
March 27, 2012
Doppler Effect ­ is a frequency shift that is the result of relative motion between the source of waves and an observer.
As a object moves the sound waves produced will be closer together as it approaches you and will be further apart as the object moves further apart. person B
person A
source
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Section 13­1.notebook
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Section 1 Review ­ all
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