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= Chapter 26: Sound =



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Chapter 26.1-The Origin of Sound
All sounds are produced by the vibrations of material objects. The source of all sound waves is vibrations

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The sound wave in these instruments are produced by the vibrating strings.

[] Vibrations in the saxophone are caused by a vibrated reed, whereas the vibrations in a flute [] are caused by a fluttering column of air at the mouthpiece. When you're singing your vocal cords are the ones doing the vibrating! Original vibrations stimulate the vibration of something larger or more massive. The frequency of the vibrating source under normal conditions, equals the frequency of sound waves produced.

__** Pitch **__= refers to how high or low sound frequencies appear to be. A high pitched sound from a piccolo has a high vibration frequency, while a low pitched sound from a fog horn has a low vibration frequency.

As you grow older your hearing range shrinks, especially at the high-frequency end. __** Infrasonic **__=sound waves with frequencies below 20 hertz __** Ultrasonic **__=frequencies above 20,000 hertz We // can't // hear either sound waves.

Chapter 26.2-Sound in Air
**__ Compression __**=a pulse of compressed air (or other matter); opposite of rarefaction → [] [] When you open a door quickly, you can imagine the door pushing the molecules next to it away from their initial positions and into their neighbors. In turn, neighboring molecules push into their neighbors and so on, like a compression wave moving along a spring, until the curtain flaps out the window. Compression is the pulse of air that moved from the door to the curtain. Therefore, the arrow represents the compression air against the window curtain which should cause the curtain to fly away from the door and out the window.

__**Rarefaction=**__ A disturbance in air (or matter) in which pressure is lowered. Opposite of compression ←

When you close a door quickly, the door pushes neighboring air molecules out of the room which produced an area of low pressure next to the door. Neighboring molecules then move into it, leaving a zone of lower pressure behind them (rarefied). Other molecules farther from the door, in turn, move into these regions which result in a pulse of rarefied air moving from the door to the curtain. The arrow represents rarefaction which causes the window curtain in return to fly toward the door. "Compressions are regions above-normal air pressure. Rarefactions are regions below-normal air pressure." []

Chapter 26.3- Media That Transmit Sound
Sound travels in many different places, usually you think air but it can also travel and solids and liquids as well. [] Sound can be heard from the ringing bell when air is inside the jar, but not when air is removed. However, sound can't travel in a vacuum. The transmission of sound requires a medium. If there is nothing to compress and expand, there can be no sound. There may still be vibration, but without a medium there is no sound!

Chapter 26.4- Speed of Sound
Have you ever noticed that you hear thunder after you see a flash of lightning?

[] This proves as evidence that sound is much slower than light. The speed of sound in dry air at 0 degrees C is about 330 meters per second. Water vapor in the air increases this speed slightly. Increased temperature increases the speed as well. Elasticity also plays a major role in the speed of sound. The speed of sound in a material does not depend of the materials density. Elasticity is the ability of a material to change shape in response to an applied force, then resume initial shape once the force is removed. [] Steel is very elastic while [] silly putty is inelastic. You would think it would be the opposite but elasticity isn't "stretchability". Some very still materials are elastic! Sound travels about fifteen times faster in steel than in air, and about four times faster in water than in air.

26.5- Loudness
The intensity of a sound is proportional to the square of the amplitude of a sound wave.

[] [] (left) An oscilloscope measures sound intensity. (right) "An oscilloscope displays period, from which frequency may be calculated."

However, loudness is a completely different thing. It differs for different people. Loudness is subjective but is related to sound intensity. [] __** Alexander Graham Bell **__: the creator of the decibel (dB), and of course the telephone, thought this would measure sound intensity greatly! Starting with 0 at the threshold of hearing for a normal ear, an increase of each 10 dB means that sound intensity increases by a factor of 10. To read how each sound intensity unit is read, you add a 0 at the end of every 10 units. If you're comparing 0 to 10 then it would increase by 10 units. However, if you're comparing 20 db and 10 db, it doesn't mean 20 db is twice as intense but 10 times as intense as 10 db. Comparing 40 db and 60 db, 60 db is 100 times as intense. []

If you want to figure it out this way you're always welcome to!!

26.6- Forced Vibration
Forced Vibration is the vibration of an object that is made to vibrate by another vibrating object that is nearby. The sounding board in a musical instrument amplifies the sound through forced vibration. So, for instance, when an unmounted tuning fork is struck the sound is faint, but when a tuning fork struck while its base is held to a tabletop the sound is relatively loud. This also applies for music boxes because they are mounted on sound boards. Without the sound board, the sound the music box mechanism makes is barely audible. []

26.7- Natural Frequency
Every object composed of an elastic material has a __**natural frequency**__ or a frequency at which an elastic object, once energized, will vibrate; Minimum energy is required to continue vibration at that frequency; Also called resonant frequency. __** Natural Frequency **__ depends on two factors: 1. The shape of the object 2. The elasticity of the object Most things- from planets to atoms and almost everything else in between- have a springiness to them and vibrate at one or more natural frequencies. []

26.8- Resonance
Resonance is a phenomenon that occurs when the frequency of forced vibrations on an object matches the object's natural frequency, and a dramatic increase in amplitude results. Resonance also means to resound or sound again. Only elastic materials vibrate, so putty does not since it is inelastic. In order for an object to resonate it needs a force to pull it back to its starting position and enough energy to keep it vibrating. [] (Above) "Stages of resonance. (a) The first compression meets the fork and gives it a tiny and momentary push. The fork bends (b) and then returns to its initial position (c) just at the time a rarefaction arrives. It keeps moving and (d) overshoots in the opposite direction. Just when it returns to its initial position (e), the next compression arrives to repeat the cycle. Now it bends farther because it is already moving." Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999. Ex: When pumping a swing, you pump in rhythm with the natural frequency of the swing. More important than the force with which you pump is timing. Even small pumps, or even small pushes from someone else, if delivered in rhythm with the natural frequency of the swinging motion, produce large amplitudes.

Ex: In 1940, four months after being completed, the Tacoma Narrows Bridge in the state of Washington was destroyed by a 40-mile-per-hour wind. The mild gale produced a fluctuating force that is said to have resonated with the natural frequency of the bridge, steadily increasing the amplitude over several hours until the bridge collapsed. []

26.9- Interference
" Dark lines show sound cancellation." []

There are two types of interference; constructive and destructive. In sound a crest of a wave corresponds to a compression, and the trough of a wave corresponds to a rarefaction and interference occurs in both longitudinal and transverse waves. "(a) Longitudinal wave displacements are parallel to direction of propagation. (b) Transverse wave displacements are perpindicular to propagation direction." []

**__Constructive interference__** occurs when the crests of one wave overlap the crests of another wave causing an increase in amplitude.

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**__Destructive interference__** occurs when the crest of one wave overlaps the trough of another wave causing a decrease in amplitude.

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Due to destructive interference, "dead spots" sometimes occur in theaters, where only a low sound can be heard.

"Reflection of sound is like reflection of light. Angle of reaction = Angle of incidence." []

26.10- Beats
__** Beats **__ are a throbbing variation in the loudness of sound caused by interference when two tones of slightly different frequencies are sounded together. When beats are in step they are more defined and louder, but when they are out of step the sound is faint. Maximum amplitude of the composite wave occurs when both waves are in phase, and minimum amplitude occurs when both waves are out of phase. [] "The interference of two sound sources of slightly different frequencies produces beats." Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.

Review Questions:
-What is the source of all sounds? -How does pitch relate to frequency? -What is the average frequency range of a young person's hearing? -Distinguish between //infrasonic// and //ultrasonic// sound -Distinguish between //compressions// and //rarefactions// of a sound wave. -How are //compressions// and //rarefactions// produced? -Light can travel through a vacuum, as is evidenced when you see the sun or the moon. Can sound travel through a vacuum also? Why or why not? -How fast does sound travel in dry air at room temperature? -How does air temperature affect the speed of sound? -How does the speed of sound in air compare with its speed in water and in steel? -Why does sound travel faster in solids and liquids than in gases? -How does loudness differ from different people? -What does it mean to say that everything has a natural frequency of vibration? -What does tuning in a radio station have to do with resonance? -Is it possible for one sound wave to cancel another? Explain. -Why does destructive interference occur when the path lengths from two identical sources differ by half a wavelength? -How does interference of sound relate to beats? -What two physics mistakes occur in a science fiction movie when you see and hear at the same time a distant explosion in outer space? -How much more intense is (a) a close whisper than the threshold of hearing? (b) a close whisper than normal breathing? -Sound waves travel at approximately 340 m/s. What is the wavelength of a sound with a frequency of 20 Hz (the lowest note we can hear as a sound)? What is the wavelength of a sound with a frequency of 20 kHz (the highest note we can hear)? -Suppose you wish to produce a sound wave that has a wavelength of 1 m in room temperature air. What would its frequency be? -An oceanic depth-sounding vessel surveys the ocean bottom with ultra sonic sound that travels 1530 m/s in seawater. Find the depth of the water if the time delay of the echo to the ocean floor and back is 8 seconds.

"Clock measures round trip time. Half the travel time, times the speed of sound gives the depth." []

-Two sounds, one at 240 Hz and the other at 243 Hz, occur at the same time. What beat frequency do you hear? -Two notes are sounding, one of which is 440 Hz. If a beat frequency of 5 Hz is heard, what is the other note's frequency?

Answers!
-Vibrations of material objects -There are directly proportional -20-20,000 Hertz -Infrasonic are sound waves with frequencies below 20 hertz and ultrasonic are sounds with frequencies above 20,000 hertz. -Compression is a pulse of compressed air and a rarefaction is a disturbance in air (or matter) in which the pressure is lowered -Compressions are produced when molecules are pushed away from their initial and into their neighbors and so on. -A rarefaction is when neighboring molecules push away from each other to produce an area of low pressure, where neighboring molecules move into. -No because there is no medium through which the sounds can vibrate -340 m/s -With each degree in air temperature above 0 degrees C sped of sound increases of about .60 m/s. -Sound travels about 15 times faster in steel than in air, and about 4 times faster in water than in air -Because they are more elastic than air. -The natural frequency of an object is a characteristic of the object's size, shape, and composition. -Tuning a radio adjusts the resonant frequency of a circuit to match the frequency of the radio waves you want to hear. -One sound wave can cancel another out if the crests of one wave line up with the troughs of the other, this is an example of destructive interference. -If two identical waves differ by exactly half of a wavelength then the crests of one wave will line up with the troughs of the other and destructive interference will result. -Beats are a result of periodic constructive and destructive interference resulting from two different frequencies being played together. -The problems with hearing a distant explosion in outer space at the same time that you see it are that first, sound would take longer than light to arrive so you should hear the explosion after you see it, and second, since there is no medium through which sound can travel you should not hear anything. -A close whisper is 100 times as intense as the threshold of hearing and 10 times as intense as normal breathing. math \displaystyle v= f x \lambda= \lambda= \displaystyle\left(\frac{v}{f}\right) = \displaystyle\left(\frac{340\displaystyle\left(\frac{m}{s}\right)}{20 Hz}\right)= 17m math
 * they are opposites!

math \lambda= \displaystyle\left\frac{v}{f}\right = \displaystyle\left(\frac{340\displaystyle\left(\frac{m}{s}\right)}{20 000 Hz}\right) = 0.017 m math

- math \displaystyle v= f x \lambda= f= \displaystyle\left(\frac{v}{\lambda}\right) = \displaystyle\left(\frac{340\displaystyle\left(\frac{m}{s}\right)}{1 m}\right)= 340 Hz math If it takes 8 seconds to reach the bottom and return then it took 4 seconds to reach the bottom.

math \displaystyle d= vt= (1530 \displaystyle\left\(\frac{m}{s}\right)(4s) = 6120 m math - math \displaystyle f_b_e_a_t= \displaystyle\left|f_1-f_2\right|= \displaystyle\left|240 Hz - 243 Hz\right| = 3 Hz math If the beat frequency is 5 Hz then the second frequency is 440 Hz ± 5 Hz so it must be either 435 Hz or 445 Hz.

Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.