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  • Define sound and hearing.
  • Describe sound as a longitudinal wave.
Photograph of a glass, half of which is shattered into small pieces by a high-intensity sound wave. The tiny glass bits are shattered all over the place.
This glass has been shattered by a high-intensity sound wave of the same frequency as the resonant frequency of the glass. While the sound is not visible, the effects of the sound prove its existence. (credit: ||read||, Flickr)

Sound can be used as a familiar illustration of waves. Because hearing is one of our most important senses, it is interesting to see how the physical properties of sound correspond to our perceptions of it. Hearing is the perception of sound, just as vision is the perception of visible light. But sound has important applications beyond hearing. Ultrasound, for example, is not heard but can be employed to form medical images and is also used in treatment.

The physical phenomenon of sound    is defined to be a disturbance of matter that is transmitted from its source outward. Sound is a wave. On the atomic scale, it is a disturbance of atoms that is far more ordered than their thermal motions. In many instances, sound is a periodic wave, and the atoms undergo simple harmonic motion. In this text, we shall explore such periodic sound waves.

A vibrating string produces a sound wave as illustrated in [link] , [link] , and [link] . As the string oscillates back and forth, it transfers energy to the air, mostly as thermal energy created by turbulence. But a small part of the string’s energy goes into compressing and expanding the surrounding air, creating slightly higher and lower local pressures. These compressions (high pressure regions) and rarefactions (low pressure regions) move out as longitudinal pressure waves having the same frequency as the string—they are the disturbance that is a sound wave. (Sound waves in air and most fluids are longitudinal, because fluids have almost no shear strength. In solids, sound waves can be both transverse and longitudinal.) [link] shows a graph of gauge pressure versus distance from the vibrating string.

Diagram of a vibrating string held fixed at both ends. The string is shown to move toward the right. The compression and rarefaction of air is shown as bold and dotted line arcs around the string.
A vibrating string moving to the right compresses the air in front of it and expands the air behind it.
Diagram of a vibrating string held fixed at both the ends. The string is shown to move toward the left. The compression and rarefaction of air is shown as bold and dotted arcs around the string.
As the string moves to the left, it creates another compression and rarefaction as the ones on the right move away from the string.
Part a of the diagram shows a vibrating string held fixed at both the ends. The string is shown to vibrate to and fro toward left and right. The compression and rarefaction of air is shown as bold and dotted arcs around the string. Part b shows a graph of pressure versus distance from the source. The pressure is along the y axis and the distance is along the x axis. The graph is a sine wave along the x axis.
After many vibrations, there are a series of compressions and rarefactions moving out from the string as a sound wave. The graph shows gauge pressure versus distance from the source. Pressures vary only slightly from atmospheric for ordinary sounds.

The amplitude of a sound wave decreases with distance from its source, because the energy of the wave is spread over a larger and larger area. But it is also absorbed by objects, such as the eardrum in [link] , and converted to thermal energy by the viscosity of air. In addition, during each compression a little heat transfers to the air and during each rarefaction even less heat transfers from the air, so that the heat transfer reduces the organized disturbance into random thermal motions. (These processes can be viewed as a manifestation of the second law of thermodynamics presented in Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency .) Whether the heat transfer from compression to rarefaction is significant depends on how far apart they are—that is, it depends on wavelength. Wavelength, frequency, amplitude, and speed of propagation are important for sound, as they are for all waves.

Diagram of an ear is shown with sound wave compressions and rare factions entering the ear as semicircular arcs of bold and dotted lines. The cross section of ear drum marked as A is shown to vibrate to and fro with a force F equals P times A.
Sound wave compressions and rarefactions travel up the ear canal and force the eardrum to vibrate. There is a net force on the eardrum, since the sound wave pressures differ from the atmospheric pressure found behind the eardrum. A complicated mechanism converts the vibrations to nerve impulses, which are perceived by the person.

Phet explorations: wave interference

Make waves with a dripping faucet, audio speaker, or laser! Add a second source or a pair of slits to create an interference pattern.

Make waves with a dripping faucet, audio speaker, or laser! Add a second source or a pair of slits to create an interference pattern.
Wave Interference

Section summary

  • Sound is a disturbance of matter that is transmitted from its source outward.
  • Sound is one type of wave.
  • Hearing is the perception of sound.

Questions & Answers

explain and draw how to measure length when using ruler, micrometer screw gauge and vernnier calliper
gift Reply
Calculate the average velocity in time interval 6sec to 12sec and determine the instantaneous velocity
Lekiisi Reply
the force is not constant in this case of tow car collide for short period of time ..why is the force is not constant?
Abel Reply
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Fortune
meaning of the term si units
Chali Reply
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Chali
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Chali
Charges(electron)
Caleb
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Caleb
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electric dynamo
albert
Electric dynamo is the sources of electric magnetic forces which utilize electromagnetic induction.
albert
A stone is dropped down a well, if it take 5 seconds to reach the water, how dip is the well
Mollamin Reply
an aircraft at as steady velocity of 70m/so eastwards at a height of 800me drops a package of supplies .a, how long will it take for the package to rich the ground? b, how fast will it be going as it lands?
Ng Reply
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Tamba
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Jide Reply
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Victor Reply
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Obaro
Good question! Physics is the study of the nature world . Does this help?
Yonn
physics is the study of matter in relations to energy.
Enoch
physics is the science of measurements
Jide
physics is a science concern with nature and properties of matter and energy
Ugomma
what is a parallelogram law of motion?
Nancy
describe how you would find the area of an irregular shaped body?
Chali
Definition for physics
Adesola Reply
It deal with matter and relation to energy
Soughie
physics is the Study of matter in relation to energy.
albert
physics is a natural science that study matter its behaviour and relation to energy.
mohammed
physic tells us more about quantities and measurement also
Kelly
life as we know it that can be measured and calculated
Jesus
what is a reference frame
Chukwu Reply
what is anatomy in relation to physics
Mubarak Reply
how does half life exist
Humble Reply
 The amount of time it takes a radioactive isotope to decay into a stable isotope is different for each radioactive isotope, and is characterized by its “half-life”. An isotope's half-life is the amount of time it takes for half the number of atoms of that isotope to decay to another isotope.
Nardine
what is the difference between Mass and weight
Pjustin
mass is constant while weight varies. unit of mass is kg, unit of weight is newton
Faith
how can a coin float in water and what principle governs it
Mercy Reply
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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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