# Physics of Sound – A Visual Representation through GIFs

In physics, sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement, through a medium such as air or water. In physiology and psychology, sound is the reception of such waves and their perception by the brain. The feature covers the physics of sound with the related educational animations.

### Longitudinal Waves

Image Courtesy: acs.psu.edu Image Courtesy: ipodphysics.com

Waves in which the motion of the individual particles of the medium is in a direction that is parallel to the direction of energy transport are termed as longitudinal waves. Sound that is transmitted through gases, plasma, and liquids is usually as longitudinal waves, also called compression waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction. The animation shows a wave moving in the form of compression and rarefaction.

### Transverse Waves

Image Courtesy: acs.psu.edu

Image Courtesy: isvr.soton.ac.uk/span> Image Courtesy: passmyexams.co.uk

Transverse waves are waves of alternating shear stress at right angle to the direction of propagation and are present in solids. If you anchor one end of a ribbon or string and hold the other end in your hand, you can create transverse waves by moving your hand up and down as you can see in the animations. Examples of transverse waves include seismic S (secondary) waves, and the motion of the electric (E) and magnetic (M) fields in an electromagnetic plane wave, which both oscillate perpendicularly to each other as well as to the direction of energy transfer.

### Speed of Sound

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The speed of sound depends on the medium that the waves pass through, and is a fundamental property of the material. The equation for measuring speed of sound is also known as Newton-Laplace equation and is expressed as

c = √(k/p)

In this equation, K = elastic modulus, c = velocity of sound, and  = density. Thus, the speed of sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium to its density. The animation shows an aircraft creating shock waves since it is travelling through the air faster than the speed of sound and which results in a sonic boom. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft.

### Doppler Effect

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The Doppler Effect is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. When the source of the waves is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrival of successive wave crests at the observer is reduced, causing an increase in the frequency. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is then increased, so the waves spread out. The animations show the Doppler shift in the two dimensions.

### Perception of Sound

Image Courtesy: aqhearing.com

Image Courtesy: pc.rhul.ac.uk

The physical reception of sound in any hearing organism is limited to a range of frequencies. Humans normally hear sound frequencies between approximately 20 Hz and 20,000 Hz (20 kHz), both limits, especially the upper limit, decrease with age. The animations show the inner working of the year.

### Boundary Behavior

As the sound waves travel through any medium, they often encounter an obstacle at the end of the medium. The sound travels through that medium to the next medium.  The interface of the two media is referred to as the boundary and the behavior of a wave at that boundary is described as its boundary behavior.  A wave exhibits following type of behaviors at the boundary.

-Reflection

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Image Courtesy: acs.psu.edu

The reflection of a sound wave can be seen in the animation whereby the sound travels back to the observer after reflecting from a nearby wall and a tree.

-Refraction

Image Courtesy: acs.psu.edu

Change in the direction of waves as they pass from one medium to another is termed as refraction of the waves.  Refraction is accompanied by a change in speed and wavelength of the waves so this means that if the medium of propagation changes, the speed of the sound wave will also change. The animation shows how a sound wave travels through two different mediums.

-Diffraction

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Image Courtesy: homepages.ius.edu

Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path. The amount of diffraction (the sharpness of the bending) increases with increasing wavelength and decreases with decreasing wavelength. In fact, when the wavelength of the wave is smaller than the obstacle or opening, no noticeable diffraction occurs. The animation shows how the sound waves bend the corners.

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