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1. Introduction
2. Wave motion
3. Mechanical waves
4. Characteristics of waves
5. Sound
6. Reflection of sound
7. Multiple reflection of sound
8. Situations making use of multiple reflection
9. Reverberation
10. Echo
11. Acoustics of buildings
12. Ultrasonic sound
13. Uses of ultrasonic waves
14. SONAR
15. Seismic waves
16. Tsunami
17. Let us assess
18. Concept map
19 . References

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After placing a paper boat in the pond the child is continuously making ripples in the water to push it away from the shore. But the boat moves only up and down and it doesn't move forward. What may be the reason ?

You might have seen ripples formed in  water when a stone is dropped into it. How does the disturbance produced by the stone in water spread to other areas? Doesn't it spread like expanding concentric circles? Let's find out

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Wave motion is the propagation of disturbances produced in one part of a medium to the other parts by the vibration of its particles. It is one of the means to transfer energy from one region of the medium to another. In a wave motion the energy received from one particle is transferred from particle to particle thus spreading all over

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The waves which need a medium for their propogation are called mechanical waves. There are mainly two types of mechanical waves

. Transverse wave
. Longitudinal wave

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A transverse wave is a wave in which the particles of the medium vibrate in a direction perpendicular to the direction of the propogation of wave

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A longitudinal wave is a wave in which the particles of the medium vibrate in a direction parallel to the direction of propogation of the wave. This creates compressions and refractions alternately in the medium

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Compressions and rarefactions of longitudinal waves 

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Compressions and rarefactions of longitudinal waves 

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Compression is that part of the longitudinal wave in which the medium of particles are closer and there is momentary decrease in volume of medium. Rarefaction is that part of longitudinal wave in which the medium of particles are apart and there is momentary increase in volume of medium.

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DIFFERENCE BETWEEN TRANSVERSE WAVES AND LONGITUDINAL WAVES

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DIFFERENCE BETWEEN TRANSVERSE WAVES AND LONGITUDINAL WAVES

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Amplitude is the maximum displacement of a particle from its mean position

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The table shows that speed of sound is different in different media.

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Wavelength is the distance advanced by the wave by the time a particle completes the vibration. This is equivalent to the distance between two consecutive particles which are in the same phase of vibration. The Greek letter λ (lambda) is used to denote wavelength. The unit of wavelength is metre (m).

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Period of a wave is the time taken for a particle in a medium to make one complete vibration. This is represented by the letter T. The unit of period is second (s)

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Frequency is the number of vibrations in one second

Frequency(f)= ( Number of vibrations)/ ( Timetaken)

If frequency and period are interrelated we can write it down as
f = 1/T

The unit of frequency is hertz (Hz)

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The speed of a wave is the distance travelled by the wave in one second.

The equation for finding speed is

Speed = distance/ time

Wavelength (λ) is the distance travelled by a wave in a time of one period (T)
so,
speed of wave= λ/T

v= (1/T)*λ

v= fλ

The unit of speed is m/s

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Sound is produced by vibrating objects. A medium is essential for the propogation of sound. Sound coming from a source produces continuous variations in the pressure on a medium. So a series of compressions and rarefactions is created in the air and these make sound waves propogate through the medium

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Sound gets reflected very well from smooth surfaces

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Sound getting reflected repeatedly from different objects is multiple reflection

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Devices like megaphones, horns musical instruments like shehanai and trumpets are made in such way that sound produced from them travels in a certain direction only without spreading to other directions.

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The ceilings of halls are given a curvature

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The curved sound boards placed behind the screen makes the sound undergo multiple reflections and spreads everywhere in the hall.

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Reverberation of Sound - Advantages & Disadvantages | Applications

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Reverberation is the persistence of sound as a result of multiple reflection

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Reverberation of Sound - Advantages & Disadvantages | Applications

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Reverberation is the phenomenon of persistence of sound after it has been stopped as a result of multiple reflections. Learn about its advantages & disadvantages along with applications

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Echo is the phenomenon of hearing a sound by reflection from a surface or obstacle, after hearing the original sound

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Acoustics of buildings is the branch of science that deals with the conditions to be fulfilled in the construction of a building for clear audibility

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Sound with a frequency greater than 20000Hz, ie, above the higher limit of audibility is called ultrasonic sound

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Class 9 | Study of Sound | Sonar | Science | SSC | CBSE | Home Revise

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A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large man-made explosion that produces low-frequency acoustic energy. Seismic waves are studied by seismologists, who record the waves using seismometers, hydrophones (in water), or accelerometers. Seismic waves are distinguished from seismic noise (ambient vibration), which is persistent low-amplitude vibration arising from a variety of natural and anthropogenic sources.

p-wave and s-wave from seismograph

Velocity of seismic waves in Earth versus depth.[1] The negligible S-wave velocity in the outer core occurs because it is liquid, while in the solid inner core the S-wave velocity is non-zero

The propagation velocity of a seismic wave depends on density and elasticity of the medium as well as the type of wave. Velocity tends to increase with depth through Earth's crust and mantle, but drops sharply going from the mantle to Earth's outer core.[2]

Earthquakes create distinct types of waves with different velocities. When recorded by a seismic observatory, their different travel times help scientists locate the quake's hypocenter. In geophysics; the refraction or reflection of seismic waves is used for research into Earth's internal structure. Scientists sometimes generate and measure vibrations to investigate shallow, subsurface structure.

Among the many types of seismic waves, one can make a broad distinction between body waves, which travel through the Earth, and surface waves, which travel at the Earth's surface.[3]: 48-50  [4]: 56-57

Body waves and surface waves

Other modes of wave propagation exist than those described in this article; though of comparatively minor importance for earth-borne waves, they are important in the case of asteroseismology.

Body waves travel through the interior of the Earth.

Surface waves travel across the surface. Surface waves decay more slowly with distance than body waves which travel in three dimensions.

Particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage.

Body waves

Body waves travel through the interior of the Earth along paths controlled by the material properties in terms of density and modulus (stiffness). The density and modulus, in turn, vary according to temperature, composition, and material phase. This effect resembles the refraction of light waves. Two types of particle motion result in two types of body waves: Primary and Secondary waves. This distinction was recognized in 1830 by the French mathematician Siméon Denis Poisson.[5]

Patterns of seismic wave travel through Earth's mantle and core. S-waves can not travel through the liquid outer core, so they leave a shadow on Earth's far side. P-waves do travel through the core, but P-wave refraction bends seismic waves away from P-wave shadow zones.

Primary waves (P-waves) are compressional waves that are longitudinal in nature. P-waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name "Primary". These waves can travel through any type of material, including fluids, and can travel nearly 1.7 times faster than the S-waves. In air, they take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite.

Secondary waves (S-waves) are shear waves that are transverse in nature. Following an earthquake event, S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground perpendicular to the direction of propagation. Depending on the propagational direction, the wave can take on different surface characteristics; for example, in the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S-waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material. Shear waves can not travel through any liquid medium,[6] so the absence of S-waves in earth's outer core suggests a liquid state.

Surface waves

Seismic surface waves travel along the Earth's surface. They can be classified as a form of mechanical surface wave. Surface waves diminish in amplitude as they get farther from the surface and propagate more slowly than seismic body waves (P and S). Surface waves from very large earthquakes can have globally observable amplitude of several centimeters.[7]

Rayleigh waves, also called ground roll, are surface waves that propagate with motions that are similar to those of waves on the surface of water (note, however, that the associated seismic particle motion at shallow depths is typically retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885.[8] They are slower than body waves, e.g., at roughly 90% of the velocity of S waves for typical homogeneous elastic media. In a layered medium (e.g., the crust and upper mantle) the velocity of the Rayleigh waves depends on their frequency and wavelength. See also Lamb waves.

Love waves are horizontally polarized shear waves (SH waves), existing only in the presence of a layered medium.[9] They are named after Augustus Edward Hough Love, a British mathematician who created a mathematical model of the waves in 1911.[10] They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity.

A Stoneley wave is a type of boundary wave (or interface wave) that propagates along a solid-fluid boundary or, under specific conditions, also along a solid-solid boundary. Amplitudes of Stoneley waves have their maximum values at the boundary between the two contacting media and decay exponentially towards away from the contact. These waves can also be generated along the walls of a fluid-filled borehole, being an important source of coherent noise in vertical seismic profiles (VSP) and making up the low frequency component of the source in sonic logging.[11] The equation for Stoneley waves was first given by Dr. Robert Stoneley (1894-1976), Emeritus Professor of Seismology, Cambridge.[12][13]

The sense of motion for toroidal ₀T₁ oscillation for two moments of time.

The scheme of motion for spheroidal ₀S₂ oscillation.Dashed lines give nodal (zero) lines. Arrows give the sense of motion.

Free oscillations of the Earth are standing waves, the result of interference between two surface waves traveling in opposite directions. Interference of Rayleigh waves results in spheroidal oscillation S while interference of Love waves gives toroidal oscillation T. The modes of oscillations are specified by three numbers, e.g., nS l m, where l is the angular order number (or spherical harmonic degree, see Spherical harmonics for more details). The number m is the azimuthal order number. It may take on 2 l+1 values from − l to + l. The number n is the radial order number. It means the wave with n zero crossings in radius. For spherically symmetric Earth the period for given n and l does not depend on m.

Some examples of spheroidal oscillations are the "breathing" mode ₀S ₀, which involves an expansion and contraction of the whole Earth, and has a period of about 20 minutes; and the "rugby" mode ₀S ₂, which involves expansions along two alternating directions, and has a period of about 54 minutes. The mode ₀S ₁ does not exist because it would require a change in the center of gravity, which would require an external force.[3]

Of the fundamental toroidal modes, ₀T ₁ represents changes in Earth's rotation rate; although this occurs, it is much too slow to be useful in seismology. The mode ₀T ₂ describes a twisting of the northern and southern hemispheres relative to each other; it has a period of about 44 minutes.[3]

The first observations of free oscillations of the Earth were done during the great 1960 earthquake in Chile. Presently the periods of thousands of modes have been observed. These data are used for constraining large scale structures of the Earth's interior.

P and S waves in Earth's mantle and core

When an earthquake occurs, seismographs near the epicenter are able to record both P and S waves, but those at a greater distance no longer detect the high frequencies of the first S wave. Since shear waves cannot pass through liquids, this phenomenon was original evidence for the now well-established observation that the Earth has a liquid outer core, as demonstrated by Richard Dixon Oldham. This kind of observation has also been used to argue, by seismic testing, that the Moon has a solid core, although recent geodetic studies suggest the core is still molten[citation needed ].

Earthquake wave paths

The naming of seismic waves is usually based on the wave type and its path; due to the theoretically infinite possibilities of travel paths and the different areas of application, a wide variety of nomenclatures have emerged historically, the standardization of which - for example in the IASPEI Standard Seismic Phase List - is still an ongoing process.[14] The path that a wave takes between the focus and the observation point is often drawn as a ray diagram. Each path is denoted by a set of letters that describe the trajectory and phase through the Earth. In general, an upper case denotes a transmitted wave and a lower case denotes a reflected wave. The two exceptions to this seem to be "g" and "n".[14][15]

c

the wave reflects off the outer core

d

a wave that has been reflected off a discontinuity at depth d

g

a wave that only travels through the crust

i

a wave that reflects off the inner core

I

a P-wave in the inner core

h

a reflection off a discontinuity in the inner core

J

an S wave in the inner core

K

a P-wave in the outer core

L

a Love wave sometimes called LT-Wave (Both caps, while an Lt is different)

n

a wave that travels along the boundary between the crust and mantle

P

a P wave in the mantle

p

a P wave ascending to the surface from the focus

R

a Rayleigh wave

S

an S wave in the mantle

s

an S wave ascending to the surface from the focus

w

the wave reflects off the bottom of the ocean

No letter is used when the wave reflects off of the surfaces

For example:

ScP is a wave that begins traveling towards the center of the Earth as an S wave. Upon reaching the outer core the wave reflects as a P wave.

sPKIKP is a wave path that begins traveling towards the surface as an S-wave. At the surface, it reflects as a P-wave. The P-wave then travels through the outer core, the inner core, the outer core, and the mantle.

The hypocenter/epicenter of an earthquake is calculated by using the seismic data of that earthquake from at least three different locations. The hypocenter/epicenter is found at the intersection of three circles centered on three observation stations, here shown in Japan, Australia and the United States. The radius of each circle is calculated from the difference in the arrival times of P- and S-waves at the corresponding station.

In the case of local or nearby earthquakes, the difference in the arrival times of the P and S waves can be used to determine the distance to the event. In the case of earthquakes that have occurred at global distances, three or more geographically diverse observing stations (using a common clock) recording P-wave arrivals permits the computation of a unique time and location on the planet for the event. Typically, dozens or even hundreds of P-wave arrivals are used to calculate hypocenters. The misfit generated by a hypocenter calculation is known as "the residual". Residuals of 0.5 second or less are typical for distant events, residuals of 0.1-0.2 s typical for local events, meaning most reported P arrivals fit the computed hypocenter that well. Typically a location program will start by assuming the event occurred at a depth of about 33 km; then it minimizes the residual by adjusting depth. Most events occur at depths shallower than about 40 km, but some occur as deep as 700 km.

P- and S-waves sharing with the propagation

A quick way to determine the distance from a location to the origin of a seismic wave less than 200 km away is to take the difference in arrival time of the P wave and the S wave in seconds and multiply by 8 kilometers per second. Modern seismic arrays use more complicated earthquake location techniques.

At teleseismic distances, the first arriving P waves have necessarily travelled deep into the mantle, and perhaps have even refracted into the outer core of the planet, before travelling back up to the Earth's surface where the seismographic stations are located. The waves travel more quickly than if they had traveled in a straight line from the earthquake. This is due to the appreciably increased velocities within the planet, and is termed Huygens' Principle. Density in the planet increases with depth, which would slow the waves, but the modulus of the rock increases much more, so deeper means faster. Therefore, a longer route can take a shorter time.

The travel time must be calculated very accurately in order to compute a precise hypocenter. Since P waves move at many kilometers per second, being off on travel-time calculation by even a half second can mean an error of many kilometers in terms of distance. In practice, P arrivals from many stations are used and the errors cancel out, so the computed epicenter is likely to be quite accurate, on the order of 10-50 km or so around the world. Dense arrays of nearby sensors such as those that exist in California can provide accuracy of roughly a kilometer, and much greater accuracy is possible when timing is measured directly by cross-correlation of seismogram waveforms.

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A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large man-made explosion that produces low-frequency acoustic energy.

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Play 1000s of free quizzes, or create your own. Challenge your friends on any device. Stream friendly.

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https://books.google.co.in/books/about/Vibrations_and_Waves.html?id=JDsPEAAAQBAJ&redir_esc=y#v=onepage&q&f=false
https://www.bookswagon.com/book/waves-mike-freelance-acoustician-goldsmith/9780198803782