- Sound waves are longitudinal mechanical waves. Sound waves are produced by compression and rarefaction of the particles of the medium.
- Sound is a form of energy that causes the sensation of hearing.
- Sound needs a medium to travel. Sound travels through gases, liquids and solids. Sound does not travel through vacuum.
- Sound waves are reflected, refracted, and diffracted, and exhibit interference
- The speed of sound is the maximum in solids, less in liquids and the least in gases.
- Sound cannot travel through vacuum. In humans, sound is produced by the voice box or the Larynx. Vocal cords in the larynx vibrate and produce sound.
- Sound wave always needs an fluid medium to be heard, though it can travel through any material medium but to have an effect on our eardrums a fluid medium is necessary.
- Sound Travel with a speed of 332 m/s at 0 C. The denser the medium, the greater the speed of sound whereas the opposite is true of light.
- The sound waves have only the characteristics of waves, whereas light has a wave particular duality in nature. They exhibit properties of both waves and particles and are said to be composed of packets of light called photons
- The speed of propagation of the sound waves is dependent on the wave frequency. This implies that there is a medium and the medium has atoms, molecules, or some structure. The dependency of the speed of propagation on frequency also implies that there is a minimum wavelength. There is a frequency cut-off.
Characteristics
of sound waves:
|
Intensity:
Intensity of sound at any point of space is defined as amount of energy passing
normally per unit area held around that point per unit time. SI unit of
intensity is watt/m2 .
Intensity of sound at a point is,
(i) inversely proportional to the
squire of the distance of point from the source.
(ii) Directly proportional to squire
of amplitude of vibration, squire of frequency and density of the medium.
Due to intensity, a sound appears loud or
faint to the ear. Actually, the sensation of the sound perceived in ear is
measured by another term called loudness which depends on intensity of sound
and sensitiveness of the ear. Unit of
loudness is bel. A practical unit of loudness is decibel (dB) which of equal to 1/10th
of bel. Another unit of loudness is phon.
Pitch: Pitch is that characteristic of sound which distinguishes a sharp
sound from a grave (dull or flat) sound. Pitch depends upon frequency. Higher
the frequency, higher will be the pitch and shriller will be the sound. Lower
the frequency, lower will be the pitch and grave will be the sound.
Quality: Quality is that characteristic of sound which
enables us to distinguish between sounds produced by two sources having the
same intensity and pitch. The quality depends upon number, frequency and
relative intensities of overtones.
Effect of
Pressure, Temperature & Humidity on Sound:
|
Effect
of pressure on speed of sound: The speed of sound is independent of pressure
i.e. speed remains unchanged by the increase or decrease of pressure.
Effect
of temperature on speed of sound: The speed of sound increases with the
increase of temperature of the medium. The speed of sound in air increases by
0.61 m/s when the temperature increased by 1 C.
Effect
of humidity on speed of sound: The speed of sound is more in humid air then in
dry air because the density of humid air is less than the density of dry air.
According to their frequency range, longitudinal
mechanical waves are divided into the following categories:
1. Audible or Sound waves
|
The
longitudinal mechanical wave which lie in the frequency range 20 Hz to 20000 Hz
are called audible or sound waves. These waves are sensitive to human ears.
These are generated by the vibrating bodies such as tuning fork, vocal cords
etc.
2. Infrasonic Waves
|
The
longitudinal mechanical waves having frequencies less than 20 Hz are called
Infrasonic. These waves are produced by sources of bigger size such as earth
quakes, Volcano eruptions, meteors, ocean waves and by elephants and whales.
Elephants have the ability to emit infrasound to communicate at distances of up
to 10 miles (12 - 35 Hz.). Even tigers emit infrasound.
Application/Uses of Infrasonic Waves:
Infrasonic
waves can carry over long distances [thousands of kilometres] and are less
susceptible to disturbance or interference than waves of higher frequencies.
A. Medical: (therapeutic devices)
- Several studies conducted in Russia
and Europe reported that infrasound has therapeutic effects.
- Infrasound peumomassage: At 4 Hz, the
progression of myopia in school children can be stabilized.
- Infrasound phonophersis in
antibacterial drugs: In treatment of patients with bacterial keratitis, it is
as effective as local instillations of the same drugs.
B. Monitoring activities of the atmosphere:
Infrasonic waves will be influenced by
the atmosphere during its propagation, which is closely related with the
distribution of temperature and wind in the atmosphere.
By measuring the propagation properties
of infrasonic waves generated by natural sources, one can detect some
characteristics and rules of the large scale meteorological motions.
C. Forecasting natural disasters:
Many disasters, such as volcanic
eruptions, earthquakes, land-slides and clear-air turbulences, radiate
infrasound in advance .
By monitoring the infrasound waves , we
can forecast these disasters.
How Bad are Infrasonic Waves:
Infrasound
is especially dangerous, due to its strong vibrations, or oscillations. They
hug the ground, travel for long distances without losing strength, and are
unstoppable.
3. Ultrasonic Waves
|
The
longitudinal mechanical waves having frequencies greater than 20000 Hz are
called Ultrasonic Waves. But certain creatures like dog, cat, bat, and mosquito
can detect these waves. Bat not only detect by also produce ultrasonic
waves. Humans cannot hear the sound of
inaudible range.
Applications of Ultrasonic Waves:
1.
For sending signals.
2.
For measuring depth of sea.
3.
For cleaning cloths, aeroplanes and machinery parts of clock.
4.
For removing lamp-shoot from the chimney of factories.
5.
In sterilizing of a liquid.
6.
In Ultra-Sonography.
7.
Doppler effect: Doppler effect to assess whether structures (usually
blood) are moving towards or away from the probe , and its relative velocity .
8.
Whales make use of ultrasounds for communication purposes. Individual
pods of whales have their own distinctive dialect of calls, similar to
songbirds.
Speed of Sound:
|
Speed
of sound is different in different mediums. In a medium, speed of sound
basically depends upon elasticity and density of medium.Speed of sound is
maximum in solids and minimum in gases.
When
sound enters from one medium to another medium, its speed and wavelength changes but frequency remains unchanged. In a
medium, the speed of sound is independent
of frequency.
Medium
|
Speed of sound (In m/s)
|
Carbon dioxide
|
260
|
Air (0 C)
|
332
|
Air (20 C)
|
343
|
Steam (100 C)
|
405
|
Helium
|
965
|
Alcohol
|
1213
|
Hydrogen
|
1269
|
Mercury
|
1450
|
Water (20 C)
|
1482
|
Sea Water
|
1533
|
Copper
|
3560
|
Iron
|
5130
|
Glass
|
5640
|
Granite
|
6000
|
Aluminum
|
6420
|
What is Audible
Range of Sound?
|
Sounds
with frequency between 20 Hz to 20,000 Hz are called audible sound. The hearing
range of human beings is between 20 hertz to 20,000 hertz. Sound with frequency
below 20 hertz and above 20,000 hertz is called sound of inaudible range.
Humans cannot hear the sound of inaudible range. Many animals, such as dogs,
cats, etc. can hear the sound with frequency above 20,000 hertz
How sound is
produced by Humans?
|
Larynx is composed of two stretched membranes;
with some gap between them. When air passes through the larynx, the membranes
or vocal cords vibrate and produce sound. That is why larynx is also known as
sound box.
Various Ways
Pleasant Sound is Produced:
|
Some
instruments produce sound due to the
-vibration
of membranes,
-vibration
of strings, and
-vibration
of an air column.
The
to and fro or back and forth motion of an object is called vibration. The
sitar, veena, violin, guitar and ektara are some stringed instruments.
The tabla, cymbals, ghatam, kartal and
manjira are some instruments that work on the vibration of a membrane.
The instruments like the flute and the
trumpet produce sound due to the vibration of an air column present in them.
Sonar and its Working Principle:
|
SONAR
means SOund NAvigation and Ranging.
Sonar is an instrument that use ultrasonic wave for sound ranging. It measures
even short time intervals quite accurately.
Sonar
works on the principle of echo. A
strong and short (ultrasonic) sound signal is sent towards the bottom of the
ocean. Echo of the signal is then detected and depth of ocean is calculated.
What is Echo?
|
The sound waves received after being reflected
from a high tower or mountains is called echo.
To hear echo, the minimum distance
between the observer and reflector should be 17m (16.6 m).
Persistence of ear (effect of sound on
ear) is 1/10.
Doppler Effect: If there is a relative motion between source of sound and observer.
The apparent frequency of sound heard by the observer is different from the
actual frequency of sound emitted by the source. This phenomenon is called
Doppler Effect.
When the frequency between the source
and observer decreases, the apparent frequency increases and vice-versa.
What is Mach
Number?
|
Mach
Number: It is defined as the ratio of speed of sound source to the speed of
sound in the same medium under the same condition of temperature and pressure.
If match number >1, body is supersonic.
If match number >5, body is called
hypersonic.
If match number < 1 -the body is said to be moving with subsonic speed.
Some reasons
commonly asked in Exam:
|
- Children and women produce sound of high frequency and their sound is shriller and of higher pitch. On the other hand, an adult male produces sound of lower frequency and his sound is less shrill and has lower pitch.
- A drum produces sound of lower frequency which is less shrill and has lower pitch, while a whistle produces sound of higher frequency which is shriller and is of higher pitch.
- A lion produces a sound of lower frequency which is less shrill and has lower pitch, while a bird produces sound of high frequency which is shriller and has higher pitch. However, sound of lion is louder than the sound of a bird.
- We know that the speed of light much more than the speed of sound. Due to this, light reaches to us faster than sound. Hence, during lightning we see the streak of light earlier than hearing the sound of thunder.
- Due to refraction, sound is heard at longer distance in nights than in day.
- Resonance: If the frequency of the imposed periodic force is equal to the natural frequency of a body, the body oscillates with a very large amplitude. This phenomenon is called resonance.
- Interference of sound: The modification or redistribution of energy at a point due to superposition of two (or more) sound waves of same frequency is called interference of sound. If two waves meet at a point in same phase, intensity of sound is maximum at that point. Such type of interference is called constructive interference. Similarly, if the two point meet at a point in opposite phase, intensity of sound at that point is minimum. Such type of interference is called destructive interference.
- Diffraction of sound: Wavelength of sound is of the order of 1 m. If an objective of that range appears in the path of sound, sound deviates at the edge of obstacle and propagates forward. This phenomenon is called diffraction of sound.
- Shock waves: A body moving with supersonic speed in air leaves behind it a conical region of disturbance which spreads continuously. Such a disturbance is called shock waves. These waves carries a huge energy and may even cracks in window panes or even damage a building.
- Bow waves: When a motor boat travels faster than sound, then waves just like shock waves are produced on the surface of water. These waves are called bow waves.
Some
Questions:
Which
of the following is true? a) Sound waves exhibit interference b) Light waves
exhibit interference c) Both the light and sound waves exhibit interference d)
Neither sound waves nor light waves exhibit interference.
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