Sound - Important Points -Properties, Characteristics, Types, Speed, Production- General Knowledge & Awareness

• 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:

Sound waves have following three characteristics.

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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