WAVE MOTION AND SOUND

Wave is a form of disturbance which travels through a material medium or vacuum due to the repeated periodic motion of the particles about their mean positions. The energy, pressure, etc are transferred from one place to another in medium or space due to wave motion.

MECHANICAL WAVES – The waves which can be propagated or produced only in a material medium, are called mechanical waves.

Mechanical waves are of two types:

1- LONGITUDINAL WAVES: The waves in which the particles of the medium vibrate in the same direction of propagation of wave are called longitudinal waves.

Longitudinal waves can be produced in all the three media such as solid, liquid, and gases. The waves which are produced in air are always longitudinal.

e.g.,

• Those waves which travel along a spring when it is pushed or pulled at one end, are longitudinal waves.
• When coils are closer to each other than normal, compressions are observed in the spring and when coils are farther apart than normal, rarefactions are observed. A long feasible spring which can be compressed or extended easily, is called slinky.

When a longitudinal wave passes through air, the density of air changes continuously, and the pressure and energy are being transferred.

2- TRANSVERSE WAVES: A waves in which the particles of the medium vibrate perpendicular to the direction of propagation of wave, are called transverse wave.

Transverse waves can be produced only in solids and liquids.

e.g.,

• Light is a transverse wave but not a mechanical wave.
• The water waves which are formed on the surface of water when a stone is dropped in it, are transverse waves.
• The waves produced by moving one end of long spring or rope up and down rapidly are transverse waves.

A transverse wave travels horizontally in a medium and the particles of the medium vibrates up and down in the vertical direction. In transverse waves, crest and trough are formed.

A crest is that part of the transverse wave which is above the line of zero disturbance of the medium. A trough is that part of the transverse wave which is below the line of zero disturbance.

Representation of the transverse wave in the displacement-distance graph is shown below:

TERMS RELATED TO WAVES

WAVELENGTH – The distance between two  consecutive compressions(C) and rarefactions(R), is called wavelength. Wavelength is the minimum distance covered by wave after which a sound wave repeats itself. It is represented by lambda. Its SI unit is metre (m).

FREQUENCY – The number of complete waves produced in one second, is called frequency of the wave. It is the number of vibrations per second. If we can count the number of compressions and rarefactions per unit time, it would be the frequency of wave. The frequency of the wave is fixed and does not change even when it passes through different substances. It is denoted by ‘nu’. The SI unit is Hertz (Hz), named in the honour of Heinrich Rudolf Hertz, who discovered photoelectric effect.

1 hertz is equal to one vibration per second.

TIME PERIOD – The time required to produce one complete wave or the time taken by two consecutive compressions and rarefactions to cross a fixed point, is called time period of the wave. It is denoted by (T). Its SI unit is second (s).

Time period of a wave is the reciprocal of its frequency, i.e.,

                    Time Period = 1/Frequency

or,           Frequency = 1/Time period

AMPLITUDE – The maximum displacement of the particles from their mean position, is called amplitude of the wave. It is used to describe the size of the wave. It is denoted by (A). Its SI unit is metre (m).

The amplitude of a wave is same as the amplitude of vibrating body producing the wave.

SPEED – The distance travelled by the wave in one second, is called the speed of the wave or velocity of the wave. Under the same physical conditions, the speed of sound remains same for all frequencies. It is represented by (v). The SI unit is metre per second (m/s or ms^-1).

RELATIONSHIP BETWEEN SPEED, FREQUENCY AND WAVELENGTH.

Speed(velocity) = Frequency X Wavelength

SOUND – The sound is defined as a vibration that propagates as an audible wave of pressure, through a medium such as a gas, liquid or solid or a sound is a vibration that propagates through a medium in the form of a mechanical wave. Sound travels fastest in solids, relatively slower in liquids and slowest in gases. All the mechanical waves that occur in nature, are called sound waves. Sound waves can not travel in vacuum.

CHARACTERISTICS OF SOUND

A sound has four characteristics:

1. LOUDNESS – It describes the strength of ear’s perception of the sound. Greater the sound energy reaching our ear per second, louder the sound will appear to be.  If the sound waves have a smaller amplitude then sound will be faint or soft but if the waves have a large amplitude, then the sound  will be loud. The loudness of sound is measured in decibel (dB). Its SI unit is Watt/m.

1. PITCH OR SHARPNESS – The pitch of the sound depends on the frequency of vibration. Greater the frequency of a sound, higher will be its pitch and lower the frequency of the sound, lower will be the pitch.

• QUALITY (Timber) – It is the characteristic of sound that differentiates between two sounds of same intensity and same frequency. The sound of single frequency is called tone. The sound produced due to the mixture of several frequencies is called a note.

2. INTENSITY – The amount of sound energy passing each second through unit area of the medium, is called the intensity of the sound. Loudness and intensity are not the same terms but these are related to each other. The SI unit of intensity is Watt/m².

SPEED OF SOUND IN DIFFERENT MEDIA

Sound propagates through a medium at a finite speed. The speed of sound depends on the properties of the medium through which it travels. It depends on the temperature of the medium. If temperature increases then speed of sound also increases. The speed of sound in air is 324 m/s at 22 ⁰C and 341 m/s at 25 ⁰C.

SPEED OF SOUND IN DIFFERENT MEDIA AT 25 ⁰C

Aluminium – 6420 m/s

Nickel – 6040 m/s

Steel – 5960 m/s

Iron – 5950 m/s

Brass – 4700 m/s

Glass(flint) – 3980 m/s

Water(sea) – 1531 m/s

Water(distilled) – 1498 m/s

Ethanol – 1207 m/s

Methanol – 1103 m/s

Hydrogen – 12284 m/s

Helium – 965 m/s

Air – 346 m/s

Oxygen – 316 m/s

Sulphur dioxide – 213 m/s

ROLE OF PHYSICAL PARAMETERS

Speed of sound is affected by the following physical parameters

1. EFFECT OF TEMPERATURE : The speed of sound in a gas is directly proportional to the square root of absolute temperature of the gas. Velocity of sound in air increases roughly by 0.61 m/s per degree celsius rise in temperature.

1. EFFECT OF PRESSURE : If temperature remains constant, then there is no effect of change in pressure on the velocity/speed of sound.
2. EFFECT OF HUMIDITY : In humid air, velocity of sound increases as compared to that in the dry air.
3. EFFECT OF WIND : If the wind is blowing, then the speed of sound changes. The speed of sound is increased if wind is blowing in the direction of propagation of sound waves.

REFLECTION OF SOUND

The bouncing back of sound when it strikes a hard surface, is known as reflection of sound. Reflection of sound does not require a smooth and shining surface like that of mirror, it can be reflected from any surface.

ECHO

When a person shouts in a big empty hall, we first hear his original sound, after that we hear the reflected sound of that shout. This reflected sound is known as echo. An echo is nothing but just a reflected sound. So, the repetition of sound caused by reflection of sound waves is called an echo. The sensation of sound persists in our brain for about 1/10 sec. i.e., the time interval between the original sound and the reflected sound must be 0.1 sec.

The minimum distance between the source of sound and the obstacle must be 17.2 m. This distance will change with the change in temperature. Echoes may be heard more than once due to successive multiple reflections. The rolling of thunder is due to successive reflections of sound from a number of reflecting surfaces, such as clouds and the land. It is used in measuring the depth of the sea and altitude of flying aircrafts, etc.

REVERBERATION

The persistence of a sound in a big hall due to repeated reflections from the walls, ceiling and the floor, is known as reverberation.

Note – A short reverberation is desirable in a concert hall, where music is being played, as it boosts the sound level. But excessive reverberation is highly undesirable because sound becomes blurred, distorted and confusing due to overlapping of different sounds. Heavy curtains are put on doors and windows to absorb sound and reduce the reverberation.

REFRACTION OF SOUND

When the sound waves move from one mechanical medium to another mechanical medium , the waves are refracted or transmitted. This phenomenon is called refraction of sound. The refracted waves deviate from the original path or the incident wave.

The main reason for occurrence of refraction of sound, is different speed of sound on different media at different temperature.

e.g., On a warm day, the air near the ground is warm than the air above, so the speed of sound waves near the ground is higher. Thus, the waves bending is away from the ground. On a cold day, the reverse happens and the sound waves bend towards the earth. Thus, on a cold day sound can be heard over large distance.

POINTS TO REMEMBER

• Frequency of ladies voice is usually higher than that of gents. Therefore, ladies voice has higher pitch(sharpness) than gents voice.
• Humming of mosquito has high pitch(high frequency) but low intensity(low loudness), while the roar of a lion has high intensity(loudness) but low pitch.
• The sound produced by different musical instruments and the singers can be distinguished from one another on the basis of their quality or timber.
• Sound waves cannot be heard on the surface of moon and in outer space because there is no air in the vacuum.
• The speed of sound in air is very slower as compared to the speed of light in air. Therefore, the flash of lighting is seen first and the sound of thunder is heard a little later.
• The ceilings of the concert halls, conference halls and cinema halls are curved, so that sound after reflection reaches all corners of the hall.