SOUND

IMPORTANT TERMS

Sound:-                  It is produced due to the vibration of objects. Vibration is the rapid to and fro motion of an object.

Human Sound:-       The sound of human voice is produced due to the vibration of the vocal cords. The vocal cords are also known as vocal folds or voice reeds.

Audible sound:-       The frequency range of human hearing is called audible range of sound. Human ear is normally sensitive to only those vibrations whose frequency lies between 20 Hz to 20000 Hz.

Infrasound:-            Sound wave with a frequency lesser than 20 Hz is called infrasound.

Ultrasound:-            Sound wave having frequency more than 20000 Hz is called ultrasound.

Pitch:-                     The sensation of frequency is commonly referred as pitch. The pitch of sound is proportional to frequency. As the frequency of vibration increases the pitch of sound also increases and vice versa.

Timbre:-                  The quality of sound is called timbre of sound. 

Tone:-                    Sound of a single frequency is called tone.

Note:-                     Sound of mixture of several frequencies is called note. A note is pleasant to listen.

Echo:-                    It is a repetition of sound produced by the reflection of sound waves from a wall, mountain, or other obstructing surface.

Sound

Sound is a form of energy like heat energy and light energy.  Sound travels in the form of wave from one place to another. It produces a sensation of hearing in our ears.

Production of sound

Sound is produced due to the vibration of an object. The motion of objects causes vibration. Vibration is a kind of rapid to and fro motion of an object about a central position.

Example:

  • It can be produced by clapping of palms, beating a table or drum.
  • Sound is produced by a guitar and flute because of vibration of guitar’s string and vibration of air column of flute respectively.
  • The sound of human voice is produced due to the vibration of the vocal cords.
  • A stretched rubber band when plucked vibrates and produces sound.

Source of sound and its propagation

  • Vibration of objects creates sound.
  • The matter or substance through which sound is transmitted is called a medium. A medium can be solid, liquid or gas.
  • Vibration creates a disturbance in the medium particles. The particles of medium pass the disturbance to the adjacent particles and the adjacent particles pass the disturbance to the next adjacent particles and so on. This phenomenon continues and the disturbance moves outward from the centre of disturbance. 
  • The particles of medium do not travel, rather only the disturbance caused by sound energy passes to the adjacent particles of the medium. So, the traveling of sound is called propagation of sound and sound propagates from one place to another in the form of waves, 

Wave

Wave is defined as the disturbance or oscillation from one location to another location accompanied by transfer of energy. There are two types of wave: electromagnetic Wave and Mechanical Wave.

Electromagnetic Wave: The waves which do not require medium for propagation are called electromagnetic waves. These waves can travel through vacuum also. For example, light waves, X-rays.

Mechanical Wave: The wave that requires medium to propagate is called Mechanical Wave. E.g., Sound wave. Sound cannot travel in the absence of a medium.

Types of Wave - On The Basis of Direction

There are two types of waves on the basis of direction of propagation: Transverse wave and Longitudinal wave.

Transverse Wave: When the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave, the wave is known as the transverse wave. For example light wave, water wave, etc.

Longitudinal Wave: When the particles of the medium vibrate along the direction of propagation of the wave then the wave is known as the longitudinal wave. For example sound wave.

Sound Wave

Sound propagates because of oscillation of particles of medium parallel to the disturbance, thus sound wave is longitudinal wave. Sound cannot travel in the absence of a medium and thus it is a mechanical wave. Sound does not travel in vacuum.

Propagation of sound through air

  • Air is the most common medium through which sound travels.
  • When a vibrating object moves forward, it pushes and compresses the air in front of it forming a region of high pressure called compression (C). The compression moves away from the vibrating object.
  • When the vibrating object moves backward, it forms a region of low pressure called rarefaction (R).
  • As the object moves to and fro rapidly, it produces a series of compressions and rarefaction in the air which makes the sound to propagate in the medium.

Characteristics of a sound wave

  • Sound wave can be described by its wavelength, frequency, amplitude, time period and velocity (speed).
  • Sound can be graphically represented as a wave. There are changes in the density and pressure as sound propagates in a medium. 
  • Compressions are the regions of high pressure and density where the particles are crowded and are represented by the upper portion of the curve called crest.
  • Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called trough.
  • The distance between two consecutive compressions (crests) or two consecutive troughs is called wave length. It is represented by the symbol λ (Greek letter lamda). Its SI unit is metre (m).
  • The number of vibrations per second is called frequency. It is represented by the symbol ٧ (Greek letter nu). Its SI unit is hertz (Hz).
  • The maximum displacement of each particle from its mean position is called amplitude. It is the height of the crest or tough. It is represented by the letter A.The S.I. unit of amplitude is metre (m).
  • The time taken to complete one vibration is called time period. It is represented by the letter T. The SI unit is second (s). Frequency= 1/(Time period) or ٧  = 1/T.
  • Pitch is the sensation (brain interpretation) of the frequency of an emitted sound and is the characteristic which distinguishes a shrill (or sharp) sound from a grave (or flat) sound. The pitch of sound depends on the frequency of vibration. If the frequency is high, the sound has high pitch and if the frequency is low, the sound has low pitch.
  • The loudness of sound depends upon the amplitude of vibration. If the amplitude is bigger, the sound is loud and if the amplitude is smaller, the sound is soft.
  • The speed of sound is different in different media. The speed of sound is more in solids, less in liquids and least in gases. The speed of sound also depends on the temperature of the medium. If the temperature of the medium is more, the speed of sound is more.
  • The distance travelled by a wave in one second is called velocity of the wave (or speed of the wave). The SI unit for the velocity of a wave is metres per second (m/s).
  • Relationship between Speed (v), frequency (٧) and wave length(λ):

                                    Speed = wave length x frequency

                                            v = λ x ٧ 

 

The velocity of sound waves depends upon the following three factors:

  • Nature of the medium: The velocity of sound is maximum in solid, moderate in liquid and minimum in gas.

·       Temperature: The velocity of sound is directly proportion to temperature.

  • Humidity present in air: An increase in humidity in air increases the velocity of sound because humid air is less in density and a decrease in humidity in air decreases the velocity of sound because dry air is high in density.

Reflection of sound

 

  • Like light, sound waves also get reflected at the surface of a solid or liquid and follow the laws of reflection.
  • Laws of reflection: (a) Angle of incidence is equal to the angle of reflection. (b) The incident ray, the reflected ray and normal at the point of incidence all lie in the same plane.

Echo

  • Echo is Phenomenon of hearing back our own sound. It is due to successive reflection from the surfaces obstacles of large size like tall building or a mountain.  If we shout or clap near a reflecting surface, we hear the same sound again. This reflecting sound which we hear is called echo.
  • If ‘t’ is the time at which an echo is heard, ‘d’ is the distance between the source of sound and the reflecting body and ‘v’ is the speed of sound. The total distance travelled by the sound is ‘2d’.

                                                     Speed of sound, v = 2d/t

Or, d = vt/2

 

   Conditions for the formation of echoes

  • To hear an echo clearly, the time interval between the original sound and the echo must be at least 0.1 s.
  • The speed of sound in air is 344 m/s.
  • The distance travelled by sound in 0.1 s = 344 m/s x 0.1 s = 34.4 m. So to hear an echo clearly, the minimum distance of the reflecting surface should be 17.2 m.

Reverberation

  • Echoes may be heard more than once due to repeated or multiple reflections of sound from several reflecting surfaces. This causes persistence of sound called reverberation. A short reverberation is desirable in a concert hall. Too much reverberation confuses the programmers and must be reduced.
  • To reduce reverberation in big halls and auditoriums, the roofs and walls are covered by sound absorbing materials like compressed fibre boards, rough plaster or draperies.

Range of Hearing

  • The audible range of sound for human beings extends from about 20 Hz to 20,000 Hz. Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. Animals like dogs, elephants, rhinoceros, whales etc. produce and hear infrasonic sound.
  • Sound whose frequency is more than 20000 Hz is called ultrasonic sound. Animals like dolphins, bats, rats, propoises etc. produce and hear ultrasonic sound.
  • Bats use reflection of ultrasonic sound waves to detect an obstacle or its prey.

Applications of ultrasound

  • It is used to clean objects like electronic components by keeping them in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the dirt particles get detached from the components.
  • It is used to detect cracks in metal blocks by sending ultrasonic waves through the metal blocks and if there are cracks, the waves are reflected back and the cracks can be detected.
  • It is used for medical diagnosis and therapy, and also as a surgical tool like breaking of small stones, formed in the kidneys, into fine grains so that they are removed through the urine.
  • Some animals use ultrasound for information exchange and for the detection and location of objects. Bats and porpoises are found to use ultrasound for navigation and to locate food in darkness or at a place where there is inadequate light for vision.

SONAR

  • It stands for SOund Navigation And Ranging. This is a device that is used to measure depth of sea bed, locate scraps, wrecks, submarines of enemies, etc. in the water by producing ultrasound. It is fitted over ships and submarines.
  • Ultrasonic sound waves are produced by transmitter of SONAR and when these waves return back to the detector, after striking the object in the sea bed, they are analyzed with the help of computer. The shape and position of objects under sea and ocean is detected on the basis of speed and nature of reflected ultrasound waves.
  • The distance of the object can be calculated by knowing the speed of sound in water and the time taken between the transmission and reception of ultrasound.

Human ear

The human ear has three main parts. Their auditory functions are as follows: External ear, Middle Ear and Internal Ear

  • The external ear is outside the body and it is also called pinna. It collects the sound from the surrounding. The collected sound passes through the auditory canal.
  • The middle ear is composed of the ear drum or tympanum and the bone ossicles. The sound waves passes through the ear canal to eardrum. The eardrum vibrates. The vibrations are amplified by the three bone ossicles of the middle ear called hammer, anvil and stirrup. The middle ear then transmits the sound waves to the inner ear.
  • The internal ear is composed of a cochlea and three semi-circular canals. In the inner ear, the sound waves are converted into electrical signals by the cochlea and sent to the brain through the auditory nerves. The brain then interprets the signals as sound.

Interesting Facts

·       Acoustics is known as the scientific study of sound waves.

·       The loudest natural sound on Earth is caused by an erupting volcano.

·       Most animals use sounds to help them detect dangers and hazards before they happen to them.

·       There is no sound in space because there is no medium for sound to travel through.

·       The sound of thunder is produced by rapidly heated air that surrounds the lightning, which expands faster than the speed of sound.

·       Sound produces a relatively low level of energy when compared to other forms of energy. Because sound produces such a low level of energy it is not used for electricity.

·       Flies cannot hear at all.