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Published in: Physics
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SOUND, CLASS IX

Arun M / Faridabad

4 years of teaching experience

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Teaches: Mathematics, IIT JEE Mains

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  1. SOUND Sub •ect : Ph sics Class : IX KEY POINTS : Board : CBSE Sound is mechanical energy which produces sensation of hearing Sound is produced by vibrating objects. Sound created by the source reaches the listener through the particles of the medium without any net transport ( movement ) of the medium. A material medium is essential for the propagation of sound. Sound is a form of energy which is emitted by a vibrating source and transmitted through a material medium producing the sensation of hearing in us. A sound wave can be considered as propagation of pressure or density variation in the medium. A sound wave which propagates as a series of compression and rarefactions is a longitudinal wave. Longitudinal Wave Motion : A wave motion is said to be longitudinal if the particles of the medium through which the wave propagates vibrate in a direction parallel to the direction of propagation of the wave. Transverse Wave Motion : A wave motion is said to be transverse if the particles of the medium through which the wave propagates vibrate in a direction perpendicular to the direction of propagation of the wave. Compression : A portion of the medium where a temporary reduction in volume and consequently an increase in density ( and pressure ) takes place when a sound wave passes through the medium is called a compression or a condensation. Rarefaction : A portion of the medium where a temporary increase in volume and consequently a decrease in density ( and pressure ) takes place when a sound wave passes through the medium is called a rarefaction. IMPORTANT TERMS CONNECTED To SOUND WAVES CREST : The portion of the medium where the density ( or pressure ) has a value larger than its average value is called a crest. TROUGH : The portion of the medium where the density ( or pressure ) has a value smaller than its average value is called a trough. AMPLITUDE : The magnitude of the maximum disturbance in the medium on either side of the mean position is called the amplitude of the wave. OSCILLATION : The change in density ( or pressure ) from maximum value to the minimum value and again to maximum value is called an oscillation. FREQUENCY : The frequency of sound wave is defined as the number of complete oscillation in density ( or pressure ) of the medium per second. It is denoted by the symbol v ( nu ). Unit of frequency is Cycles per second ( s-1 ) or Hertz
  2. TIME PERIOD ( T ): The time taken for one complete oscillation in density ( or pressure ) of the medium is called the time period of the wave. It is also defined as time taken by its two consecutive compressions or rarefaction to cross a fixed point. Unit is sec. WAVELENGTH ( ) : The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength of the wave. It is also equal to the distance travelled by it in its Time Period ( T ). Unit is m. RELATION BETWEEN FREOUENCY AND TIME PERIOD : T=l/v OR v=1/T OR v. T = 1 RELATION BETWEEN FREOUENCY , WAVELENGTH AND SPEED OF SOUND . speed of sound = frequency (v) x Wavelength (1) Speed of sound is finite and is much less than the speed of light. Speed of sound in solids > speed of sound in liquids > speed of sound in gases. Speed of sound increases with increase in temperature. Sources that move faster than the speed of sound are said to have SUPERSONIC speeds. CHARACTERISTICS OF SOUND : LOUDNESS : The sensation produced in the ear which enables us to distinguish between a loud and a faint sound is called loudness. It depends on the sensitivity of the ear. Loudness is directly proportional to the square of the Amplitude. Loudness varies inversely as the square of the distance from the source of sound' Loudness varies directly as the square of the frequency of the source. It has a subjective existence. INTENSITY : The intensity of sound at any point in space is defined as the amount of energy passing per unit time per unit area in a direction perpendicular to the area. It is a physical quantity that can be easily measured. It does not depend upon the sensitivity of the ear. It has an objective existence.
  3. PITCH . Pitch is that characteristic of sound which helps in differentiating between a SHRILL sound from a GRAVE ( Flat or Dull ) sound. The pitch increases with the increase in frequency and vice versa. High and low pitched sounds are called TREBLE and BASS. It has a subjective existence. OUALITY or TIMBER: The characteristic of a sound which distinguishes it from another sound of same pitch and loudness is called quality or timber. The quality of sound is determined by its wave form. A sound of SINGLE frequency is called a TONE ( or PURE NOTE ). A sound which is produced due to a mixture of several frequencies is called NOTE. It has a subjective existence. REFLECTION OF SOUND . • The angle of reflection is always equal to the angle of incidence. • The incident wave, the reflected wave and the normal ( at the point of incidence ) all lie in the same plane. ECHO An echo is the phenomenon of repetition of sound of a source by reflection from an obstacle. The sensation of sound lasts in our brain for 0.10 sec. This property is called persistence of hearing. For hearing a distinct echo, the minimum distance of the obstacle from the source of sound should be 17.2 m ( Speed of sound in air at 200 C is 344 m / sec. ) Relation between d ( distance of obstacle from the source ), u ( Speed of sound ) and t ( time taken by sound to reach back the listener after reflection ) is given by 2d = u x t REVERBERATION . The phenomenon of persistence or prolongation of audible sound after the source has stopped emitting sound is called reverberation. The time for which reverberation persists until it becomes inaudible is called reverberation time. Please note a certain amount of reverberation is desirable as because it will enrich the sound and make it more pleasant.
  4. RANGE OF FREOUENCIES : , AUDIBLE SOUND WAVES Waves with frequencies between 20 Hz to 20 KHz. , INFRASOUND WAVES : Waves with frequencies below 20 Hz , ULTRASOUND WAVES : Waves with frequencies above 20 KHz. USES OF ULTRASOUND . INDUSTRIAL USES Cleaning instruments and electronic components, Plastic Welding, Detecting flaws and cracks in metal blocks. MEDICAL USES : Echocardiography, Ultrasonography, Surgical uses. SONAR ( SOund Navigation And Ranging ) : A sonar is a device which measures the distance, direction and speed of objects lying under water using ultrasonic waves. Relation between d ( distance of the object that reflected the ultrasound ), u ( Speed of sound through sea water ) and t ( time interval between the transmission and reception of the reflected ultrasound waves ) is given by 2d = u x t HUMAN EAR : Divided into THREE parts OUTER EAR Consists of PNNA and AUDITARY CANAL ( Including EAR DRUM ). It collects sound waves and transmits these to the ear drum. MIDDLE EAR : Consists of three bones HAMMER, ANVIL and STIRRUP. It amplifies these sound waves about 60 times. INNER EAR : Consists of COCHLEA, BASILAR MEMBRANR and three semi- circular CANALS. It amplifies sound energy into electrical energy and conveys to the Brain as nerve impulses for interpretation.

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