sound, class 9

 

Sound: it is a kind of energy which gives us sensation of hearing.

• The sound is produced by vibrating objects.
• They travel from one place to another in the form of waves. Hence, the name sound waves.
• Sound waves are longitudinal mechanical wave.

 Wave and particle motion of waves

• Mechanical waves are waves that travel through a material medium.
• It is of two types: depending on the direction of motion of the particle of the medium and the wave propagation:

• Transverse waves

  • Particle motion is to perpendicular the direction of wave motion.
  • This type of wave is a mechanical wave called a transverse wave. E.g.: Light, or even  Mexican wave in a stadium.

  Longitudinal waves

  • When the particles of the medium travel parallel to the direction of the wave motion by means of successive compression or rarefaction.
  • It is also a mechanical wave.
  • Example: a slinky

  A longitudinal wave consists of successive compression and rarefaction that is formed due to continuous to and fro motion of a vibrating object.

  
  

  A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are, and there is a momentary reduction in volume of the medium. It is a region of high pressure and high density.

  
  

  A rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium. It is a region of low pressure and low density
•  Sound is produced by vibrating the objects and it is carried in all directions with the help of a medium
•  Sound travels through air when air molecules vibrate. If air molecules are considered as small balls, a sound wave travels through air by pushing these balls close to each other and then pulling them away from each other. The air molecules come together in areas called compressions and they lie away from each other in areas called rarefactions. The air molecules do not move from their place but only vibrate about their original position. This vibrating effect propagates and allows sound to travel through air.

• Speed of sound: Speed of sound is different in different medium. the speed is basically depend on the density and elasticity of material.

  ·         It is maximum in solid and minimum in gases.

  ü  When sound enter  from one medium to another medium its speed and wavelength change but frequency remain unchanged.

  ü  In a medium the speed of sound is independent of frequency.

  ·         speed of sound increase as the density of material increase. As more near the particle faster the energy transmits.

  ·         Speed of sound in air: 343 m/s

  ·         Speed of sound in water: 1480 m/s

  ·         Speed of sound in steel: 5100 m/s Effect of pressure on sound: pressure does not affect the speed of sound. speed of sound remain unchanged by increase or decrease of pressure.

  ü  Effect of temperature: speed of sound increase with increase in temperature of medium.

  ü  The speed of sound in air increases by 0.61 m/s when temperature is increases by 1^o C.

  ü  Effect of humidity: the speed of sound is more in humid air then dry air due to increase in density.

   

  Sound Properties

  ·         Sound needs a medium to travel. It cannot travel in a vacuum.

  ·         Sound needs a medium to propagate. The matter or material through which sounds propagates is called a medium.

  How do humans produce sound?

  • The sound produced in the voice box called larynx located at the upper end of the windpipe.
  • 2 vocal cords get stretched across in the voice box. Has a slit, through which air is forced out by the lungs.
  • Muscles attached to vocal cords make it tight or loose.

   

  Hearing: We hear sound through our ears.

  The eardrums of our ears sense the vibrations produced by a vibrating object and send them to the brain as the stimulus. This process is called a hearing.

   

  Human ear: Human ear is made of three parts:

  ·         Outer ear – This is the visible part called pinna. It collects sound waves and directs them to the ear tube, at the end of which lies the ear drum. When sound strikes, the ear drum vibrates and passes sound to the middle ear.

  ·         Middle ear – It is a cavity containing three important bones, placed in a manner that they can move with the vibrating ear drum and transmit sound to the inner ear.

  ·         Inner ear – It contains fluid, which vibrates with transmission of sound and excites tiny hair in inner ear. The hair transform these vibrations into electrical impulses and these are transmitted to the brain through auditory nerve.

   
  

  

  
  

  

• Amplitude, frequency and time period of vibrations

  • Wave length: the horizontal distance between two consecutive crest or trough of a wave is called wavelength.

  It is measured in meter(m) and represented by lamda(  )

  • Amplitude (A): The magnitude of disturbance in the medium on either side of the mean value is called as Amplitude (A). Larger the amplitude, louder the sound. It is measured in meter
  • Frequency: The number of oscillations per second is called frequency. Expressed in Hertz (Hz) and represented by mu ()
  • Time period: Time taken for one complete oscillation to travel across a point. T = 1/f. (Seconds)

  Speed of sound= wavelength * ferquency

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  Audible and inaudible sounds: according to the frequency range sound is classified as :

  • Audible sound = 20Hz to 20 kHz known as the Sonic range. These are audible by human ear.
  • 0 to 20 Hz (inaudible) → infrasonic sound: These waves are produced by bigger source such as earthquake, volcanic eruption, elephant, whale, etc.
  • 20 Hz to 20 kHz (inaudible) → Ultrasonic sound. Dog, cat, bat, mosquito can detect these sound but bat can produce these sound.

   

  Characteristics of sound:

  1.    Intensity of sound: it is defined as amount of sound wave passing normally per unit area in given instance of time.

  SI unit- watt/m²

  ü  Intensity of sound is inversely proportional to square of distance of point from the source.

  ü  It is directly proportional to the square of amplitude of vibration, square of frequency and density of medium.

  2.    The loudness of Sound: the sensation of a sound perceived in ear is measured by loudness.

  ü  It depend on intensity of sound, sensitivity of ear and amplitude of wave.

  ü  Larger the amplitude of vibration louder is the sound.

  ü  Unit of loudness id bel. And its practical unit is decibel (dB).

  ü  1 dB = 1/10 bel.

  ü  Another unit is phon.

  ü  Loudness is directly proportional to square of amplitude .

  ü  Loudness = amplitude ²

  3.    Pitch of the Sound: it distinguish a sharp(shrill) sound from grave (dull or flat) sound.

  o   it is depend on the frequency.

  o   Higher the frequency of vibration, the higher is the pitch, or shrill of the sound.

  ü  Shrillness: The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we can say that sound is shrill.

  4.    Quality of sound: it is the characteristic of a sound that enable us to distinguish between two sound of same pitch and loudness produced by two different sources. It depends upon the waveform of the sound.

  ü  Quality of sound is called its tone.

  ü  Some instrument produces a single frequency sound like fork these frequency are called basic or fundamental frequency.

  ü  While some produce mixture of frequency like guitar. These mixtures of frequency are called harmonics. Harmonics provide richness to sound.

   

  Loudness and Pitch

  • Volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.
  • The number of oscillations per unit time. Directly proportional to frequency.
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  • Echo: the phenomenon of hearing the sound after some time just after producing it is called echo. Echo can be heard in mountain or from high building. Echo is created due to reflection of sound.

    To hear echo the minimum distance between the observer and reflector should be 17.2 m (at 21⁰C)

    Persistence of ear (effect of sound on ear) is 1/10 sec.

     

    Calculation for the minimum distance for echo:

    Let the distance between thr source and the wall is Xm, then sound have to reach the source after reflection from wall, so the total distance covered by the sound is 2X

    Speed of sound at 21⁰C is 344 m/s .

    Time to cover 2x distance= 1/10 sec= 0.1 sec

    Speed = distance / time

    344 = 2X/ 0.1,

    X= 344* 0.1/2 = 17.2 m

     

    Reverberation: the hearing of sound after reflection until the sound become inaudible in a big hall is called reverberation.

    To reduce the reverberation the walls and ceiling are made rough our covered with rough surface, damper material etc.

    Application of multiple reflections:

    1.    Loud box, musical instrument like sahanai: the shape is conical which directed the sound in single direction and increase the intensity of sound which reaches to longer distance.

    2.    Stethoscope: it is a medical instrument used by sound to detect the sound of heart and lungs. Sound reach to ear by multiple reflections in pipe.

    3.    The hall of concert of is made dome shaped so that the sound after reflection reach to the audience on back side.

    4.    The speakers are kept toward the stage so that after reflection the sound wave reaches to maximum audience.

     

    Noise: Unpleasant sounds are called noise.

    Oscillation motion: The to and fro motion of an object is called oscillation motion.

     

    Noise Pollution: The presence of excessive or unwanted sound in the atmosphere is called noise pollution.

    Major causes of noise pollution are sounds of vehicles, explosions including the bursting of crackers, machines, loudspeakers etc.

    Presence of excessive noise in the surroundings may cause many health-related problems e.g., lack of sleep, hypertension and anxiety, Can cause hearing impairment, sleeplessness and also hypertension.

    Plantation on the roadside and elsewhere is the best source to reduce the noise pollution.

    Shock wave: a body moving with supersonic speed in air leaves behind it a conical region of disturbance which spread continuously. Such a disturbance is called shock wave. This wave has huge energy and may even make cracks in window panes or even damage a building.

    Bow waves: when a motor boat in a sea travels faster than the sound, then waves just like wave are produced on the surface of water. These waves are called bow waves.

    Noise and music: 

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