Sound Class 9 Notes Science Chapter 11

Production of Sound

Sound is a form of energy that produces a sensation of hearing in our ears.  

• Sound is created when objects vibrate or move back and forth.
• Tuning forks and actions like plucking, scratching, rubbing, blowing, or shaking objects can produce sound.
• Our voice produces sound through vibrations in the vocal cords.
• These activities make objects vibrate, which leads to the production of sound.

Propagation of Sound

• Sound is produced when objects vibrate, setting nearby particles in motion. 
• It travels through different mediums like solids, liquids, and gases, moving from the source to the listener. 
• As an object vibrates, it disturbs the surrounding particles, transferring energy in a wave-like motion. 
• However, the particles themselves do not move forward; instead, they pass on the vibration. 
• Sound waves are mechanical waves that require a medium for transmission, as they involve the movement of particles within the medium.
• Thus, sound waves can't travel through vacuum.  

Sound Propagation in Air:

• When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C), as shown in the following figure. This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

Compression(C) and Rarefaction(R) of sound

• As the object moves back and forth rapidly, a series of compressions and rarefactions are created in the air. These make the sound wave that propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure.

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Sounds Waves are Longitudinal Waves

• Sound travels through compressions and rarefactions in the medium.
• These compressions and rarefactions create longitudinal waves.
• In longitudinal waves, particles move parallel to the direction of the wave.
• The particles oscillate around their resting position without changing their location.
• Sound waves are an example of longitudinal waves.

Characteristics of a Sound Wave

• Wavelength: The distance between two consecutive peaks (crest) or troughs is called wavelength. Wavelength is represented by λ (lambda) and its SI unit is metres.
• Frequency: The number of oscillations of sound waves per unit time is called its frequency. A number of peaks or troughs per unit of time will give frequency. It is represented by ν (nu) and its SI unit is Hertz (Hz).
• Time Period: The time taken to complete one oscillation is called its time period. Its unit is second and is represented by T.
• Relation of frequency and period: Period is the reciprocal of Frequency.

• Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A. Its unit is a meter.
• Softness or Loudness of Sound: If the amplitude is smaller then the sound will be softer and if it is larger then the sound will be louder. Higher amplitude helps the sound wave travel faster.
•  Speed of sound: It is the distance in which compression or a rarefaction travels per unit of time.
  Speed of sound is given by:So, Speed = Wavelength × Frequency 

Speed of Sound In Different Media

• The speed of sound changes across different materials at the same temperature.
• When the temperature rises, the speed of sound in the material also goes up. Thus, the temperature of the medium affects the speed of sound.
• For instance, the speed of sound in air is 331 m/s at 0°C and 344 m/s at 22°C.
• The speed of sound lowers when moving from solids to gases, depending on the medium's characteristics.
• The speed of sound is mainly influenced by the medium and its temperature, not remaining constant across all frequencies.
• Tables show the speeds of sound at specific temperatures in different materials.
  
  Speed of Sound in Different Media at 25º C
  

Reflection of Sound

• Sound Reflection: Sound bounces off surfaces similar to light, following the law of reflection.
• Angle of Reflection: The incident and reflected angles are equal relative to the normal.
• Plane of Reflection: Incident sound, reflected sound, and normal lie in the same plane.
• Obstacle Requirement: A large surface is needed for effective sound reflection.
• Echo Formation: A minimum 0.1-second gap is required between the original sound and its echo for clear perception.

Reflection of Sound

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Echo

If we shout or clap near a suitable reflecting object like a tall building or a mountain, we hear the same sound again after a short time. This repeated sound is known as an echo.

• Persistence of Sound: The brain retains sound for 0.1 seconds, requiring a 0.1s gap for a clear echo.
• Minimum Distance for Echo: With sound speed at 344 m/s, the reflecting surface must be at least 17.2 m away.
• Temperature Effect: Echo distance varies with air temperature.
• Multiple Echoes: Successive reflections cause repeated echoes.

Reverberation

A sound made in a large hall continues to be heard due to repeated bouncing off the walls until it fades away and cannot be heard anymore. This continuation of sound in a space like an auditorium occurs because of many reflections and is known as reverberation.

• Unwanted Reverberation: Excessive sound reflection in large halls is undesirable.
• Sound-Absorbing Materials: Ceilings and walls use compressed fibreboard, rough plaster, and draperies to absorb sound.
• Seating Materials: Chosen for their sound absorption properties.

Reverberation of Sound

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Uses of Multiple Reflections of Sound

• Directional Sound Devices: Megaphones, loudhailers, horns, and musical instruments like trumpets and shehanais use conical openings to direct sound waves towards the audience.
• Stethoscope: Uses multiple reflections to guide internal body sounds, like heartbeats, to a doctor's ears.
• Acoustic Design in Halls: Curved ceilings and soundboards in concert, conference, and cinema halls reflect sound evenly, ensuring clear audio distribution

Applications of Ultrasound