9th Physics Sound

 

 Class 9th Science -Physics

ICSE/ CBSE Notes-

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SOUND Sound is a form of energy which produces a Sensation of hearing in our ears. when a source of sound  vibrates , it creates periodic disturbance in the medium near it. This disturbance travels in  the form of wave in any medium.

Properties of sound

• sound produces when a body vibrate, so we can say that each vibrating body produces sound.

• sound can not travel in vacuum , ie it needs medium for it propagation.

• sound can travel in solid ,in liquids as well as in gases.

• speed of sound is maximum in solids and minimum in gases.

• speed in solids  >  speed in liquids  > speed in gases.

• sound obeys the law of reflection.

• The loudness of sound is measured by DECIBELS. it show the sound pressure level.

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Terms used in Sound

Terms used in sound are given as -

1-Amplitude  

The maximum displacement of vibrating body from its mean position is called  Amplitude. It is denoted by 'A'

• If any stationary body struck with less force ,it has less amplitude.
• If any stationary body struck with large force ,it has large amplitude.

2- Frequency

The number of vibrations produced by a vibrating body in one second is called frequency . It is denoted by letter  'f'

• SI unit of frequency is  hertz ,it represented by 'Hz'
• when vibrating body produce one vibration in one second , it frequency is said to be 1 Hz
• Bigger unit of frequency are kilohertz (KHz) , megahertz (MHz)
• 1 kHz = 1000 Hz      and   1 MHz  =  1000000 Hz
• The voice of child and woman has higher frequency
• The sound of flute has a higher frequency as compared to the sound of drum.
• The shrill sound has higher frequency than a bass sound. 

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3- Time Period 

The time in which a vibrating body complete one cycle is called time period . it is denoted by 'T'

 

T = 1/f

Time  period =1/ Frequency

4- Wave length

Distance between two consecutive Trough or Crest is called  wave length of the sound, it is denoted 

by 'λ' 

λ = v /f

wave length = speed of sound wave / frequency

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1- Subsonic vibration or Infrasonic vibration

• If the frequency of the vibration is less than 20 Hz ,such vibrations are called subsonic vibration.
• Human ear can not realize them.
• frequency <  20 Hz

2- Sonic vibration OR sound vibration

• If the frequency of the vibration is  between  20Hz to  20,000 Hz ,such vibrations are called sonic vibration.
• Human ear can realize then easily.
•  20 Hz <  frequency <  20,000 Hz
• It is also called audible wave

3- Ultrasonic vibration or ultrasonic

• If the frequency of vibration is above 20,000 Hz such vibration are called ultrasonic vibration.
• Human ear can not perceive this.
• frequency > 20,000 Hz

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Note-

• Human  can hear up to frequency  20,000 Hz
• Dog can hear  up to  frequency  40,000Hz
• Bats (चमगादड़ ) can produce vibration in the frequency range of 50 Hz to 80,000 Hz.
• Bats produce ultrasonic  vibration when they flying at night , which can not be heart by human.
• Dolphins are also produce ultrasonic sound to locate their prey.

sound insulation or sound absorption

• Shining objects like mirrors,metals ,hard objects like buildings ,stones are reflect the sound.

• Loose texture objects are absorb the sound ie curtains ,carpets, gunny bags , straw etc

• Auditoriums and cinema hall walls are coated with  some  rough materials, so they do not reflect any sound.

• These places are very big there fore echoes are farmed ,thus the quality of sound heard by people become very poor.

• Due to same reason the sound recording studios are coating by sound proof materials.

• The reflection of sound take place only from hard and polished surface 

• In sound reflection, the angle of incidence is equal to the angle of reflection.

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Echoes

• The repetition of sound by reflecting from high building or any high place is called Echo.
• Human ear can hear two sounds separately only with in 1/10 of a second.