CBSE Notes Class 8 Science Chapter 10 Sound

Sound is a type of energy that moves through different mediums—like air, water, and solids—by means of vibrations. It is generated when an object vibrates, producing waves that travel through these mediums. Sound is essential in our daily lives, serving important functions in communication, music, and even in medical fields, such as in ultrasound technology.

1.0Definition of Sound

• Sound is produced by a vibrating body, hence a vibrating body would always produce sound, sometimes the vibration is clearly observable and the sound is strong whereas in some cases the vibration is small and the sound is feeble.
• Example-When the diaphragm of a drum is struck, it vibrates and produces sound, When the air column vibrates within a flute it produces sound.

2.0Propagation of Sound

• Sound can propagate through solids, liquids and gases. It cannot propagate through vacuum.
• Sound can propagate through solids, Hence the doctor checks us using the stethoscope. He can hear the sound from within our body because sound travels through solids.
• Sound can propagate through liquids, in fact this is how aquatic animals communicate with each other.
• Sound can propagate through gases, this is the reason why we are able to hear a person standing near us speak.

3.0Generation of Sound by Humans

• In humans sound is produced because of the vibration of the voice box called larynx. The larynx is situated at the upper end of the windpipe.
• It comprises two stretched membranes called vocal cords with a narrow slit between them, when we say air passes through those stretched membranes they produce different sounds in accordance with stretching.

Sound Heard by Ears:

• The shape of our outer ear is like a funnel, sound waves enter inside. It travels down a canal and at the end of it, touches a thin stretched membrane called the eardrum.
• The eardrum vibrates to the effect of sound. The ear passes on these vibrations to the inner ear and then there to the brain which processes it.

4.0Various Terms Used In Sound

1. Frequency-The number of oscillations per second is called the frequency of oscillation.

• Frequency is expressed in Hertz(Hz)
• Frequency=No. of Oscillation Time(Seconds)=
• Frequency determines the shrillness or the pitch of the sound. Higher the frequency, higher the pitch or it is said that the sound is shrilling.
• Lower the frequency ,lower the shrill or Pitch. Example-Babies voice has more frequency and thereby more shrilling than an adult voice.

2. Amplitude-It is a measure of the intensity of the sound, Amplitude determines the loudness of the sound.

• Loudness is proportional to the square of amplitude of the vibration body producing sound.

Loudness=KAmplitude2   K=Proportionality Constant

•  Loudness is measured in a unit called Decibel(dB)
• The more the amplitude of vibration, the louder the sound is.
• The lesser the amplitude of vibration, the sound is feeble.

5.0Audible and Inaudible Sound

• The human ear can recognize sound in the range of 20 Hz -20 KHz. These are called audible sounds. Example-Music, Instruments, Cracker
• Frequencies below 20 Hz and greater than 20 KHz cannot be heard by the human ear. These are called inaudible sounds. Example-It is believed that bat calls are in the inaudible ultrasonic range. Dogs can hear frequencies greater than 20 KHz so they are used by the police to respond to such whistles.
• Ultrasound scans produce sounds in this range.

6.0Difference between Noise and Music

• Music refers to pleasant sounds which we would like to hear. Example-Musical instruments like flute, guitar, violin, etc.
• Sound which are unpleasant to hear are termed as noise. Example-Honking of vehicles, Loud-crackers, Explosions

7.0Noise Pollution

• Presence of excessive or unwanted sounds in the environment around us is called Noise Pollution.

Sources of Noise Pollution

• Vehicles honking on the road
• Explosions
• Bursting Crackers
• Loud Speakers
• Noise pollution may leads to health related problems such as anxiety, stress, lack of sleep

Measure to limit Noise Pollution

• Avoid or minimize use of crackers.
• Industry and factory units which produce a lot of noise should be set up away from residential areas