8th Class Science Sound Sound

     Sound

Sound plays an important role in our lives. Sound waves help us to communicate with each other. Sound is produced due to the vibration of an object. If we strike a steel plate, sound comes out until the vibration of the steel plate stops. Therefore, sound is produced when a body strikes on another body.  

            Voice box in Human Body

The upper part of wind pipe has voice box which produces sound in the human body. Two vocal cords across, the voice box or larynx, leave a narrow slit between them for the passage of air. Sound is produced when lungs force air through the slit and the vocal cords vibrate.

Look at the following picture of voice box (Larynx) in the human body:

Sound cannot travel in vacuum, it requires medium to travel. The medium can be anything. For example, air, water, solid, liquid, etc. The intensity of sound is called loudness. The speed of sound in a medium depends on the property of the medium. Dense medium allows the sound to travel fast. Whereas, light medium allows the sound to travel at low speed. Sound travels faster in a water body in comparison to air, because the density of air is less than the density of water. The speed of sound depends on the temperature of the medium also. It increases on increasing the temperature. The speed of sound in air at 20°C is 343 metre per second whereas the speed of sound in air at 0°Cis 331 meter per second.

Sound waves have two different properties, with which we can obtain the intensity or amplitude and frequency of the sound waves. The number of oscillations per second is called the frequency. The unit for the measurement of frequency is hertz or Hz. Sound waves, less than the frequency of 20 Hz cannot be detected by the human ear and is called infrasonic sound. Sound waves, greater than the frequency of 20,000 Hz., also cannot be detected by the human ear. This is called ultra frequency sound. Therefore, the human ear can detect the audio frequency ranging from 20 Hz. to 20, 000 Hz. Loudness of sound is measured in decibel as given below.

         Reflection of Sound

When sound waves strike .on an object, it is reflected back by the surface and also diffracts in different directions. The property of sound waves to diffracts in different directions enable us to hear the sound.

         Echo

Echo is produced when sound waves reach the listener a few moments after the production of sound. The ear of human cannot listen an echo if the delay between the produced sound and reflected sound is less than \[\frac{1}{10}\] of one second. The echo effect also depends on the distance between the sound producing body and reflecting object. The distance between the sound producing body to the reflecting object must be more than 17.50 metres. Echo effect is used to listen the heart function of human body by a stethoscope. It is also used for analyzing the depth of the sea. Some materials do not allow the sound waves to reflect, therefore, they are called sound absorbers. For example porous materials like sandstone.

Look at the following picture to understand, how the echo effect is produced:

           Human Ears

Ear of human body is divided into three parts, outer ear, middle ear and inner ear.

Middle ear has liquid for conducting the sound waves into the inner part of the ear. When sound waves enter into our ears, athin membrane which is called eardrum catch the sound waves through ear canal. Sound waves vibrate the eardrum and eardrum sends the vibration to its hammer (one of the three tiny bones in the ear). The vibration of hammer causes the anvil (a tiny bone touching the hammer) to vibrate. Vibration of anvil is passed on to the stirrup and then to the cochlea through liquid filled sack. Cochlea sends the signal to the brain.

Look at the following picture of human ear:

      The speed of sound depends on the physical features of the medium. Which one of the following mediums allows the sound to travel with maximum speed?

(a) Milk

(b) Water

(c) Air

(d) Diamond

(e) None of these

Answer (d)

Explanation

Sound travels fast in dense medium. Therefore, option (D) is correct and rest of the options is incorrect.  

      Sound travels fast in dense medium. Therefore, option (D) is correct and rest of the options is incorrect.

Which one of the following parts of human ear sends the signal from the liquid filled sack to the brain?

(a) Cochlea

(b) Lens

(c) Ear canal       

(d) All of these

(e) None of these

Answer: (a)

Explanation 

Cochlea sends the signal to the brain. Therefore option (A) is correct and rest of the options is incorrect.

Light always travels in straight line.

Bending of a beam of white light, when it enters from a medium into another medium, is called refraction of light.

According to the laws of reflection, angle of incident is equal to the angle of reflection.

Human ear is divided into three parts, upper, middle and inner parts..