Sound class 8

Vibration in bodies produces sound. For example, tuning fork, stretched strings, plastic rulers etc. produce sound when allowed to vibrate.

Stem and Prongs. Stem is the handle of the tuning fork. Prongs are the two arms in its U-shaped section.

When a vibrating tuning fork touches the surface of the water, waves are produced on the water surface.

Voice box or larynx produces sound in our bodies. Two vocal cords are stretched across it leaving a narrow slit between them. To produce sound, air is forced to pass through the slit to vibrate vocal cords.

I will touch my throat and feel the hard bump that appears to move when I swallow the food. This bump is voice box.

Sound vibrations pass through a material medium and moves from its one end to another. This way, sound moves from one place to another through a medium. Medium is must for movement of the sound. Sound cannot move through vacuum.

Distance travelled by sound in unit time is its speed. Mathematically, \[\text {Speed of Sound}= \frac {\text{Distance travelled by sound}}{\text{Time taken}}\]

Sound moves in various mediums with varying speed. Its speed is high in solid medium but low in gaseous medium. The speed of sound in liquid medium is moderate.

Rapid to-and-fro motion of a body about its mean position is vibration. For example, vibration produced when a steel glass falls on the ground.

Slow to-and-fro motion of a body about its mean position is oscillation. For example, swing etc.

When a body in oscillation, moves from one extreme position to another and come back to it again, it completes one oscillation. We also call it cycle.

The maximum displacement of an oscillating body from its mean position is amplitude. It is the displacement between its mean position and extreme positions. We denote it by symbol A.

The time taken by an oscillating body to complete one oscillation is its Time Period. We denote it by symbol T. Its SI unit is second.

Number of oscillations a body completes in one second is Frequency of an oscillation. We represent it by symbol \(\nu\) or \(f\). Its SI unit is Hertz (Hz).
1 Hz = 1 cycle per second

Frequency, \(\nu\) = \(\frac {1}{\text {Time Period}, T}\)

We identify a sound by its Loudness, Pitch and Quality or Tone.

Loudness of a sound depends on its amplitude. Higher is the amplitude, more is the loudness. SI unit of the sound is decibel (dB).
Loudness above 80 decibels is painful to our hearings. Higher value of loudness can damage it too.

The shrillness of a sound is its pitch. It depends on the frequency of the sound. Higher the frequency, higher is the pitch. The voices of children and women have high pitch while that of men have low pitch.

Voices of children and women have high pitch while that of men have low pitch. That’s why, children and women have shrill voices.

Quality of sound refers to how pleasant or unpleasant the sound is. The pitch, loudness of the overtones decides the quality. Another name of quality of sound is Tone.

Our outer ears collect sound vibrations and direct it to the eardrum. It causes the eardrum to vibrate. The vibrations are passed on to the oval window for amplification. It transfers the vibrations to the liquid in Cochlea to which auditory nerves are connected. It sends the messages to the brain.

Ears are delicate. Its eardrum can be damaged if no proper care is taken. Few suggestions for proper care are:
1. Pointed object should not be inserted in our ears. It can pierce the eardrum and damage it.
2. One should never shout loudly into someone’s ear.
3. One should never hit anyone hard on ear or near ear.

No. We cannot hear all types of sound. We can hear only those sounds which have frequencies ranging between 20 Hz and 20,000 Hz. This frequency range is also called as audible range.
However, an infant of one year old can hear sounds up to 35,000 Hz.

Sonic sound – The sound in audible range i.e. having frequencies between 20 Hz and 20,000 Hz.
Ultrasonic sound – The sound having frequencies higher than 20,000 Hz.
Subsonic sound or infrasonic sound – The sound having frequencies lower than 20 Hz.

Many animals like dog, leopard, monkey, and deer etc. can hear ultrasound (sound having frequencies more than 20,000 Hz).
Some birds and animals like bat and dolphin produce and use ultrasound to locate their preys or find obstacle in their paths.

Music – Sound that are pleasant to our ears and minds are music or musical sound. The loudness of such sound lies between 10 dB and 30 dB. The sound produced by flute, guitar etc. is music.
Noise – Sound that our ears and minds feel as irritating and unpleasant is noise. The sound level of such sound is more than 80 dB. Noise includes sound produced by factory, airplane etc.

No. Human ear is comfortable with particular types of sound only. On the basis of comfort, we can divide sound into two broad parts, namely music and noise.

Sound pollution refers to unwanted and disturbing sound that negatively affects health, well-being of human beings, animals and the environment. We also call it noise pollution.

High blood pressure, anxiety, hearing problems, sleeping disorders, headache, and migraine are some diseases that Sound pollution causes.

Yes, sound pollution can be reduced or its impact can be limited. Some of the measures include:
Plantation (growing trees),
using machines that make less sounds,
developing industries in areas away from human habitation,
keeping low sound while listening music,
using sound-proofing materials in the areas prone to sound pollution etc.