Key Learning Points- Vibrations, Larynx (Voice box), Medium of sound propagation, Human ear, Properties of sound- Amplitude, Time period and Frequency, Audible range of sound, Noise and music , Noise pollution
Introduction- In our surrounding we hear sound of different varieties. Some of them are sweet and pleasant while some others are loud and irritating. The chirping of birds, ringing of bell, sound of flute etc. are all sounds. Some sounds are louder and some others are shrill. The voice box (larynx) in humans produces sound. The sound is produced because of vibration of any object. Sound has different properties and uses. Sound propagates maximum in solids, then in liquids and least in gases.
Sound is produced by vibration- Vibrations are the to and fro movement of any membrane or object. It is a periodic motion. When rubber band is plucked, striking of a steel disc by spoon, striking a pan, Jaltarang etc. sound is produced.
The musical instruments have their vibrating parts which produce sound on striking.
The musical instruments are classified in three types on the basis of their type of sound producing part-
1. Stretched/Vibrating String- Veena, Sitar, Guitar, Violin, Ektara etc.
2. Stretched/Percussion membrane- Tabla, Drums, Pakhawaj etc.
3. Vibrating air column- Flute, harmonium, Mouth organ, Ghatam, Jaltarang, Pepa etc.
Sound Produced by Humans- When we speak whisper, buzz or scream we can feel the vibrations by keeping our hand on throat. These vibrations are produced by our voice box (larynx) when we speak. The voice box is found in neck as protruded hard bump. It is the upper end of wind pipe. Voice box is a thin slit of trachea (windpipe) through which air passes. On both side of the voice box, vocal cords are found. They vibrate on passing of air. The muscles attached to vocal cords also help in its vibration. The thin and tight vocal cords produce high pitch shrill sound. The loose and thick vocal cords produce low pitch hoarse voice.
Medium of Propagation of sound- The sound is a mechanical wave that requires any medium to travel from one place to other. Through any medium it transfers it’s energy. Sound cannot travel through vaccum. The density of solids is more than liquids and gases; therefore, sound propagates easily in solid medium. Similarly, it propagates comparatively faster in liquids than gases.
Speed of Sound in Iron- Approximately 5000 m/s
Speed of Sound in water- Approximately 1400 m/s
Speed of Sound in Gases/ Air- 332 m/s
Speed of Sound in Solids > Speed of Sound in Liquids > Speed of
Sound in Gases
Human ear- We hear the sound through our
ears. The shape of ear pinna is like a funnel. When sound enters in ear, it
goes into the ear (auditory) canal. At the end of auditory canal a tightly
stretched thin membrane called eardrum (tympanic membrane) is found. The
eardrum vibrates due to sound vibrations. Then vibrations go to inner ear. The
3 smallest bones (malleus, incus and stapes) help in increasing the vibrations.
These vibrations are changed into electric impulses/signals that go to brain
through auditory nerves.
Cleaning of
ear and safety of eardrum is essential. The injured eardrum is not repairable.
It causes leakage of pus.
Properties of sound- There are
three main characteristics of a sound wave. These are Amplitude, Time period
and Frequency.
The to and
fro motion is called as vibration or oscillatory or periodic motion or simple
harmonic motion. The time taken to complete one oscillation is called as time
period. Sound wave is also a vibration. The two sounds are different from each
other due to their qualities like amplitude and frequencies.
A. Amplitude-Amplitude of a sound wave is the
maximum displacement of medium’s particles. It is represented by symbol ‘A’. Since, it a
displacement of particles therefore it’s S.I. unit is meter ‘m’. The loudness
of sound is determined by amplitude of sound. Higher is the amplitude, louder
is the sound. When the amplitude is smaller , sound will be feeble.
Loudness of
the sound is proportional to the square of the amplitude of the vibration
producing the sound. Loudness is directly proportional to the square of
amplitude - Loudness ∝A2.
It means when amplitude becomes twice, the
loudness of sound increases by four times. The unit of loudness of sound is
decibel (dB). Sounds above 80 dB are painful to hear by humans.
B.
Frequency
– Number of oscillations per second is called frequency. The S.I. unit of sound
is Hertz (Hz) / Cycle per second. Frequency of sound determines the shrillness
or pitch of the sound. The shrill sounds have more frequencies. The hoarse
sounds have less frequency. The sound of drum has low frequency and high
amplitude but the sound of whistle has very high frequencies and comparatively
low amplitude. The roar of lions has low frequencies and chirping of birds has
high frequencies. Galton’s Whistle
AUDIBLE AND INAUDIBLE SOUNDS- For humans, the range of audible frequencies
is between 20 Hz to 20,000 Hz. The sound below 20 Hz is called as infrasonic
sound and sound above 20, 000 Hz is called ultrasonic sound. Both, infrasonic
and ultrasonic sound cannot be listened by human ear. Dogs can listen sound
above 20 K Hz (or 20 ,000 Hz). So, police uses Galton’s Whistle to keep dogs away or give
command. Ultrasound is used in medical diagnosis. Infrasonic sound is produced
by Whale, Elephant, Rhinoceros, Hippopotamus etc. Ultrasonic sound is produced
by Bat, Grasshopper, Frog, Whale, Dolphins etc.
Music-
Pleasant sound is called music. It is melodious and pleases us. It has certain vibrations that have
particular and repeated sound. These sounds have fixed amplitude generally and
frequencies.
Noise-
Unpleasant sound is called noise. It disturbs us. It has different amplitude
and different frequencies.
NOISE POLLUTION- The presence of
excessive undesirable sound in the environment is called as noise pollution.
The sources of noise pollution are- speakers, aeroplanes, jet planes, machines,
crackers etc. The noise pollution may become deaf, causing anxiety,
sleeplessness, hypertension (high blood pressure) and health disorders are
caused.
The silencing devices in machines,
vehicles, home appliances, aircrafts etc must be installed to reduce noise
pollution. The sound absorbing wood panels and cotton curtains, clothes etc.
are good to reduce noise pollution. This is the reason why empty room causes
echo on speaking while room with clothes, curtain and wooden furniture does not
produce echo.
The reduce hearing impairment can be
improved by hearing aids.
Q.2 Which of the following voices is likely to have minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Ans- (c) A man
Q.3 I which of the following statements, tick T against those which are true, and F against those which are false:
(a)
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