Page 146 - Understanding NCERT Science 09
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In these instruments, a tube followed
by a conical opening reflects sound
successively to guide most of the
sound waves from the source in the
forward direction towards the
audience.
2. Stethoscope is a medical instrument
used for listening to sounds produced
within the body, mainly in the heart or
lungs. In stethoscopes the sound of the
patient’s heartbeat reaches the doctor’s
ears by multiple reflection of sound, as
shown in Fig.11.11.
Fig. 11.13: Sound board used in a big hall.
Q uestion
1. Why are the ceilings of concert
halls curved?
Fig.11.11: Stethoscope 11.4 Range of Hearing
The audible range of sound for human beings
3. Generally the ceilings of concert halls,
conference halls and cinema halls are extends from about 20 Hz to 20000 Hz (one
Hz = one cycle/s). Children under the age of
curved so that sound after reflection
reaches all corners of the hall, as five and some animals, such as dogs can hear
up to 25 kHz (1 kHz = 1000 Hz). As people
shown in Fig 11.12. Sometimes a
curved soundboard may be placed grow older their ears become less sensitive to
behind the stage so that the sound, higher frequencies. Sounds of frequencies
below 20 Hz are called infrasonic sound or
after reflecting from the sound board,
spreads evenly across the width of the infrasound. If we could hear infrasound we
hall (Fig 11.13). would hear the vibrations of a pendulum just
as we hear the vibrations of the wings of a bee.
Rhinoceroses communicate using infrasound
of frequency as low as 5 Hz. Whales and
elephants produce sound in the infrasound
range. It is observed that some animals get
disturbed before earthquakes. Earthquakes
produce low-frequency infrasound before the
main shock waves begin which possibly alert
the animals. Frequencies higher than 20 kHz
are called ultrasonic sound or ultrasound.
Ultrasound is produced by animals such as
dolphins, bats and porpoises. Moths of certain
families have very sensitive hearing equipment.
Fig. 11.12: Curved ceiling of a conference hall. These moths can hear the high frequency
SOUND 135
Rationalised 2023-24
