RANGE OF HEARING
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RANGE OF HEARING
Sound of Class 9
RANGE OF HEARING
Human are not good to hear the sound.The human ear is able to hear sound in a frequency range of about 20Hz to 20kHz only. We are not able to hear sounds of frequencies less than 20Hz of more than 20kHz, these limits vary from persons to person and with age. Children can her sounds of somewhat higher frequencies, say upto 30 kHz. With age, our ability to hear high frequency sound diminishes. For the elder, the upper limit often falls to 10-12 kHz. We take 20Hz-20 kHz as the audible range for a average person.
Even in the audible range the human ear is not equally sensitive for all frequency. it is most sensitive to frequencies around 2000-3000 Hz.
Sound of frequencies less than 20 Hz is known as infrasonic sound or infrasound. Sound of frequency greater than 20 kHz is known as ultrasonic or ultrasound.
Different animals have different ranges of audible frequencies. A dog can hear sound of frequencies upto about 50 kHz and a bat upto about 100 kHz. Dolphins can hear sounds of even higher frequencies. Animals such as elephants and whales can hear sounds of frequencies less than 20 Hz. Some fishes can hear sounds of frequencies as low as 1-25 Hz.
SPEED OF SOUND IN DIFFERENT MEDIA:
When we strike an object with a hammer the sound of hammering is heard a short while after the actual impact. Similarly, the sound of cracker is heard only after it has exploded.
The flash of lightning is seen first and thunder is heard later on. These observations reveal that
- Sound travels with a finite speed.
- Sound travels with a speed which is much less than the speed of light.
Thus, speed of sound depends upon:
- The elasticity and density of the medium through which it propagates.
- Temperature of the medium.
So, we can conclude that:
Speed of sound in solids > Speed of sound in liquids > Speed of sound in gases.
However, in certain solids, the speed (v) of sound is much less than that even in gases as v (for vulcanized rubber) = 54 m/s and v (for hydrogen) = 1284 m/s. The speed of sound in lead (a solid) = 1332 m/s and in sea water (a liquid) = 1531 m/s. The speed of sound in methyl alcohol (a liquid) = 1103 m/s and in hydrogen (a gas), its value = 1284 m/s.
The speed of sound increases with increase in temperature of the medium. In air, it increases roughly by 0.61 m/s with rise of 1°C in temperature. The speed of sound in air at 0°C is 331 m/s and at 22°C, it is 344 m/s.
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