Page 10 - 10-phy-11 Sound

11. Sound eLearn.Punjab

Example 11.1:
Calculate the intensity levels of the (a) faintest audible sound (b) rustling of leaves.

Solution:

(a) Intensity level of faintest audible sound can be calculated by substituting

Therefore, I = I0 = 10-12 Wm-2 in Eq. (11.5).
Intensity level of faintest audible sound = 10 log 10-12 /10-12 dB

= 0 dB

(b) As the intensity of the rustle of leaves is = 10-11 Wm-2,

therefore,

Intensity level due to rustling of leaves = 10 log10-11 /10-12 dB

= 10 log10 dB

= 10 dB

11.3 REFLECTION (ECHO) OF SOUND

When we clap or shout near a reflecting surface such as Interesting Information
a tall building or a mountain, we hear the same sound
again a little later. What causes this? This sound which A blue whale’s 180 dB rumble is the loudest
we hear is called an echo and is a result of reflection of animal sound ever recorded. Whale sounds
sound from the surface. also appear to be part of a highly evolved
communication system. Some whales are
When sound is incident on the surface of thought to communicate over hundreds
a medium it bounces back into the first and may be thousands of kilometers. This
medium. This phenomenon is called echo or is possible, in part, because sound waves
travel five times faster in water than in air. In
reflection of sound. addition, the temperature characteristics of
ocean water — decrease in temperature with
depth — create a unique sound phenomenon.

The sensation of sound persists in our brain for about 0.1s. To hear a clear echo, the time interval
between our sound and the reflected sound must be at least 0.1s. If we consider speed of sound
to be 340 ms-1 at a normal temperature in air, we will hear the echo after 0.1s. The total distance
covered by the sound from the point of generation to the reflecting surface and back should be
at least 340 ms-1×0.1s = 34.0 m. Thus, for hearing distinct echoes, the minimum distance of the
obstacle from the source of sound must be half of this distance, that is, 17 m. Echoes may be heard
more than once due to successive or multiple reflections.

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