Tag: propagation of sound waves

Questions Related to propagation of sound waves

On a hot, dry summer day a boy is standing between plane parallel vertical cliffs separated by $75m$. He is $30$m away from one of the cliffs. Consider speed of sound in air on that hot day to be $360m/s$. The boy claps loudly and hearts its successive echoes. The time in seconds at which he hears the first four echoes are respectively

physics propagation of sound waves longitudinal vs transverse wave sound and light comparison of speed of sound with speed of light

  1. $\displaystyle \frac{1}{6}, \frac{1}{4}, \frac{5}{12}, \frac{5}{12}$

  2. $\displaystyle \frac{1}{6}, \frac{1}{4}, \frac{7}{12}, \frac{2}{3}$

  3. $\displaystyle \frac{1}{4}, \frac{1}{3}, \frac{5}{12}, \frac{5}{12}$

  4. $\displaystyle \frac{1}{6}, \frac{1}{4}, \frac{1}{3}, \frac{5}{12}$

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Correct Option: A
Explanation:

First echo : When sound goes to the cliff 1 and returns to his ear

Distance traveled by sound, $S = 30+30 = 60$ m
Time taken, $t _1 = \dfrac{60}{360} = \dfrac{1}{6}$ s

Second echo : When sound goes to the cliff 2 and returns to his ear
Distance traveled by sound, $S = 45+45 = 90$ m
Time taken, $t _2 = \dfrac{90}{360} = \dfrac{1}{4}$ s

Third echo : When the reflected sound from cliff 1 goes to the cliff 2 and returns to his ear
Distance traveled by sound, $S = 30+75+45 = 150$ m
Time taken, $t _3 = \dfrac{150}{360} = \dfrac{5}{12}$ s

Fourth echo : When the reflected sound from cliff 2 goes to the cliff 1 and returns to his ear
Distance traveled by sound, $S = 45+75+30 = 150$ m
Time taken, $t _4 = \dfrac{150}{360} = \dfrac{5}{12}$ s

The ratio of the velocity of body and velocity of sound is known as

physics propagation of sound waves sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

  1. laplace number

  2. positive number

  3. stable number

  4. mach number

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Correct Option: D
Explanation:

Definition of Mach Number is $Ma=\frac{V _{body}}{V _{\mbox{sound in that medium}}}$.
Option "D" is correct.

The ratio of the speed of a body to the speed of sound is called:

physics propagation of sound waves sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

  1. Mach number

  2. Doppler Ratio

  3. Sonic index

  4. Refractive index

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Correct Option: A
Explanation:

Mach number of a body is the ratio of its speed to that of local sound.
The local speed of sound, and thereby the Mach number, depends on the condition of the surrounding medium, in particular the temperature and pressure. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow. 

What is the ratio of the speed of sound in neon and water vapor at the same temperature. It is nearest to :

physics propagation of sound waves sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

  1. 2.5

  2. 2

  3. 1.5

  4. 1

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Correct Option: D
Explanation:

According to the Avogadro's law , at same temperature and pressure the number of molecules of different gasses are equal. Therefore density will not change in either case of neon or water vapor. And the speed of sound waves is a function of density and temperature. So in this case doesn't matter.
Option "D" is correct. 

The ratio $(v)$ of velocities of sound in dry air and humid air is

physics propagation of sound waves sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

  1. $v<1$

  2. $v>1$

  3. $v=1$

  4. Zero

Show answer
Correct Option: A
Explanation:

Because water molecules have less mass than the average air molecules $(N _2 (28 \text{amu}),$ while water $H _2O (18 amu)).$ But It will happen if and only if the pressure is unchanged. Given that dry and humid air are both under the same pressure then we have smaller density for humid air.
And velocity of sound waves$,v,$ in a medium is given by,


$v=\sqrt{\dfrac{\gamma E}{d}}$, here $\gamma = \dfrac{C _P}{C _V}$, 

$E$ is elasticity of the medium, and $d$ is the density.

Since, density of air decreases with humidity increases. therefore velocity of sound increases in humid air.
Option "A" is correct.