Intensity of Sound Waves

Q. In expressing sound intensity, we take ${10}^{-12}$ $W/m^2$ as the reference level. For ordinary conversation, the intensity level is about ${10}^{-6}$ $W/m^2$. Expressed in decibel, this is

1. ${10}^6$
2. 6
3. 60
4. $loge\left({10}^6\right)$

Q. A circular plate of area $0.8{\text{cm}}^2$ is kept at a distance of $2\text{m}$ from a source of sound of power $\pi \text{W}$. Find the amount of energy received by the plate in $4\text{s}$.

1. $50\mu \text{J}$
2. $30\mu \text{J}$
3. $20\mu \text{J}$
4. $15\mu \text{J}$

Q. Calculate the ratio of intensity of wavetrain A to wavetrain B.

1. $4$
2. $1$
3. $3$
4. $0.5$

Q. The pressure amplitude in a medium through which sound is travelling is $1.5\times {10}^{-3}{\text{N/m}}^2$ and intensity is ${10}^{-6}{\text{W/m}}^2$. If the pressure amplitude is increased to $4.5\times {10}^{-3}{\text{N/m}}^2$ by increasing the volume of sound then the intensity will be

1. $6\times {10}^{-6}{\text{W/m}}^2$
2. $7\times {10}^{-6}{\text{W/m}}^2$
3. $8\times {10}^{-6}{\text{W/m}}^2$
4. $9\times {10}^{-6}{\text{W/m}}^2$

Q. An increase in intensity level of 1 dB implies an increase in intensity of (given anti $lo{g}_{10}0.1=1.2589)$

1. $1\%$
2. $3.01\%$
3. $26\%$
4. $0.1\%$

Q.

A fan at a rock concert is 30 m from the stage, and at this point the sound intensity level is 110 dB.How much energy is transferred to her eardrums each second? (Assume eardrum to be circular with a radius of 10mm)

1. $\pi \times {10}^{-5}\frac{J}{S}$

2. $2\pi \times {10}^{-5}\frac{J}{S}$

3. $0.1\frac{J}{S}$

4. None of these

Q. A point source emits sound of the same power in all directions in a non-absorbing medium. Two points $P$ and $Q$ are at a distance of $9\text{m}$ and $25\text{m}$ respectively from the source. The ratio of the amplitude of the waves at $P$ and $Q$ is

1. $\frac{3}{5}$
2. $\frac{5}{3}$
3. $\frac{9}{25}$
4. $\frac{25}{9}$

Q. A bomb blast gives a sound of intensity $0.8{\text{W/m}}^2$ and frequency $1.5\text{kHz}$. If the density of air is $1.25{\text{kg/m}}^3$ and speed of sound in air is $330\text{m/s}$, then the amplitude of the sound wave is approximately

1. $4.4\mu \text{m}$
2. $5.5\mu \text{m}$
3. $6.6\mu \text{m}$
4. $7.7\mu \text{m}$

Q.

A source of sound S and a detector D are placed at some distance from one another. A big cardboard is placed near the detector and perpendicular to the line SD as shown in figure. It is gradually moved away and it is found that the intensity changes from a maximum to a minimum as the board is moved through a distance of 20 cm. Find the frequency of the sound emitted. Velocity of sound in air is $336\frac{m}{s}$?

1. 1680 Hz

2. 420 Hz

3. 840 Hz

4. None of these

Q. Find the displacement amplitude of particles of air of density $1.5{\text{kg/m}}^3$ if the intensity and frequency of sound are $8\times {10}^{-6}{\text{W/m}}^2$ and $5000\text{Hz}$ respectively.
[speed of sound in air $=330\text{m/s}$]

1. $5.7\text{nm}$
2. $4.7\text{nm}$
3. $3.7\text{nm}$
4. $2.7\text{nm}$

Q. A point source of power $50\pi \text{W}$ is producing sound waves of frequency $1876\text{Hz}$. What is the pressure amplitude at a distance $\sqrt{330}\text{m}$ from the point source?
[Speed of sound is $330\text{m/s}$ and density of air is $1{\text{kg/m}}^3$]

1. $5{\text{N/m}}^2$
2. $6{\text{N/m}}^2$
3. $7{\text{N/m}}^2$
4. $8{\text{N/m}}^2$

Q. Curves of two waves propagating in the same medium is shown. Here, $y$ is displacement in $\text{m}$ and $t$ is time in $\text{s}$. Find the ratio of their average intensities?

1. $25:16$
2. $9:16$
3. $25:9$
4. $4:9$

Q. For a point source of sound, the graph of intensity $\left(I\right)$ versus square of distance from the source $\left(r^2\right)$ is best represented by

1. 
2. 
3. 
4. 

Q. A straight line source of sound of length $10\text{m}$ emits pulse that travels radially outward from the source. A detector is located at a perpendicular distance of $7\text{m}$ from the source. The total power emitted by the source in the form of sound wave is $2.2\times {10}^4\text{W}$. If the surface area of the acoustic detector is $1.2{\text{cm}}^2$, then the power (in $\text{mW}$) intercepted by the detector is
(Assume waves reaches surface of detector perpendicularly and take $\pi =\frac{22}{7}$)

1. $5$
2. $6$
3. $7$
4. $8$

Q. If the frequency of sound produced by a siren increases from $400\text{Hz}$ to $1200\text{Hz}$ keeping the amplitude constant. The ratio of the intensity of sound produced at frequency $1200\text{Hz}$ and $400\text{Hz}$ will be

1. $1:1$
2. $1:3$
3. $3:1$
4. $9:1$

Q. Match the following columns. Here, $I$ represent intensity, $A$ amplitude and $r$ distance from the source.

Column $\text{I}$	Column $\text{II}$
$a.$ $I$ due to point source	$p.$ $\propto r^{-1/2}$
$b.$ $A$ due to point source	$q.$ $\propto r^{-1}$
$c.$ $I$ due to line source	$r.$ $\propto r^{-2}$
$d.$ $A$ due to line source	$s.$ $\propto r^{-4}$

1. $a-r,b-q,c-q,d-p$
2. $a-p,b-q,c-r,d-s$
3. $a-q,b-q,c-s,d-r$
4. $a-r,b-p,c-q,d-r$

Q. A bomb blast gives a sound of intensity $0.8{\text{W/m}}^2$ and frequency $1.5\text{kHz}$. If the density of air is $1.25{\text{kg/m}}^3$ and speed of sound in air is $330\text{m/s}$, then the amplitude of the sound wave is approximately

1. $4.4\mu \text{m}$
2. $5.5\mu \text{m}$
3. $6.6\mu \text{m}$
4. $7.7\mu \text{m}$

Q. An increase in intensity level of 1 dB implies an increase in intensity of (given anti $lo{g}_{10}0.1=1.2589)$

1. $1\%$
2. $3.01\%$
3. $26\%$
4. $0.1\%$