Decibels

Q. A sound absorber attenuates the sound level by $20dB$. The intensity decreases by a factor of

1. $1000$
2. $10000$
3. $10$
4. $100$

Q. At a distance of $10\text{m}$ from a point source, loudness is found to be $20\text{dB}$. The distance from the source where loudness is $0\text{dB}$ is

1. $20\text{m}$
2. $50\text{m}$
3. $100\text{m}$
4. $\infty$

Q. A two-fold increase in intensity of a wave implies an increase of (Given: ${\text{log}}_{10}2=0.3010$)

1. $2\text{dB}$
2. $10\text{dB}$
3. $3.01\text{dB}$
4. $0.5\text{dB}$

Q. The intensity level $1\text{m}$ away from a point source is $40\text{dB}$. Threshold intensity of hearing is ${10}^{-12}{\text{W/m}}^2$. If there is no loss of power in air, then intensity level at $10\text{m}$ from the source is

1. $10\text{dB}$
2. $20\text{dB}$
3. $30\text{dB}$
4. $40\text{dB}$

Q. Calculate the change in intensity level when the intensity of sound increases by ${10}^6$ times its original intensity.

1. $50\text{dB}$
2. $40\text{dB}$
3. $30\text{dB}$
4. $60\text{dB}$

Q. A $12.5eV$ electron beam is used to bombard gaseous hydrogen at room temperature. What series of wavelengths will be emitted?

Q. Determine the ratio of perimeter of $2^{nd}$ and $3^{rd}$ Bohr orbit in ${\text{He}}^{+}$ atom $(Z=2)$.

1. $\frac{9}{4}$
2. $\frac{16}{9}$
3. $\frac{9}{16}$
4. $\frac{4}{9}$

Q. A sound absorber decreases a sound's loudness from 60 dB to 20 dB. Intensity of sound decreases by a factor of

1. 10000
2. 10
3. 1000
4. 100

Q. How many times does the electron of hydrogen atom go round the first Bohr orbit in one $\text{second}$?
$[m=9.1\times {10}^{-31}\text{kg},e=1.6\times {10}^{-19}\text{C}{\epsilon }_0=8.85\times {10}^{-12}{\text{F m}}^{-1},h=6.6\times {10}^{-34}\text{J s}]$

1. $6.6\times {10}^{15}$
2. $6.6\times {10}^{10}$
3. $6.6\times {10}^{13}$
4. $6.6\times {10}^5$

Q. How many times does the electron of hydrogen atom go round the first Bohr orbit in one $\text{second}$?
$[m=9.1\times {10}^{-31}\text{kg},e=1.6\times {10}^{-19}\text{C}{\epsilon }_0=8.85\times {10}^{-12}{\text{F m}}^{-1},h=6.6\times {10}^{-34}\text{J s}]$

1. $6.6\times {10}^5$
2. $6.6\times {10}^{10}$
3. $6.6\times {10}^{15}$
4. $6.6\times {10}^{13}$

Q. If the intensity is increased by a factor $x$, sound level is increased by $20\text{dB}$. Then, the value of $x$ is

1. $10$
2. $100$
3. $1000$
4. $0.10$

Q. A two-fold increase in intensity of a wave implies an increase of (Given: ${\text{log}}_{10}2=0.3010$)

1. $2\text{dB}$
2. $10\text{dB}$
3. $3.01\text{dB}$
4. $0.5\text{dB}$

Q. The ratio of intensities between two coherent sound sources is $9:1$. The difference of loudness in decibels $\left(\text{dB}\right)$ between maximum and minimum intensities when they interfere in space is

1. $10\log 4$
2. $10\log 9$
3. $10\log 3$
4. $\log 3$

Q. Two stereo speakers are separated by a distance of 2.4 m. A person stands at a distance of 3.2 m as shown directly in front of one of the speakers. Find the frequencies in audible range for which the listener will hear a minimum sound intensity :
Speed of the sound in air is $320m{s}^{-1}$

1. $160(2n+1)$
2. $320(2n+1)$
3. $200(2n+1)$
4. $100(2n+1)$

Q. Intensity level of sound whose intensity is ${10}^{-8}w{m}^{-2}$ is ______ dB.

1. $8$
2. $4$
3. $40$
4. $80$

Q. How many times more intense is $90\text{dB}$ sound than $40\text{dB}$ sound?

1. $5$
2. $50$
3. $500$
4. ${10}^5$

Q. At a distance of $10\text{m}$ from a point source, loudness is found to be $20\text{dB}$. The distance from the source where loudness is $0\text{dB}$ is

1. $20\text{m}$
2. $50\text{m}$
3. $100\text{m}$
4. $\infty$

Q. Figure shows an air filled, acoustic interfero meter, used to demonstrate the interference of sound waves. Sound source $S$ is an oscillating diaphragm; $D$ is a sound detector, such as the ear or a microphone. Path $SBD$ can be varied in length, but path $SAD$ is fixed. At $D,$ the sound wave coming along path $SBD$ interferes with that coming along path $SAD.$ In one demonstration, the sound intensity at $D$ has a minimum value of 100 units at one position of the movable arm and continuously climbs to a maximum value of 900 units when that arm is shifted by $1.65cm.$ Find the frequency of the sound emitted by the source.

Q. The energy per unit area associated with a progressive sound wave will be doubled if:

1. The amplitude of the wave is doubled
2. The amplitude of the wave is increased by 50%
3. The amplitude of the wave is increased by 41%
4. The frequency of the wave is increased by 50%

Q. A sound source is moving towards stationary listener with $\frac{1}{10}$th of the speed of sound. The ratio of apparent to real frequency is:

1. ${\left(\frac{9}{10}\right)}^2$
2. $\frac{10}{9}$
3. $\frac{11}{10}$
4. ${\left(\frac{11}{10}\right)}^2$

Q. When a sound is going away from a stationary observer with the velocity equal to that of sound in air, then the frequency heard by observer is n times the original frequency. The value of n is

1. 0.5
2. 0.25
3. 1
4. no sound is heard

Q. If the velocity of sound in air is $336m/s$. The maximum length of a closed pipe that would produce a just audible sound will be :

1. $3.2cm$
2. $4.2m$
3. $4.2cm$
4. $3.2m$

Q. The pitch of a sound source appears to be 20% decreased to an observer. The velocity of source with respect to the observer will be

1. 0.825 m$s^{-1}$
2. 8.25 m$s^{-1}$
3. 82.5 m$s^{-1}$
4. 330 m$s^{-1}$

Q. In an experiment to measure the absorption coefficient by stationary method, $i_1$ is the current measured due to maximum amplitude of sound and $i_2$ is the current due to minimum amplitude of sound, then:

1. $a=\frac{i_1{i}_2}{(i_1+i_2{)}^2}$
2. $a=\frac{4i_1{i}_2}{(i_1+i_2{)}^2}$
3. $a=\frac{(i_1-i_2{)}^2}{(i_1+i_2{)}^2}$
4. $a=\frac{(i_1-i_2)}{(i_1+i_2)}$

Q. Find the amplitude of motion of the air in the path of a $40\text{dB},1000\text{Hz}$ sound wave.
[Assume that the density of air is $1.3{\text{kg/m}}^3$ and speed of sound in $340\text{m/s}$].

1. $1\text{cm}$
2. $1\mu \text{m}$
3. $1\text{nm}$
4. $1\text{mm}$

Q. Show that the sound level, $\beta$, can be written in terms of the pressure amplitude, $\Delta P_M$, as
$\beta \left(dB\right)=20\log \frac{\Delta P_M}{\Delta P_{M_0}}$
where $\Delta P_{M_0}$ is the pressure amplitude at some reference level.

Q.

$16$g of a radioisotope reduces to one gram in one hour. Its half life period in minutes is-

1. $15$

2. $30$

3. $45$

4. $60$

Q. Beats are heard due to interference of two sound waves. The intensity of sound changes from maximum to minimum in $100ms$. Therefore, the difference in frequencies of the two waves is

1. $10Hz$
2. $5Hz$
3. $2.5Hz$
4. $20Hz$