Physics · Science General

Acoustics and Sound Waves

2,006 Questions

Acoustics and sound waves deal with mechanical vibrations traveling through media like air and water. Key concepts include wave reflection, beats, echoes, the Mach number, and infrasound. These physics fundamentals are regularly tested in general science sections of multiple competitive exams.

Sound wave propagationEchoes and reflectionWave interferenceMach numberInfrasound frequency

Acoustics and Sound Waves Questions

Multiple choice intensity and loudness waves physics

Which of the following statements are incorrect?

  1. Wave pulses in string are transverse waves

  2. Sound waves in a air are transverse waves of compression and rarefaction
  3. The speed of sound in air at $20^{o}C$ is twice that at $5^{o}C$

  4. A 60 dB sound has twice the intensity of a 30 dB sound
Reveal answer Fill a bubble to check yourself
B,C,D Correct answer
Explanation

Case B : Sound wave in air is the longitudinal wave. so B is not true

Case C because of higher temperature means molecules having higher energy and moving faster than in cold temperature

case D :
 $L = 10  log \dfrac{I}{I _o}$

$\dfrac{60}{30} = \dfrac{10  log\dfrac{I _1}{I _o}}{10  log\dfrac{I _2}{I _o}}$

$2log\dfrac{I _2}{I _o} = log \dfrac{I _1}{I _o}$

so $I _2 = 2I _1$ is not true.

Multiple choice intensity and loudness waves physics

If the loudness changes from 30 dB to 60 dB. What is the ratio of the intensities in two cases?

  1. 10,000

  2. 1000

  3. 100

  4. 10
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Let the intensities be  $I _1$ and  $I _2$ for loudness $L _1 = 30   dB$ and $L _2 = 60  dB$ respectively.
Loudness of sound       $L  = 10  log _{10} \dfrac{I}{I _o}$   where  $I$ is the intensity of the sound wave
$\implies   L _2  - L _1  = 10  log _{10} \dfrac{I _2}{I _1}$
Thus                  $60  -  30  = 10  log _{10} \dfrac{I _2}{I _1}$
$\implies  \dfrac{I _2}{I _1} = 10^3$

Multiple choice intensity and loudness waves physics

The intensity level of sound having intensity $I$ is defined in term of $I _0$. The threshold intensity of hearing is

  1. Intensity level = $\displaystyle\frac{I}{I _0}$ decibels
  2. Intensity level = $I\times I _0$ decibels
  3. Intensity level = $\displaystyle\frac{I}{I _0}\times 100$ decibels
  4. Intensity level = $10\log _{10}\left(\displaystyle\frac{I}{I _0}\right)$ decibels
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The intensity level of sound having intensity I is defined in term of $I _0$. The threshold intensity of hearing is $I _{dB} = 10 \log _{10}({\dfrac{I}{I _0}})$.

Multiple choice intensity and loudness waves physics

An increase in the intensity level of one decibel implies an increase in intensity of :

  1. $1\mbox{%}$
  2. $3.01\mbox{%}$
  3. $26\mbox{%}$
  4. $0.1\mbox{%}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The Decibel scale is given by

L = $10log(\frac{I}{I{ _{o}}})$ ; $I{ _{o}}$ is the intensity at threshold for hearing.

Let the reading be X;

X = $10log(\frac{I{ _{1}}}{I{ _{0}}})$;

X + 1 =  $10log(\frac{I{ _{2}}}{I{ _{0}}})$;

Subtracting the equations we get

$\Rightarrow $1 = 10($log\dfrac{I{ _{2}}}{I{ _{0}}} - log\dfrac{I{ _{1}}}{I{ _{0}}}$)

$\Rightarrow $1 = 10($log\dfrac{I{ _{2}}}{I{ _{1}}}$)

$\Rightarrow $$log\dfrac{I{ _{2}}}{I{ _{1}}}$ = $ \dfrac{1}{10}$

$\Rightarrow $$\dfrac{I{ _{2}}}{I{ _{1}}}$ = $10^{0.1}$ ; $10^{0.1}$ = 1.26;

$\Rightarrow $$\dfrac{I{ _{2}}}{I{ _{1}}}$ = 1.26;

$\Rightarrow $$I{ _{2}}$=1.26$I{ _{1}}$; And hence a 26% increase

Multiple choice intensity and loudness waves physics

The relation between the objective measurement of intensity of sound, $I$ and the subjective sensory response called loudness $L$ is given by :

  1. $\displaystyle I = Klog L$
  2. $\displaystyle L = K log I$
  3. $\displaystyle L = I$

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

loudness($L$) in terms of  intensity ($I$) is 

$L=K\log(I)$
Where $L$ is the loudness, $I$ is the intensity and $K$ is a constant of proportionality.

so the answer is B.

Multiple choice intensity and loudness waves physics

Most of the human ears cannot hear sound of intensity less than :

  1. $\displaystyle 10^{-12} Wm^{-2}$
  2. $\displaystyle 10^{-6} Wm^{-2}$
  3. $\displaystyle 10^{-3} Wm^{-2}$
  4. $\displaystyle 1 Wm^{-2}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A human ear can hear a sound of minimum intensity $10^{-12} W/m^2$. This intensity sound corresponds to loudness $0 \ dB$. So, we say that a human ear cannot hear a sound of loudness $0 \ dB$.
A human ear can hear a sound of maximum intensity $1 \ W/m^2.$

Multiple choice intensity and loudness waves physics

The power of a loud speaker is increased from 20 W to 400 W. What is the intensity increase as compared to the original value?

  1. 13 dB

  2. 7 dB

  3. 4 dB

  4. 2 dB
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
As intensity of wave is directly proportional to power, i.e $I  \propto   P$
Loudness of sound, $L =  10  log _{10} \dfrac{I}{I _o}$
$\implies    L _2 - L _1 = 10   log _{10} \dfrac{I _2}{I _1}$    
Thus, $  L _2 - L _1 = 10   log _{10} \dfrac{P _2}{P _1}$  
$  L _2 - L _1 = 10   log _{10} \dfrac{400}{20}$  

$  L _2 - L _1 = 10   log _{10} 20        $             $(log _{10} 20 = 1.3 )$ 

$\implies  L _2 - L _1 = 13    dB$
Multiple choice intensity and loudness waves physics

At what sound level headache begins?

  1. 120-125 dB

  2. 100-105 dB

  3. 80-85 dB

  4. 60-65 dB
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

headache starts at 80 dB sound for a normal person.

Multiple choice intensity and loudness waves physics

The power , or loudness, of sound is measured on a scale of increased by a factor of 100$(=10^2$), the sound is called twice as loud and when it increases 10,000 $(=10^4)$ times it is called four times as loud. The exponent of ten is called a bel and one decible is one tenth of a bel. Zero decible is chosen as the intensity of the slowest sound which is just audible or is on the threshold of hearing whereas the intensity of loudest sound is about 170 decibel. A sound of 60 decibel is ......... times more intense than a sound of 40 decibel :

  1. 20

  2. 100
  3. $\displaystyle 10^{20}$

  4. none of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Loudness of two sounds are given as  $L _2 = 60 \ dB$ and $L _1 = 40 \ dB$

Loudness of sound  $L = 10 \log _{10}\dfrac{I}{I _o}$
$\implies$  $L _2 - L _1 = 10\log _{10}\dfrac{I _2}{I _1}$

Or  $60 - 40  = 10\log _{10}\dfrac{I _2}{I _1}$

Or  $\log _{10}\dfrac{I _2}{I _1} = 2$
Or  $\dfrac{I _2}{I _1} = 10^2 = 100$
Thus $60dB$ sound is $100$ times more intense than $40dB$ sound.

Multiple choice intensity and loudness waves physics

The power , or loudness, of sound is measured on a scale of increased by a factor of 100$(=10^2$), the sound is called twice as loud and when it increases 10,000 $(=10^4)$ times it is called four times as loud. The exponent of ten is called a bel and one decible is one tenth of a bel. Zero decible is chosen as the intensity of the slowest sound which is just audible or is on the threshold of hearing whereas the intensity of loudest sound is about 170 decibel. Sound intensities above a level of nearly descibels produce a feeling of pain :

  1. 60

  2. 120

  3. 140

  4. 170
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A human ear can hear a sound of maximum loudness of $120 \ dB$. Sound intensities above nearly $120 \ dB$ produce pain in the human ear.

Multiple choice intensity and loudness waves physics

The intensity of ordinary conversation is rated as nearly 

  1. 10 decibel

  2. 50 decibel

  3. 40 decibel

  4. 60 decibel
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Human ear can hear sound from 0dB to 140dB. Intensity of ordinary conversation is rated as nearly 60dB.

Multiple choice intensity and loudness waves physics

Two waves of the same pitch have amplitude in the ratio 1 : 3. What will be the ratio of their loudness:

  1. 1:3

  2. 3:1

  3. 1:9

  4. 9:1
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Loudness depends on the square of the amplitude of the wave. 


That is, $loudness\propto { \left( amplitude \right)  }^{ 2 }$

Thus, when the ratio of amplitudes of the wave is 1:3, the loudness becomes 1:9.

Multiple choice intensity and loudness waves physics

In expressing sound intensity, we take $ 10^{-12} Wm^{-2} $ as the reference level. For ordinary conversion, the intensity level is about $ 10^{-6} Wm^{-2} $. Expressed in decibel, this is :

  1. $ 10^6$
  2. $ 6$
  3. $ 60 $
  4. $ log _e(10^6) $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Intensity of reference level  $I _o = 10^{-12} Wm^{-2}$

Measured intensity  $I = 10^{-6} W m^{-2}$
Using  $L = 10\log _{10}\dfrac{I}{I _o}$  decibels
$\therefore  $  $L = 10\log _{10}\dfrac{10^{-6}}{10^{-12}}$   decibels
Or  $L = 10 \ log _{10} 10^{6} = 10\times 6 = 60$  decibels

Multiple choice intensity and loudness waves physics

Decibel (dB) is a unit of loudness of sound. It is defined in a manner such that when amplitude of sound is multiplied by a factor of $\sqrt{10}$, the decibel level increase by 10 units. Loud music of 70 dB is being played at a function. To reduce the loudness to a level of 30 dB, the amplitude of the instrument playing music to be reduced by a factor of

  1. $0$
  2. $10\sqrt{10}$
  3. $100$
  4. $100\sqrt{100}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

We know that Loudness of Sound is given by equation,
$\beta  = 10\log \left ( \frac{1}{I _{0}} \right ) dB$
where $I _{0} = 10^{-12} Wm^{-2} $ is the reference unit.
Intensity $ I \propto a^{2} $
where $a$ = amplitude.
Solving we get, the amplitude of the instrument playing music to be reduced by a factor of $100$.

Multiple choice intensity and loudness waves physics

The threshold of hearing for the human ear $ 10^{-12} Wm^{-2} $. This is taken as the standard level. The intensity of sound is $ 1Wm^{-2} $. It has intensity (in $db$)

  1. $10^{12} db $
  2. $12\ db$
  3. $240\ db$
  4. $120\ db$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Intensity in $dB$:
$I _{dB} = 10 \log _{10}(\dfrac{I}{I _0})=10 \log _{10}(\dfrac{1}{10^{-12}})=10\times 12 \log _{10}10=120$ $dB$