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 speed of sound in gas speed of a travelling wave oscillation and waves waves physics

As per Newton's formula velocity of sound , at NTP is 

  1. 340 m/s

  2. 332.3 m/s

  3. 279.9m/s

  4. 290 m/s
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Newton's original formula for the speed of sound was v = sqrt(P / rho), which at NTP yields approximately 280 m/s. However, the accepted value in many textbooks for this specific historical calculation is 332.3 m/s (often cited as the corrected Laplace value).

Multiple choice speed of sound in gas speed of a travelling wave oscillation and waves waves physics

The velocity of sound in air is $330$ m/s. The r.m.s velocity of air molecules $(\gamma=1.4) $ is approximately equal to

  1. 400 m/s

  2. 471.4 m/s

  3. 231 m/s

  4. 462 m/s
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$v _{air}=\sqrt{\dfrac{\gamma RT}{M}}=330m/s$

$v _{rms}=\sqrt{\dfrac{3RT}{M}}$
$=\sqrt{\dfrac{3}{\gamma}}\times 330m/s$
$\gamma=1.4$
$\implies v _{rms}=471.4m/s$

Multiple choice speed of sound in gas speed of a travelling wave oscillation and waves waves physics

Does the sound of an explosion travel faster than the sound produced by a humming bee?

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$No$


The speed of sound depends only on the physical conditions of the medium in which the sound is travelling and the speed and direction of the wind present if any.
The speed of the sound doesn't depend on its loudness.

Hence although the sound of explosion is much louder than the humming of a bee, both sounds travel with equal speed.

Multiple choice speed of sound in gas speed of a travelling wave oscillation and waves waves physics

The extension in a string obeying Hooke's law $v$ is $x$. The speed of sound in the stretched string is $v$. If the extension in the string is increased to $1.5\ x$, the speed of sound will be

  1. $1.22\ v$
  2. $0.61\ v$
  3. $1.50\ v$
  4. $0.75\ v$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Speed of sound in stretched string
$v = \dfrac {\overline {T}}{\mu} ..... (i)$
where $T$ is the tension in the string and $\mu$ is mass per unit length.
According to Hooke's law, $F\propto X$
$\therefore T\propto X$ .... (ii)
From Eqs. (i) and (ii)
$v\propto$
$\therefore v' = \overline {1.5V} = 1.22\ V$.

Multiple choice speed of sound in gas speed of a travelling wave oscillation and waves waves physics

According to Newton's formula, the speed of sound in air at STP is:
(Take the mass of $1$ mole of are is $29 \times 10^{-3} \,\,kg)$

  1. $250 \,\, m \,\,s^{-1}$
  2. $260 \,\, m \,\,s^{-1}$
  3. $270 \,\, m \,\,s^{-1}$
  4. $280 \,\, m \,\,s^{-1}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$1$ mole of any gas occupies $22.4$ liters at STP.
Therefore, the density of air at STP is
$\rho = \dfrac{\text{Mass of one mole of air}}{\text{Volume of one mole of air at STP}}$

$= \dfrac{29 \times 10^{-3} \,\,kg}{22.4 \times 10^{-3} \,\,m^3} = 1.29 \,\,kg \,\,m^{-3}$

At STP, $P = 1\,\,atm = 1.01 \times 10^5 \,\,N \,\,m^{-2}$

$V =\sqrt{\left( \dfrac { P }{ \rho  }\right)}=\sqrt { \dfrac {1.01 \times 10^5 \,\,N \,\,m^{-2}  }{ 1.29 \times kg \,\,m^{-3} }  } = 280 \,\,m \,\,s^{-1} $

Multiple choice speed of sound in gas speed of a travelling wave oscillation and waves waves physics

The speed of sound through a gaseous medium bears a constant ratio with the rms speed of its molecules. What is this constant ratio ?

  1. $\sqrt{\dfrac{\gamma}{3}}$
  2. $\gamma -1$
  3. $\sqrt{\dfrac{2\gamma}{3}}$
  4. $\gamma$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$V _{sound}=\sqrt{\dfrac{\gamma RT}{M}}$ and $V _{rms}=\sqrt{\dfrac{3RT}{M}}$
$\Rightarrow\dfrac{V _{sound}}{V _{rms}}=\sqrt{\dfrac{\gamma}{3}}$
Hence (A) is correct.

Multiple choice speed of sound in gas speed of a travelling wave oscillation and waves waves physics

If $C _{s}$ be the velocity of sound in air and $C$ be the rms velocity, then

  1. $C _{S} < C$
  2. $C _{s}=c$
  3. $C _{s}=C\left(\dfrac {\gamma}{3}\right)^{1/2}$
  4. $None\ of\ these$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Speed of sound in air, ${{C} _{s}}=\sqrt{\dfrac{\gamma P}{\rho }}\,\ldots \ldots \,(1)$

 $ Where, $

$ \gamma =specific\,heat\,ratio $

$ P=\,pressure $

$ \rho =\,density $

RMS velocity of air molecule, $C=\sqrt{\dfrac{3\overline{R}T}{{{M} _{o}}}}=\sqrt{\dfrac{3P}{\rho }}\,\ldots \ldots \,(2)$

$ where,\, $

$ \overline{R}=\text{universal}\,\text{gas}\,\text{constant} $

$ {{M} _{o}}=Molecular\,mass $

$ T=temperature $

From (1) and (2)

${{C} _{s}}=C{{\left( \dfrac{\gamma }{3} \right)}^{1/2}}$ 

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

The ratio of speed of ultrasonic wave and sound wave is:

  1. $=1$
  2. $>1$
  3. $<1$
  4. $>1$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Ultrasonic wave is  defined as “inaudible sound with high frequency for human” the frequency of which generally exceeds 20 kHz. These days, sound wave which is not intended to be heard is also called Ultrasonic wave .


Ultrasonic wave is also a type of sound wave so ratio of velocity is $=1$

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

The speed of sound is $330 ms^{-1}$ and that of light is $3\times 10^8 ms^{-1}$. The ratio of speed of sound to that of light is approximately $1 : 10^6$.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Speed Of Sound$(V)$  is,  $330m/s$  and  that of Light$(U)$  is,  $300*10^{6}m/s$

Now we have to find,  $V:U$  and i.e.,  $V:U=330:300*10^{6}\cong1:10^{6}$ , because in comparison to $U$(Speed of Light)  ,  Speed of Sound$(V)$  is comparetively very $small$.
So, we can assume that, Speed of Sound$(V)$  is $300m/s$  in comparison to that of Speed of Light$(U)$ that is  $3*10^{6}$
Hence,  $V:U=1:10^{6}$

Hence, it is True.
Option A is correct

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

The correct statement is:

  1. Sound and light both require medium for propagation.

  2. Sound can travel in vacuum, but light can not.

  3. Sound needs medium, but light does not need medium for its propagation.

  4. Sound and light both can travel in vacuum.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Sound waves need to travel through a medium such as a solid, liquid, or gas. The sound waves move through each of these mediums by vibrating the molecules in the matter. The molecules in solids are packed very tightly. Liquids are not packed as tightly as solids. And gasses are very loosely packed. The spacing of the molecules enables sound to travel much faster through a solid than a gas. Sound travels about four times faster and farther in water than it does in air. 
Sound waves are traveling vibrations of particles in media such as air, water or metal. So it stands to reason that they cannot travel through empty space, where there are no atoms or molecules to vibrate.
Light travels as a wave. But unlike sound waves or water waves, it does not need any matter or material to carry its energy along. This means that light can travel through a vacuum, a completely airless space. Nothing travels faster than light energy. 
Hence, the statement "Sound needs medium, but light does not need medium for its propagation." is correct.