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

Name the characteristic of the sound affected due to a change in its amplitude.

  1. Loudness.

  2. Wavelength.

  3. Frequency.

  4. Waveform.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Amplitude is the size of the vibration, and this determines how loud the sound is.  Larger vibrations make a louder sound.
The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration. A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distance from the source increases.
Hence, loudness of the sound is affected due to the change in its amplitude.

Multiple choice intensity and loudness waves physics

As we move away from the source along with the amplitude which of the following quantity decreases as well ?

  1. waveform

  2. loudness

  3. wavelength

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

The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration. A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distance from the source increases. Loudness is measured in units called decibels. 
Loudness depends on the square of the amplitude of the wave. That is, $loudness{ \propto \left( amplitude \right)  }^{ 2 }$.
Hence, As it moves away from the source its amplitude as well as its loudness decreases.

Multiple choice intensity and loudness waves physics

By reducing the amplitude of a sound wave, its :

  1. Pitch increases

  2. Loudness decreases

  3. Loudness increases

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

The loudness of sound is directly proportional to amplitude of sound wave. 


loudness $\propto$ amplitude

Hence on reducing amplitude the loudness also decreases. 

Multiple choice intensity and loudness waves physics

Unit for measuring the loudness of sound is _________________.

  1. Decibel

  2. Pascal

  3. Newton

  4. Newton-metre
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Unit for measuring the loudness of sound is decibel.
The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration. A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distance from the source increases. Loudness is measured in units called decibels. 

Multiple choice intensity and loudness waves physics

Sound levels are measured in

  1. Decimeters

  2. Decibels

  3. Deciliters

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

Answer is B.

The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration. A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distance from the source increases.
Sound levels is measured in units called decibels. 

Multiple choice intensity and loudness waves physics

If the amplitude of the sound is made 3 times its initial value, then its loudness will become _______________ times its initial value.

  1. 9

  2. 4

  3. 16

  4. 25
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

If the amplitude of the sound is made 3 times its initial value, then its loudness will become nine times its initial value.
Loudness depends on the square of the amplitude of the wave. 
That is, $Loudness\propto { (Amplitude) }^{ 2 }$.
Thus, when amplitude of the wave is tripled, the loudness increases nine times.

Multiple choice intensity and loudness waves physics

The loudness of sound is measured in _______ .

  1. decibels

  2. metres

  3. kilograms

  4. seconds
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The loudness of sound is measured in units called decibels (dB). A decibel unit expresses the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 100 dB, which is near the level most people find uncomfortably loud.

Multiple choice intensity and loudness waves physics

Identify the scale which can be used to measure the intensity of sound?

  1. decibel

  2. acoustic

  3. ultrasound

  4. infrasound

  5. Hertz
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Intensity (or loudness) of sound         $L =10 \log _{10} \dfrac{I}{I _o}$

It is measured in decibel  (dB)

Multiple choice intensity and loudness waves physics

A normal human being can hear sound having an intensity level of maximum ...................

  1. 50 dB

  2. 80 dB

  3. 100 dB

  4. 150 dB
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Decibel levels help us to understand the loudness of a sound with respect to a reference level. A decibel level of zero dB represents the lowest level of sound and a level of 80 dB represents the level of pain for humans.

Multiple choice intensity and loudness waves physics

How many order of magnitude more powerful is 90 dB sound than 40 dB sound?

  1. 5

  2. 50

  3. 500
  4. $ {10}^ {5} $
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$90 = 10 log (\dfrac{I _{1}}{I})$

$40 = 10 log (\dfrac{I _{2}}{I})$

$90 - 40 = 10 log (\dfrac{I _{1}}{I _{2}})$

$\dfrac{I _{1}}{I _{2}} = 10^{5}$

Multiple choice intensity and loudness waves physics

If the intensity of sound is increased by a factor of 30 , by how many decibels is the sound level increased ?

  1. 12 dB

  2. 14.77 dB

  3. 10 dB

  4. 13 dB
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
The intensity of sound in increased by factor $30$
Let $2$ be the intensity of previous sound
Intensity of recent sound $I=30I$
$\beta =10 \log\dfrac{I}{I _o}$
$\beta _1=10 \log\dfrac{I}{I _o}$ where intensity of sound is $ I$
$\beta _1=10 \log\dfrac{30I}{I _o}$ when intensity of sound is $I$
Increased in sound level $\Rightarrow \beta _2-\beta _1$
$=10 \log\dfrac{30I}{I _o}-10\log \dfrac{I}{I _o}$
$=10 \log 30$
$10 \log \dfrac{30 I}{I}$
$=14.77d\beta$.
Multiple choice intensity and loudness waves physics

When a person wears a hearing aid, the sound intensity level increases by 30 dB. The sound intensity increases by  

  1. e$^{3}$
  2. 10$^{3}$

  3. 30
  4. 10$^{2}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let the intensity of the sound without hearing Aid is $I _0$, and the 


intensity after wearing hearing aid is $I$, then from the formula, 

$L _{db}=10 \log _{10}{\dfrac{I}{I _0}}$

$30=10 \log _{10}{\dfrac{I}{I _0}}$

$\dfrac{I}{I _0}=10^3$

$I=10^3 \times I _0$

Option "B" is correct.

Multiple choice intensity and loudness waves physics

Spherical sound waves are emitted uniformly in all directions from a point source. The variation in sound level SL as a function of distance 'r' from the source can be written as 

  1. SL = -b log r$^{a}$
  2. SL = a - b (log r)$^{2}$

  3. SL = a - b log r
  4. SL = a - b/r$^{2}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

we know $SL = 10 log \dfrac{I}{I _o}$


$I = \dfrac{P}{4\pi r^2}$  where P is the power of the source

$SL = 10  log\dfrac{P}{4\pi r^2I _o} = 10  log \dfrac{P}{4\pi I _o} - 10  log  r^2 = 10  log \dfrac{P}{4\pi I _o} - 20  log  r = a - b  log  r$

Multiple choice intensity and loudness waves physics

The intensity level of two sounds are 100 dB and 50 dB. What is the ratio of their intensities?

  1. 10$^{1}$
  2. 10$^{3}$
  3. 10$^{5}$
  4. 10$^{10}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Loudness of two sounds is given as  $L _2 = 100 \ dB$ and $L _1 = 50 \ dB$

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  $100 - 50   = 10\log _{10}\dfrac{I _2}{I _1}$

Or  $\log _{10}\dfrac{I _2}{I _1} = 5$
Or  $\dfrac{I _2}{I _1} = 10^5 $

Multiple choice intensity and loudness waves physics

A source of sound emits 200 W power which is uniformly distributed over a sphere of radius 10 m. What is the loudness of sound on the surface of sphere? 

  1. 70dB

  2. 107dB

  3. 80dB

  4. 112dB
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Intensity is given by:
$I=\dfrac{W}{4\pi{r}^2}$
$I=\dfrac{200}{4\pi\times 100} =0.159W/m^2$

In terms of decibels, $I=10\log _{ 10 }{ I } +120 dB$
$I=112dB$