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 physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

In order to reduce the loudness of a sound we have to:

  1. decrease its frequency of vibration of the sound

  2. increase its frequency of vibration of the sound

  3. decrease its amplitude of vibration of the sound

  4. increase its amplitude of vibration of the sound
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Loudness of the sound  $L = 10 \log _{10}\dfrac{I}{I _o}$
where $I \propto A^2$   ($A$ is the amplitude of vibration of sound) 
Thus the loudness of sound varies directly as the square of the amplitude of vibration of the sound source.
Hence, the in order to reduce the loudness of sound, we have to decrease the amplitude of vibration of the sound.
Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

A 1cm long vibrates with a fundamental frequency of 256Hz. If the length is reduced to 1/4cm keeping the tension unaltered, the new fundamental frequency will be

  1. 64

  2. 256

  3. 512

  4. 1024
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
We know that $n\alpha\cfrac{1}{l}$
Thus $\cfrac{n _2}{n _1}=\cfrac{l _1}{l _1}\\n _2=\cfrac{l _1}{l _2}(n _1)=\cfrac{1\times256}{(1/4)}=256\times4=1024Hz$
Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

Two identical sound $A$ and $B$ each a point in the same phase. The resultant sound is $C$. The loudness of $C$ is $n$ $dB$ higher than the loudness of $A$

  1. $2$
  2. $3$
  3. $4$
  4. $6$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

When two identical sound sources of the same phase are combined, the intensity doubles (I_total = 2 * I). The change in decibels is 10 * log10(I_total / I) = 10 * log10(2) = 10 * 0.3 = 3 dB. The question states the loudness is n dB higher, and 6 dB is often cited for coherent sources, but for incoherent sources, it is 3 dB. Given the options, 6 dB is likely expected if the sources are coherent.

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

If the intensity of sound increases by a factor of $10^5$ , the increases in the intensity level is

  1. $5db$
  2. $10db$
  3. $25db$
  4. $50db$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The change in sound intensity level in decibels is calculated using the formula delta L = 10 log10(I2/I1). Given that the intensity increases by a factor of 10^5, substitute this into the formula to get 10 log10(10^5) = 10 * 5 = 50 dB. Therefore, the increase in the intensity level is 50 db.

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

In a class of $100$ students each shouting at $100 dB$. Find noise level of class

  1. $10 dB$
  2. $100 dB$
  3. $12 dB$
  4. $120 dB$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The sound level of N identical independent sources each at level L is given by the formula L_total = L + 10 log10(N). Here, N = 100 students and each shouts at 100 dB. Calculating the addition gives 100 + 10 log10(100) = 100 + 10 * 2 = 120 dB.

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

The fundamental frequency of a closed organ pipe is same as the first overtone frequency of an open pipe. The length of open pipe is $20\ cm$. The length of closed pipe is

  1. $25\ cm$
  2. $12.5\ cm$
  3. $13.33\ cm$
  4. $200\ cm$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Given $:- $
Fundamental Frequency for the Open orange pipe $= ν/2l'$
where$, ν =$ Frequency and l' is the length of the open orange pipe$.$
Third Harmonic of the orange pipe $= 3ν/4l$
where$, ν =$ frequency 
and $l$ is the length of the Closed orange pipe $= 20 cm.$
Now$,$ According to the Question$,$
$3ν/4l = ν/2l'$
$⇒ 3/(4 × 20) = 1/2l'$
$⇒ 3/80 = 1/2l'$
$⇒ 2l' = 80/3$
$⇒ l' = 40/3$
$⇒ l' = 13.33 cm.$
Hence,
option $(C)$ is correct answer.
Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

If the amplitude of a wave is doubled, what will be the effect on its loudness ?

  1. increase four times

  2. increase two times

  3. decrease four times

  4. decrease two times
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

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


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

Thus, when amplitude of the wave is doubled, the loudness becomes four times.

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

Which of the following factors change when a sound gets louder?

  1. intensity

  2. Speed of the sound wves

  3. frequency

  4. decibel level

  5. wavelength

  6. amplitude
Reveal answer Fill a bubble to check yourself
A,D,F Correct answer
Explanation

When a sound gets louder , its amplitude increases , as intensity $I\propto A^{2}$ (amplitude) , therefore intensity of sound also increases , which in turn increases level of intensity i.e. decibel .

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

With the increase in frequency a sound becomes louder. True or false

  1. True

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

Increased frequency means decreased wavelength and higher pitch/ sharpness. So increase in frequency makes sound more sharper while higher amplitude or higher intensity makes louder sound. 

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

A vibrating tuning fork is first held in the hand and then its end is brought in contact with a table. Which of the following statement(s) is/are correct in respect of this situation?

  1. The sound is louder when the tuning fork is held in hand.

  2. The sound is louder when the tuning fork is in contact with table.

  3. The sound dies away sooner when tuning fork is brought in contact with the table.

  4. The sound remains for a longer duration when turning fork is held in hand.
Reveal answer Fill a bubble to check yourself
B,C,D Correct answer
Explanation

when the tuning fork is held in hand there is only air in contact to the fork and sound waves force the air particles to vibrate but when the fork is in contact with the table the sound waves will travel in table as well as air which is in contact with table top. The sound dies sooner as the waves travel much faster in solids and liquids than in gases.

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

The difference of sound level between two point is $30$ decibel. The ratio of pressure amplitude between points is : 

  1. $10$
  2. $20$
  3. $30$
  4. $32$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The sound level difference in dB is 20 * log10(P2/P1) for pressure amplitude. Given 30 dB = 20 * log10(ratio), then log10(ratio) = 1.5, so the ratio is 10^1.5, which is approximately 31.62, rounding to 32.

Multiple choice physics sound time period, frequency and amplitude of sound amplitude, time period and frequency of a vibration nature and characteristics of sound waves

The pitch of the sound produced by a stretched string gets higher as the tension .....................

  1. Increases

  2. decrease

  3. same

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

Pitch depends on string length and string tension.

  1. If the string's mass per unit length remains constant, for longer string, high tension will produce high pitch.
  2. If the string's length remains constant, for heavier string, the higher the tension will generate higher pitch.