Physics · Science General

Acoustics and Sound Waves

2,160 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 study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

The minimum distance between the source of sound and the reflecting surface necessary to cause echo is

  1. $1.7\ m$
  2. $17\ m$
  3. $7\ m$
  4. $70\ m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The human ear can hear two sounds distinctly only if there is a time interval of $\frac{1}{10}$seconds between them. Now, the speed of sound in air at $20^0$C is $340m/s$, so the distance traveled in $\frac{1}{10}$seconds will be: 
Distance = Speed X time 
$=340 \times \frac{1}{10} \= 34m$

Thus, it is possible to hear the echo at the minimum distance of $\frac{34}{2}= 17m$ between the source of sound and reflecting surface. 

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

The minimum distance to hear a clear echo is (V is the velocity of sound)

  1. 2V/5

  2. 5V/2

  3. V/20

  4. 5V
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Minimum frequency to the human ear is 20
$\therefore T= \dfrac {1} {F}$
$T=\dfrac{1} {20}$
$d= V \times t$
$d= V\times \dfrac {1} {20}$
$\therefore$ The minimum distance is$ \dfrac {V} {20}$, where V is speed of the sound.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A man, standing between two cliffs, claps his hands and starts hearing a series of echoes at intervals of one second. If the speed of sound in air is $340ms^{-1}$, the distance between the cliffs is?

  1. $680$m
  2. $1700$m
  3. $340$m
  4. $1620$m
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let the distance between the two cliffs be d. Since, the man is standing midway between the two cliffs, then the distance of man from either end is $d/2$.
The distance travelled by sound (in producing an echo)
$2\times \displaystyle\frac{d}{2}=v\times t\Rightarrow d=340\times 1=340$m.

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A signal source starts from rest and moves away from a stationary wall with an acceleration of $0.5 m/s^2$. After what time in secs will we hear an echo

  1. $1.2 $
  2. $8.2 $
  3. $18.2 $
  4. $4.2 $
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Distance required for an echo to be heard is 17 m. Thus, $17 =  0 + \dfrac{0.5 t^2}{2} \implies t= 2\sqrt{17}=8.2 s$. 

The correct option is (b)

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A source of sound is kept at a distance of 10 m from a wall. What distance should it be moved further, so that an echo is heard

  1. 10 m away from the wall

  2. 7 m away from the wall

  3. 7 m towards the wall

  4. 10 m towards the wall
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

To hear an echo, the distance between the source and the reflector should be atleast 17 m. Thus the source should be moved 7 m away from the wall

The correct option is (b)

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A source of sound moves with an uniform velocity away from a wall. An echo is heard at 4th second from its beginning position, what is the speed of the source

  1. 10 m/s

  2. 7 m/s

  3. 4.25 m/s

  4. 1 m/s
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

distance moved by the source = 17 m for the echo to be heard

Thus, $17 = v(4) \implies v = 17/4 = 4.25 $ m/s

The correct option is (c)

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

The minimum distance between the sound and the reflecting surface, in order to hear an echo, must be

  1. $0.65\space m$
  2. $1.65\space m$
  3. $16.5\space m$
  4. $165\space m$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let the distance between the source of sound and the obstacle be $x$ metres .

Thus the distance travelled by sound to reach back to the source   $d = 2  x$
As the minimum time required to hear an echo is equal to $ 0.1  sec$
As velocity of sound in air    $ v = 330   m/s$
Thus  $d = v  t$
$2  x = 330  \times 0.1             \implies x = 16.5   m$

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

Velocity of sound in the atmosphere of a planet is 600m$s^{-1}$. The minimum distance between the source of sound and the obstacle to hear echo should be

  1. 60 m

  2. 25 m

  3. 30 m

  4. 17 m
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let the distance between the source of sound and the obstacle be $x$ metres .

Thus the distance travelled by sound to reach back to the source   $d = 2  x$
As the minimum time required to hear an echo is equal to $ 0.1  sec$
Thus  $d = v  t$
$2  x = 600  \times 0.1             \implies x = 30   m$

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A source of sound emitting a tone of frequency 200 Hz moves towards an observer with a velocity v equal to the velocity of sound if the observer also moves away from the source with the same velocity v, the apparent frequency heard by the observer is

  1. $50 Hz$
  2. $100 Hz$
  3. $150 Hz$
  4. $200 Hz$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

According to the Doppler effect, if both the source and observer move at the same velocity in the same direction, the relative velocity between them is zero. Therefore, the frequency heard remains the same as the source frequency.

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A table is revolving on its axis at 5 revolutions per second. A sound source of frequency 1000 Hz is fixed on the table at 70 cm from the axis. The minimum frequency heard by a listener standing at a distance from the table will be (speed of sound = 352 m/s)

  1. 1000 Hz

  2. 1066 Hz

  3. 941 Hz

  4. 352 Hz
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$\begin{array}{l} For\, \, source\, \, vS=r\omega =0.70\times 2\pi \times 5=22\, m/s \ the\, \, source\, \, is\, \, receding\, \, the\, \, man.\, it\, \, is\, \, given\, \, by\, \, { \eta _{ \min   } }=n\frac { v }{ { v+{ v _{ s } } } } Hz \ Minimum\, \, frequency\, \, is\, \, heard\, \, when=1000\times \frac { { 352 } }{ { 352+22 } } =941 \end{array}$

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A person clapped his hands hear cliff and heard a echo after 5s. The minimum distance of the cliff from the person if the speed of sound is take as 346 m/s 

  1. 17.11 m

  2. 117.2 m

  3. 173 m

  4. 865 m
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The total distance traveled by the sound is speed times time: 346 m/s * 5 s = 1730 m. Since the sound travels to the cliff and back, the distance to the cliff is half of the total distance: 1730 / 2 = 865 m.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A rail engine approaches a hill at a speed of $54\ kmph$ whistling a sound of frequency $160\ Hz$. The frequency of echo heard by the engine driver is (velocity of sound in air $335\ ms^{-1})$.

  1. $167.5\ Hz$
  2. $152.5\ Hz$
  3. $175\ Hz$
  4. $145\ Hz$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The engine is moving toward the hill at 15 m/s (54 km/h). The frequency of the echo is f' = f * (v + vs) / (v - vs), where v is speed of sound and vs is speed of engine. f' = 160 * (335 + 15) / (335 - 15) = 160 * 350 / 320 = 175 Hz.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A driver blows the horn when he is at a distance of $1020m$ from a stiff. He hears the echo after $11/2sec$. At what rate is he approaching the cliff. (velocity of sound in air $=340m/s$)

  1. $72m/s$
  2. $31m/s$
  3. $31mph$
  4. $60kmph$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Distance = 1020 m. Total sound distance = 2 × 1020 = 2040 m. If echo is heard after 1.5 s (assuming 11/2 means 1.5), time available for driver approach = (2040/340) - 1.5 = 6 - 1.5 = 4.5 s. Speed = 1020 / 4.5 = 31 m/s (approximately).