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 range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

The grandparents and parents of two-year old girl are playing with her in a room. A sound source produces a $28 kHz$ sound. Who in the room is most likely to hear the sound?

  1. Grandparent

  2. Little girl

  3. Both

  4. None of them
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
The frequency range of hearing in infants is more than in adult. As the infant gets mature, this range decreases. So two year girl will be more likely to hear that sound.
So the answer is B.
Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

The sound intensity is 0.008 $W/m^2$ at a distance of 10 m from an isotropic point source of power of the source is approximately :

  1. 2.5 watt

  2. 0.8 watt

  3. 8 watt

  4. 10 watt
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Intensity I = P / (4 * pi * r^2). Given I = 0.008 and r = 10, P = I * 4 * pi * r^2 = 0.008 * 4 * 3.14 * 100 = 0.008 * 1256 = 10.048 watts, which is approximately 10 watts.

Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

A train running at 108 km/hr towards east whistles at a frequency of 800 Hz. The frequencies heard by a passenger sitting in the train and a person standing near the track whom the train has just passed (Speed of Sound = 330 m/s) 

  1. 800 Hz, 733 Hz

  2. 740 Hz, 800 Hz

  3. 800 Hz, 880 Hz

  4. 800 Hz, 750 Hz
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For a passenger sitting inside the train, there is no relative motion, so the frequency heard remains the source frequency of 800 Hz. For a person standing near the track whom the train has just passed, the train is moving away, so the observed frequency is f' = f * (v / (v + v_s)) = 800 * (330 / (330 + 30)) = 733 Hz.

Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

The driver of a car travelling at the speed of $30m/sec$ towards a hill sounds a horn of frequency $600\ Hz$. If the velocity of sound in air is $330m/s$, the frequency of reflected sound as heard by the driver is

  1. $500\ Hz$
  2. $550\ Hz$
  3. $555.5\ Hz$
  4. $720\ Hz$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The frequency heard by the hill is f' = f * (v / (v - vs)) = 600 * (330 / (330 - 30)) = 600 * (330 / 300) = 660 Hz. This reflected sound acts as a source moving towards the driver, so f'' = f' * ((v + vo) / v) = 660 * ((330 + 30) / 330) = 660 * (360 / 330) = 720 Hz.

Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

A sound wave of pressure amplitude $5$ pascal propagates through the air medium. The normal pressure of air is $1.0\times10^5 N/m^2$. The difference between maximum and minimum pressure in the medium is 

  1. $5\times10^5 N/m^2$
  2. $10\times10^5 N/m^2$
  3. $10 N/m^2$
  4. $None\ of\ these$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The pressure amplitude is the difference between the maximum pressure and the ambient pressure. The difference between maximum and minimum pressure is twice the pressure amplitude: 2 * 5 = 10 N/m^2.

Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

A siren emitting a sound of frequency $1000 Hz$ moves away from you towards a cliff at a speed of $10 m/s$ [speed of sound in air $330 m/s$] Then. 

  1. The frequency of the sound you hearing coming directly from the siren is $ \dfrac {33}{34} \times 1000 Hz $.
  2. The frequency of the sound you hearing coming directly from the siren is $ \dfrac {33}{32} \times 1000 Hz $.
  3. The frequency of the sound you hear reflect off the cliff $ \dfrac {33}{32} \times 1000 Hz $.
  4. The frequency of the sound you hear reflect off the cliff $ \dfrac {33}{34} \times 1000 Hz $.
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

A person moving in a car with a velocity of $36kmph $ towards a fort wall blows a horn. If he hears the echo after $3s$, the distance of the wall from him. when he blows then horn (velocity of sound $340ms^P{-1}$)

  1. $340m$
  2. $1050m$
  3. $700m$
  4. $525m$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The sound travels to the wall and back. Let d be the distance when the horn is blown. The wall is stationary. The sound travels distance d + (d - v_car * t) = 2d - v_car * t. 2d - v_car * t = v_sound * t. 2d = (v_sound + v_car) * t. v_car = 36 km/h = 10 m/s. 2d = (340 + 10) * 3 = 350 * 3 = 1050. d = 525 m.

Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

Bats detect the obstacles in their path by receiving the reflected

  1. infrasonic waves

  2. ultrasonic waves

  3. micro waves

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

Answer is B.

Most bats produce echolocation sounds by contracting their larynx (voice box). A few species, though, click their tongues. These sounds are generally emitted through the mouth. Echolocation calls are usually ultrasonic, ranging in frequency from 20 to 200 kilohertz (kHz), whereas human hearing normally tops out at around 20 kHz.
While using echolocation, the bats emit a series of short, high-frequency sounds from their mouths or nostrils that bounce off objects and surfaces and then return to the animals ears. Since high-frequency waves do not diffract, or bend, extensively, these ultrasonic vibrations provide bats with accurate maps of their surroundings. 

Multiple choice physics range of hearing sound: production of sound audible, infrasonic and ultrasonic sounds different sounds

Sound of frequency below $20 \ Hz$ is called

  1. audio sounds

  2. infrasonic

  3. ultrasonic

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

Sound waves are categorized on the basis of their frequency range.

Sound of frequency below $20 \ Hz$ is called as infrasonic. Sound of frequency above $20 \ kHz$ is called as ultrasonic whereas sound of frequency in between $20 \ Hz$ to $20 \ kHz$ is called as audio sound.

Multiple choice

Why doesn't a duck's quack echo?

  1. Pitch

  2. Frequency

  3. Volume

  4. No one actually knows
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The common claim that a duck's quack does not echo is a persistent urban legend. Scientific studies have shown that it does echo, but the sound is often difficult to distinguish due to the specific frequency and decay of the quack.

Multiple choice

The sound of a snap, is...

  1. Transverse wave symmetry

  2. Your bones rubbing together

  3. Your middle finger hitting the palm of your hand

  4. The sound of your skin scrapping against itself
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The sound of a finger snap is primarily caused by the middle finger hitting the palm of the hand at high speed, creating a sudden pressure wave.