Sound Quiz: Clerkship GS
A little practice. A step forward.
10 questions on sound for WBPSC Clerkship everyday science. 📖 Read the detailed topic
What kind of wave is a sound wave?
In air, sound travels as compressions (dense regions) and rarefactions (thin regions), with particles vibrating along the direction of travel, so it is a longitudinal wave. In a transverse wave the particles vibrate at right angles to the direction, as in light or water ripples. Electromagnetic waves need no medium, but sound does.
- Sound is a mechanical wave and needs a medium
- In air it is a longitudinal wave
- Dense part = compression, thin part = rarefaction
- Light is transverse and needs no medium; sound is longitudinal and needs one
- Sound = compressions and rarefactions = along the direction (longitudinal)
In which of the following can sound NOT travel?
Sound is a mechanical wave that moves by vibrating particles. A vacuum has no particles, so sound cannot travel through it. Air (gas), water (liquid) and steel (solid) all carry sound.
- Sound travels in solids, liquids and gases
- Sound cannot travel in a vacuum
- No sound can be heard in space
- Light travels through a vacuum; sound does not
- No particles, no sound
In which of the following media is the speed of sound the highest?
As a rule, sound travels fastest in solids, slower in liquids and slowest in gases. In iron it is about 5950 m/s, in water about 1498 m/s and in air about 346 m/s. Hydrogen is a light gas, so sound is faster in it than in air (about 1284 m/s), but still far slower than in iron.
- Order of speed: solid > liquid > gas
- About 346 m/s in air at 22°C
- Speed in air rises with temperature
- A denser medium is not always slower: solids are more elastic, so sound is faster in them
- Fast in solids, slow in gases
| Medium | Speed (m/s) |
|---|---|
| Air (22°C) | 346 |
| Distilled water (25°C) | 1498 |
| Iron (25°C) | 5950 |
Speed of sound
What is the audible range of human hearing?
A healthy human ear hears sounds of about 20 Hz to 20,000 Hz (20 kHz). Below 20 Hz is infrasound, above 20 kHz is ultrasound. The last option is the ultrasound range, not the audible range.
- Audible range: 20 Hz to 20 kHz
- Below 20 Hz: infrasound
- Above 20 kHz: ultrasound
- The ability to hear high frequencies falls with age
- Twenty to twenty thousand
| Type | Frequency | Example |
|---|---|---|
| Infrasound | below 20 Hz | Elephants, whales, earthquake waves |
| Audible sound | 20 Hz – 20,000 Hz | Human speech and music |
| Ultrasound | above 20,000 Hz | Bats, dolphins, medical scans |
Which animal uses ultrasound to find its way and catch prey in the dark?
A bat emits high-frequency ultrasonic waves and listens for the echoes that bounce back from obstacles and prey. Elephants use infrasound to communicate over long distances. Tigers and chimpanzees do not navigate by ultrasound.
- Bats and dolphins use ultrasound
- Elephants and whales use infrasound
- Ultrasound is also used in medical sonography
- Infrasound (elephants) and ultrasound (bats) are easily mixed up
- Bat = Ultra high, Elephant = Infra low
On what does the loudness of a sound depend?
Loudness depends on the amplitude of the wave: the larger the amplitude, the louder the sound. It is measured in decibels (dB). Frequency decides the pitch, while wavelength and speed do not directly set the loudness.
- Loudness depends on amplitude
- Loudness is measured in decibels (dB)
- Pitch depends on frequency
- A loud sound is not always a high-pitched one: a drum is loud but low-pitched
- Loudness = Amplitude, Pitch = Frequency
On what does the pitch of a sound depend?
Pitch depends on frequency. A higher frequency gives a shriller (higher) sound and a lower frequency gives a deeper one. That is why a typical female voice sounds shriller than a male voice. Changing the amplitude changes loudness, not pitch.
- Higher frequency = higher pitch
- Lower frequency = lower pitch
- The unit of frequency is hertz (Hz)
- More amplitude makes a sound louder, not shriller
- Pitch = Frequency (both start with a 'p/f' pair)
| Property | Depends on | Unit |
|---|---|---|
| Loudness | Amplitude | decibel (dB) |
| Pitch | Frequency | hertz (Hz) |
For an echo to be heard distinctly, the minimum distance between the source and the reflector is about (speed of sound in air about 340 m/s)?
The ear retains a sound for about 0.1 s, so the echo must return after at least 0.1 s to be heard separately. In that time sound covers $340 \times 0.1 = 34$ m there and back, so the one-way distance is half of it, about 17 m. At 8.5 m the echo returns in only 0.05 s and blends with the original sound.
- Minimum time gap: about 0.1 s
- Minimum distance: about 17 m
- An echo is a reflection of sound
- 34 m is the round-trip path, not the distance to the wall
- 0.1 s, 34 m there and back, 17 m one way
One-way distance $= \frac{340 \times 0.1}{2} = 17$ m.
A ship sends ultrasonic waves using SONAR and the echo from the sea floor returns after 4 seconds. If the speed of sound in water is 1500 m/s, what is the depth of the sea?
In 4 s the sound goes down and comes back, so the total path is $1500 \times 4 = 6000$ m. The depth is half of that: $\frac{6000}{2} = 3000$ m. The 6000 m option is the round-trip path, not the depth.
- Depth $= \frac{v \times t}{2}$
- SONAR = Sound Navigation And Ranging
- SONAR finds sea depth and submarines
- Using the full time gives 6000 m, which is the wrong answer
- Halve the time, then multiply by the speed
$$d = \frac{v \times t}{2} = \frac{1500 \times 4}{2} = 3000 \text{ m}$$
Why is the thunder heard after the lightning flash is seen?
Lightning and thunder happen together. Light travels at about $3 \times 10^{8}$ m/s while sound in air travels at about 340 m/s. Light reaches us almost instantly, sound takes time, so we see the flash first and hear the thunder later.
- Speed of light is about $3 \times 10^{8}$ m/s
- Speed of sound in air is about 340 m/s
- Both happen at the same moment; only arrival times differ
- The events are simultaneous; the delay is in the travel time
- Light first, sound later
📊 Topic Analysis
10 questions on sound for WBPSC Clerkship everyday science.
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