Sound Archives

31. The speed of a body that has Mach number more than 1 is

The speed of a body that has Mach number more than 1 is

supersonic

subsonic

300 m/s

about 10 m/s

This question was previously asked in

UPSC NDA-1 – 2017

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Mach number is defined as the ratio of the speed of an object in a medium to the speed of sound in that medium.
– Mach number = 1: Speed of the object equals the speed of sound (sonic).
– Mach number < 1: Speed of the object is less than the speed of sound (subsonic). - Mach number > 1: Speed of the object is greater than the speed of sound (supersonic).
– Mach number > 5: Speed of the object is much greater than the speed of sound (hypersonic).
The question asks for the speed of a body that has a Mach number more than 1, which corresponds to supersonic speed.

– Mach number is a dimensionless quantity representing the ratio of speed to the speed of sound.
– Mach > 1 means supersonic speed.
– Mach < 1 means subsonic speed. - Mach = 1 means sonic speed.

The speed of sound varies depending on the medium (e.g., air, water) and its properties (e.g., temperature, pressure). In dry air at 15°C at sea level, the speed of sound is approximately 340 m/s (around 1225 km/h or 761 mph). So, a speed of 300 m/s (Option C) is close to Mach 1 but not necessarily supersonic, and the statement about 10 m/s (Option D) is clearly subsonic.

32. Ultrasonic waves of frequency 3 × 10⁵ Hz are passed through a medium w

Ultrasonic waves of frequency 3 × 10⁵ Hz are passed through a medium where speed of sound is 10 times that in air (Speed of sound in air is 300 m/s). The wavelength of this wave in that medium will be of the order of

1 cm

10 cm

100 cm

0·1 cm

This question was previously asked in

UPSC NDA-1 – 2015

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The correct option is A. The wavelength will be of the order of 1 cm.

The relationship between wave speed (v), frequency (f), and wavelength (λ) is given by v = fλ.
Given frequency f = 3 × 10⁵ Hz.
Speed of sound in air v_air = 300 m/s.
Speed of sound in the medium v_medium = 10 × v_air = 10 × 300 m/s = 3000 m/s.
We need to find the wavelength λ in the medium. Using the formula λ = v_medium / f:
λ = 3000 m/s / (3 × 10⁵ Hz) = 3000 / 300000 m = 1/100 m = 0.01 m.

To convert meters to centimeters, we multiply by 100:
0.01 m × 100 cm/m = 1 cm.
The wavelength of the ultrasonic wave in the given medium is 1 cm.

33. Infrasonic sounds have frequencies

Infrasonic sounds have frequencies

above 25 kHz

between 20 kHz and 25 kHz

below 20 Hz

between 20 Hz and 20 kHz

This question was previously asked in

UPSC Geoscientist – 2023

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The correct answer is C) below 20 Hz.

Sound waves are classified based on their frequency ranges relative to human hearing. The typical range of hearing for a healthy young human is from approximately 20 Hertz (Hz) to 20,000 Hertz (20 kHz). Sounds with frequencies below this audible range are called infrasonic sounds.

Sounds with frequencies above the audible range (above 20 kHz) are called ultrasonic sounds. Animals like elephants, whales, and dolphins can produce and perceive infrasonic sounds, often used for long-distance communication. Sources of infrasound include earthquakes, volcanic eruptions, weather phenomena like storms, and large machinery.

34. Reverberation of sound ensures

Reverberation of sound ensures

a single refraction

a single reflection

multiple reflections

multiple refractions

This question was previously asked in

UPSC Geoscientist – 2023

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Reverberation is the phenomenon where a sound persists in an enclosed space after the original sound source has stopped. This happens because sound waves reflect off the surfaces (walls, ceiling, floor, objects) in the room and these reflections reach the listener’s ear shortly after the initial sound and continue to arrive as they bounce multiple times. It is the collection of these multiple, closely spaced reflections that creates reverberation, not a single reflection (which would be a distinct echo if the delay is long enough) or refraction.

– Reverberation is the persistence of sound.
– It occurs in enclosed spaces.
– It is caused by multiple reflections of sound waves.
– Contrast with echo, which is a single, delayed reflection.

The degree of reverberation depends on the size and shape of the space and the acoustic properties of the surfaces. Highly reflective surfaces (like concrete or glass) lead to strong reverberation, while sound-absorbing materials reduce it. Reverberation time is the time it takes for the sound intensity to decrease by 60 dB after the source stops.

35. Which one of the following statements with regard to ultrasonic waves

Which one of the following statements with regard to ultrasonic waves is NOT correct?

Ultrasonic waves have frequency above 20 kHz

Ultrasonic waves are longitudinal waves

Ultrasonic waves can travel through water

Ultrasonic waves can travel through vacuum

This question was previously asked in

UPSC Geoscientist – 2021

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Ultrasonic waves are sound waves with frequencies above the upper audible limit of human hearing (typically above 20 kHz). Like all sound waves, they are mechanical waves, meaning they require a material medium (solid, liquid, or gas) to propagate. They cannot travel through a vacuum.

Mechanical waves transport energy through the vibration of particles in a medium. In a vacuum, there are essentially no particles to vibrate, so mechanical waves cannot propagate.

Ultrasonic waves are longitudinal waves, meaning the particle displacement is parallel to the direction of wave propagation, similar to how sound travels in air. They are used in various applications, including medical imaging (ultrasound), sonar, and industrial cleaning.

36. Which one of the following statements is NOT true about the sound

Which one of the following statements is NOT true about the sound waves?

Sound waves are longitudinal in nature

Sound waves can propagate through water

Speed of sound waves in steel is more than its speed in air

Sound waves can propagate through vacuum

This question was previously asked in

UPSC Geoscientist – 2020

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The statement that is NOT true about sound waves is that they can propagate through vacuum. Sound waves are mechanical waves, meaning they require a material medium (solid, liquid, or gas) through which to travel. They cause vibrations in the particles of the medium, and these vibrations are transmitted from one particle to the next. In a vacuum, there are essentially no particles to vibrate, so sound cannot travel through it.

Sound waves are mechanical waves and require a medium (solid, liquid, or gas) to propagate.

Sound waves in fluids (like air and water) are primarily longitudinal. Sound can propagate through various media, including water (as demonstrated by underwater sound) and solids like steel. The speed of sound varies depending on the medium; it is generally fastest in solids (due to stronger inter-particle forces), slower in liquids, and slowest in gases. The speed of sound in steel is significantly higher than its speed in air.

37. A sound wave having frequency of 300 Hz is travelling in an unknown me

A sound wave having frequency of 300 Hz is travelling in an unknown medium. Its wavelength is not known. It travels a distance equal to 150 times its wavelength in time t. The value of t is :

0.5 s

1 s

1.5 s

2 s

This question was previously asked in

UPSC CDS-2 – 2024

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The time taken for the sound wave to travel the given distance is 0.5 s.

– The speed of a wave (v) is given by the product of its frequency (f) and wavelength (λ): v = f * λ.
– The distance travelled (d) is given as 150 times the wavelength: d = 150λ.
– The relationship between speed, distance, and time (t) is v = d / t.
– Combining these equations, we get: f * λ = (150λ) / t.
– We can cancel out λ from both sides (assuming λ is non-zero): f = 150 / t.
– We are given the frequency f = 300 Hz.
– Rearranging the equation to find t: t = 150 / f.
– Substituting the value of f: t = 150 / 300 = 0.5 s.

This calculation assumes the speed of the sound wave is constant in the unknown medium. The wavelength of the sound wave in the medium is not required to find the time, as it cancels out in the calculation.

38. In which one among the following mediums would the speed of sound be m

In which one among the following mediums would the speed of sound be maximum ?

Distilled water

Sea water

Drinking water

Methanol

This question was previously asked in

UPSC CDS-2 – 2024

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The speed of sound depends on the properties of the medium through which it travels, specifically its density and compressibility (bulk modulus). Sound travels fastest in solids, then liquids, and slowest in gases. Among the given liquids, sea water has dissolved salts, which makes it slightly denser and less compressible than distilled or drinking water. Methanol is less dense than water. Generally, sound travels faster in denser and less compressible liquids. Therefore, the speed of sound is highest in sea water among the given options.

– Speed of sound increases with the stiffness (bulk modulus) of the medium and decreases with density, though the effect of stiffness is usually more significant in liquids and solids.
– Dissolved salts in sea water increase its density and bulk modulus compared to fresh water.

Typical speed of sound in air at 20°C is about 343 m/s. In fresh water at 20°C, it’s about 1482 m/s. In sea water at 20°C and typical salinity, it’s about 1522 m/s. In methanol at 20°C, it’s about 1100 m/s. This confirms that sound travels faster in sea water than the other liquid options provided.

39. In which of the following media is the speed of sound the maximum?

In which of the following media is the speed of sound the maximum?

Glass

Stainless steel

Water

Oxygen

This question was previously asked in

UPSC CDS-2 – 2023

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The speed of sound is generally highest in solids, lower in liquids, and lowest in gases. Within solids, the speed of sound depends on the material’s density and elasticity (stiffness). Among the given options, Stainless steel is a solid with high density and stiffness, allowing sound waves to travel fastest through it compared to glass (another solid), water (liquid), and oxygen (gas).

The speed of sound is determined by the medium’s properties, specifically its bulk modulus (a measure of stiffness) and density. The formula for the speed of sound is approximately $v = \sqrt{B/\rho}$ for liquids and solids, where B is the bulk modulus and $\rho$ is the density. Solids generally have much higher bulk moduli than liquids and gases, leading to higher sound speeds, even though they might also have higher densities. Among solids, stiffer materials tend to have higher sound speeds.

Approximate speeds of sound at room temperature:
– Oxygen (Gas): ~316 m/s
– Water (Liquid): ~1480 m/s
– Glass (Solid): ~4500 – 6000 m/s (varies with type)
– Stainless Steel (Solid): ~5790 m/s (in bulk)
Comparing these values, stainless steel has the highest speed of sound among the given options.

40. Sonic boom is produced when a source of sound travels at a speed :

Sonic boom is produced when a source of sound travels at a speed :

greater than the speed of sound.

greater than the speed of light.

lesser than the speed of sound.

equal to the speed of sound.

This question was previously asked in

UPSC CDS-2 – 2023

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The correct answer is A) greater than the speed of sound.

A sonic boom is a loud sound produced when an object, such as an aircraft, travels through the air faster than the speed of sound. When an object moves at supersonic speed (Mach number greater than 1), it creates shock waves as it compresses the air ahead of it. These shock waves propagate outwards and reach the listener’s ears as a ‘boom’.

The speed of sound varies depending on the medium and its temperature and pressure. At sea level in standard atmospheric conditions (15°C), the speed of sound in air is approximately 343 meters per second (or 1,236 kilometers per hour, or 767 miles per hour). Objects travelling at speeds less than the speed of sound are called subsonic, at the speed of sound are transonic or sonic (Mach 1), and greater than the speed of sound are supersonic or hypersonic (Mach > 5).