10. Very Short Question Answer: Sound Waves: Characteristics and Applications

Q1: How is sound produced?

Ans: Sound is produced by vibrating objects. Vibration refers to the periodic to and fro motion (oscillations) of an object. The object that produces sound is called the source of sound.

Q2: What is a medium? Can sound travel through vacuum?

Ans: The material through which sound propagates is called a medium. Sound cannot travel through vacuum because it needs a material medium – solid, liquid, or gas – to propagate.

Q3: What is a compression and what is a rarefaction in a sound wave?

Ans: A compression (C) is a region of air with higher density than average, formed when the source pushes air particles forward. A rarefaction (R) is a region of air with lower density than average, formed when the source pulls back. Sound consists of alternating compressions and rarefactions.

Q4: What is a sound wave and what type of wave is it?

Ans: A sound wave is a disturbance consisting of a series of alternating compressions and rarefactions propagating through a medium, without the actual flow of particles of the medium. Sound is a longitudinal mechanical wave because the particles of the medium vibrate parallel to the direction of wave propagation.

Q5: Define wavelength of a sound wave and give its SI unit.

Ans: The distance between two consecutive crests or two consecutive troughs in the graphical representation of a sound wave is called its wavelength, denoted by $\lambda$. Its SI unit is the metre (m).

Q6: Define frequency of a sound wave. What is its SI unit?

Ans: The number of density oscillations at a fixed point per unit time is called the frequency $(\nu )$ of the sound wave. Its SI unit is per second $({\text{s}}^{-1})$, also called a hertz (Hz).

Q7: Define time period of a sound wave and write its relation with frequency.

Ans: The time taken for one complete density oscillation at a fixed point is called the time period $(T)$ of the wave. Its SI unit is second (s). The relation between time period and frequency is:$$\nu =\frac{1}{T}$$

Q8: Write the formula relating speed, wavelength, and frequency of a sound wave.

Ans: The speed $(v)$ of a sound wave is related to its wavelength $(\lambda )$ and frequency $(\nu )$ by the formula:$$v=\lambda \times \nu$$That is, speed = wavelength $\times$ frequency.

Q9: In which state of matter does sound travel the fastest and the slowest?

Ans: Sound travels fastest in solids and slowest in gases. For example, the speed of sound in steel is about $5000{\text{m s}}^{-1}$, in water about $1500{\text{m s}}^{-1}$, and in air about $340{\text{m s}}^{-1}$ at $15{}^{\circ }\text{C}$.

Q10: What is the amplitude of a sound wave and how is it related to the energy of the wave?

Ans: The amplitude of a sound wave is the maximum change in the density of air in a compression (or rarefaction) compared to the average density. A wave with a larger amplitude carries more energy and sounds louder than a wave with a smaller amplitude.

Q11: Define intensity of sound.

Ans: The amount of sound energy passing through a unit area perpendicular to the direction of propagation of the sound wave in a unit time is called the intensity of sound. Intensity decreases as we move farther away from the source.

Q12: What is pitch? How is it related to frequency?

Ans: Pitch is how frequency is perceived by humans. High pitch sounds (like a whistle or siren) have higher frequency, while low pitch sounds (like thunder or an aircraft rumble) have lower frequency.

Q13: What is the audible range of sound for humans? Define infrasonic and ultrasonic waves.

Ans: The human audible range is from $20\text{ Hz}$ to $20,000\text{ Hz }(20\text{ kHz})$. Sound waves with frequency below $20\text{ Hz}$ are called infrasonic waves, and those with frequency above $20\text{ kHz}$ are called ultrasonic waves.

Q14: What is an echo? What is the minimum time gap required to hear an echo?

Ans: An echo is the reflection of sound from a hard surface of a distant object that comes back to the listener after some time. The minimum time gap between the original sound and the reflected sound for it to be heard as a separate echo is $0.1\text{ s}$.

Q15: What is the minimum distance required from a reflecting surface to hear an echo? Take speed of sound as 340 m s−1.

Ans: The sound must travel to the wall and back in $0.1\text{ s}$. Total distance covered $=340\times 0.1=34\text{ m}$. Since this is the to and fro distance, the minimum distance from the reflecting surface is:$$\text{Minimum distance}=\frac{34}{2}=17\text{ m}$$

Q16: What is reverberation? How is it different from an echo?

Ans: Reverberation is the persistence of sound caused by multiple reflections from the walls of a large hall or auditorium, where the reflected sounds arrive with a time difference of less than $0.05\text{ s}$. Unlike an echo, where the reflected sound is heard as a separate sound, in reverberation the reflections merge with the original sound.

Q17: What is echolocation? Name two animals that use it.

Ans: Echolocation is the ability to locate objects using reflected sound waves. Bats emit short bursts of ultrasonic waves and sense the echoes to determine the position of obstacles and prey. Dolphins and whales also use echolocation for navigation and hunting.

Q18: What is SONAR? On what principle does it work?

Ans: SONAR stands for Sound Navigation And Ranging. It works on the principle of echolocation – ultrasonic waves are sent into water and the reflected waves are analysed to determine the distance, direction, and speed of underwater objects such as submarines or shipwrecks.

Q19: A sonar signal returns after 0.90 s. If the speed of sound in seawater is 1530 m s−1, how far is the underwater object?

Ans: Time taken to reach the object $={\displaystyle \frac{0.90}{2}}=0.45\text{ s}$.$$\text{Distance}=v\times t=1530{\text{m s}}^{-1}\times 0.45\text{ s}=688.5\text{ m}$$The object is $688.5\text{ m}$ away.

Q20: Give any two applications of ultrasonic waves in medicine and industry.

Ans: (i) In medicine, ultrasonic waves are used for imaging internal organs without surgery, a technique called ultrasonography. They are also used to break kidney stones into smaller pieces that can be passed out of the body. 
(ii) In industry, ultrasonic waves are used for ultrasonic welding and for cleaning delicate machine parts, as well as for detecting defects inside metal blocks.