Characteristics of Sound – Long Answer Questions (Class 9 Science, Physics)

Medium Level (Application & Explanation)

Q1. Explain the nature of sound waves and how they travel through a medium.

Answer:
Sound is a form of mechanical energy that travels as a wave. Specifically, sound in air and most common media travels as longitudinal waves, where particles of the medium move back and forth parallel to the direction of wave travel. These particle motions create alternating regions of compression (particles close together) and rarefaction (particles spread out). Sound requires a medium (solid, liquid or gas) to travel because the wave depends on particle interactions; it cannot travel through vacuum. As the source vibrates, it sets nearby particles in motion; these particles push neighbors, and the disturbance propagates outward. The speed, amplitude, and frequency of the wave depend on the medium and the source’s vibration. Understanding these features helps explain echoes, loudness and pitch in everyday life.

Q2. Describe the role of amplitude in determining the loudness of a sound and how amplitude relates to sound energy.

Answer:

Amplitude is the maximum displacement of particles from their rest position in a sound wave.
A larger amplitude means particles oscillate more strongly, which corresponds to greater energy carried by the wave.
When amplitude increases, the sound is perceived as louder by the ear. Loudness is a subjective sensation; physical intensity depends on the square of amplitude.
Sound intensity is measured in watts per square metre, while loudness is commonly given in decibels (dB).
Two sounds with the same frequency but different amplitudes will have the same pitch but different loudness.
In real situations, distance from the source and absorption by the medium reduce amplitude, lowering perceived loudness.

Q3. Explain frequency and how it determines the pitch of a sound. Include the human audible range.

Answer:
Frequency is the number of complete vibrations or waves that pass a point in one second and is measured in Hertz (Hz). Higher frequency means more cycles per second and is perceived as a higher pitch; lower frequency is perceived as a lower pitch. The human audible range is roughly 20 Hz to 20,000 Hz; sounds below this are infrasound and above are ultrasound. Musical instruments produce notes by vibrating at specific frequencies — changing string length, tension or air column length alters frequency and thus pitch. Frequency does not change when sound changes medium (neglecting Doppler effect); speed and wavelength may change so that v = fλ still holds. Frequency is therefore the key factor determining what we identify as pitch.

Q4. Using the relation v = fλ, explain how wavelength changes with frequency and speed, and give a numerical example.

Answer:

The relation v = fλ connects speed (v), frequency (f) and wavelength (λ). For a given medium, v is fixed, so wavelength λ = v / f.
If frequency increases, wavelength decreases proportionally; conversely, a lower frequency yields a longer wavelength.
When sound moves into a different medium, v changes. If the frequency remains constant, λ changes according to the new speed.
Example: In air at room temperature, v ≈ 343 m/s. For a note of 256 Hz (middle C), λ = 343 / 256 ≈ 1.34 m. If the same frequency moves to water where v ≈ 1500 m/s, λ ≈ 1500 / 256 ≈ 5.86 m. This shows how wavelength depends on both frequency and medium speed.

Q5. Why does sound travel fastest in solids, slower in liquids, and slowest in gases? Give examples.

Answer:
Sound speed depends on two properties of the medium: density and elasticity (stiffness). In solids, particles are tightly packed and the material is more elastic, so disturbances transfer quickly from particle to particle, giving highest speed (e.g., steel ≈ 5000 m/s). In liquids, particles are less tightly packed than solids but closer than gases, so speed is intermediate (water ≈ 1500 m/s). In gases, particles are far apart and collisions are less frequent, so sound travels slowest (air at 20°C ≈ 343 m/s). Although higher density can reduce speed, the elastic property dominates: solids’ high stiffness makes sound propagation fastest despite higher density.

High Complexity (Analytical & Scenario-Based)

Q6. Design an experiment using the echo method to measure the speed of sound. Describe procedure, calculations and sources of error.

Answer:

Set up a flat reflecting surface (large wall or cliff) at a measured distance d from a person with a stopwatch.
The observer produces a sharp sound (clap) and starts the stopwatch when the sound is made. They stop the watch when the echo returns. The measured time t_total is the round-trip time for sound to travel 2d.
Calculate speed using v = 2d / t_total. For accuracy, repeat several trials and use the average time.
Ensure distance d is measured precisely and use a large flat reflector to get a clear echo.
Main error sources: reaction time of the observer with the stopwatch, wind or temperature gradients changing v, and inaccurate distance measurement. Use electronic timers or record audio to reduce human reaction error. State temperature to compare with expected speed.

Q7. You strike a tuning fork and then touch its base to a wooden table. Why does the sound become louder on the table? Explain the physics involved.

Answer:
When a tuning fork vibrates in air, it radiates sound by moving surrounding air. Touching its base to a wooden table couples the fork’s vibrations to the table, turning the table into a larger vibrating surface. This increases the effective area that pushes air, improving energy transfer into the air. The table’s larger surface and matching acoustic impedance help radiate more sound power, so the sound is louder. Also, the table may resonate at frequencies close to the fork’s frequency, increasing amplitude by resonance. In short, better mechanical coupling and larger radiating area produce louder sound and improved efficiency of sound radiation.

Q8. Explain why low-frequency (bass) sounds travel farther outdoors than high-frequency sounds. Mention absorption, diffraction and atmospheric effects.

Answer:

Low-frequency sounds have longer wavelengths which are less easily absorbed by air molecules and obstacles; therefore they lose less energy over distance.
Long wavelengths diffract around obstacles (buildings, trees) more easily than short wavelengths, so bass can propagate around corners and through openings.
Atmospheric absorption is frequency dependent: higher frequencies are absorbed more due to viscous and thermal losses in air, and molecular relaxation effects, so treble fades faster.
Wind and temperature gradients can refract high frequencies differently, often scattering them.
Consequently, in open spaces or during concerts, bass notes are heard farther and more clearly at large distances than high-pitched notes.

Q9. You talk to a friend underwater and the voice sounds different. Explain the physical reasons and how human perception is affected.

Answer:
Underwater, sound speed (~1500 m/s) is much higher than in air, so wavelengths for the same frequency are longer. Human ears are adapted to air; when underwater, sound reaches the inner ear largely through bone conduction and body tissues rather than through the eardrum. Also, there is a strong impedance mismatch between water and the ear’s air-filled structures, causing poor transmission of higher frequencies and attenuating certain components. The result is a muffled, lower-quality sound with changed timbre; pitch may seem similar but clarity drops. Devices like hydrophones convert pressure waves in water to electrical signals to reproduce sound properly. Speaking through a tube or using underwater communication systems helps maintain intelligibility.

Q10. If you shout from a mountain, what factors determine how far your voice can be heard? Analyze environmental and physical influences.

Answer:

Frequency content: Lower frequencies travel farther due to lower absorption and better diffraction around obstacles.
Amplitude: Louder shouts (higher amplitude) can be detected farther away.
Air density and temperature: On mountains, thinner air (lower density) slightly reduces sound intensity, but temperature gradients (inversions) can refract sound and either carry it farther or trap it.
Wind: Wind direction and speed can carry sound farther downwind and reduce range upwind.
Humidity: Higher humidity reduces absorption at some frequencies, slightly increasing range.
Obstacles and terrain: Cliffs can reflect sound giving echoes; vegetation and irregular ground absorb and scatter sound.
Background noise: Wind or ambient natural sounds reduce audibility. To be heard farther, use lower-frequency, louder calls, and face downwind or use reflecting surfaces to direct sound.