Exploration Class 9 Science Chapter 10: Sound Waves: Characteristics and Applications — NCERT Solutions

Explanation: (i) When the struck tuning fork is brought near the ear, a soft humming sound is clearly audible because its prongs are vibrating. (ii) When a vibrating prong just touches the water surface, the water is disturbed - it splashes and circular ripples spread out, as seen in Fig. 10.4(c). This shows the prong is moving rapidly to and fro even though the motion is too fast to see directly. (iii) Together these observations conclude that sound is produced by vibrating objects: as long as the prongs vibrate, sound is produced, and the vibration is confirmed by the disturbance created on the water.

Explanation: In Fig. 10.19 the density curve starts on the mean (dashed) line at 0, rises to a crest, crosses the mean line at 1.5 cm, dips to a trough and returns to the mean line at 3.0 cm. So one complete wave (one crest + one trough) occupies 3.0 cm, i.e. wavelength lambda = 3.0 cm; this also matches the crest-to-crest distance on the graph. Therefore half the wavelength = 3.0/2 = 1.5 cm, which is the distance between two successive crossings of the mean density line.

Explanation: The loud sound travels through the air as a wave and strikes the stretched sheet, setting it into vibration. The vibrating sheet tosses the grains lying on it, so they are seen jumping up and down. Since the grains are made to move without anything touching them, the experiment shows that sound carries energy from the source to the sheet through the air.

Explanation: (i) On plucking, the rubber band moves rapidly to and fro (it appears blurred) and simultaneously a sound is heard, showing the vibrating band is producing the sound. (ii) The moment the band stops vibrating, the sound also stops - no vibration, no sound. (iii) Increasing the tension makes the band vibrate faster (higher frequency), so the sound becomes shriller (higher pitch); decreasing the tension gives a duller, lower-pitch sound. (iv) When the band is stretched between two fingers and plucked near the ear, a sound is still produced because the band still vibrates, but it is much feebler - the open box in the earlier setup acts as a sound board, its walls and the air inside also vibrate and make the sound louder.

Explanation: In humans, sound is produced in the larynx (voice box), situated in the throat at the upper end of the windpipe, which has two vocal cords stretched across it with a narrow slit between them (Fig. 10.3). When we speak or sing, air from the lungs is forced through this slit and the vocal cords vibrate, producing sound; muscles attached to the cords change their tightness and the gap between them, which changes the type of voice. On placing the fingers gently on the throat while speaking, a trembling or shaking sensation (vibration) is felt. This indicates that the source of the sound is the vibrating vocal cords - again confirming that sound is produced by vibrations.

Explanation: In the dot pictures, a compression is a region where the particles of the medium are crowded together (high density), so every dark, closely packed band in (a) and (b) is labelled C; a rarefaction is a region where the particles are spread apart (low density), so every lighter, sparse band is labelled R. For graphs (c) and (d), the horizontal axis is labelled distance and the vertical axis density, with the dashed horizontal line representing the mean (undisturbed) density. The curve is drawn as a smooth wave oscillating about this mean line: it rises to a crest (maximum density) exactly below each compression and falls to a trough (minimum density) exactly below each rarefaction of the wave directly above it. One compression plus one adjacent rarefaction makes one complete wavelength, so the spacing of crests in (c) and (d) must match the spacing of the C regions in (a) and (b) respectively - the wave whose compressions are closer together gets a curve with more cycles in the same length (shorter wavelength).

Explanation: The assertion correctly describes the outcome of the bell jar experiment, which demonstrates that sound cannot travel in a vacuum. The reason states the scientific principle that sound, being a mechanical wave, requires a material medium to propagate. The reason perfectly explains why the assertion is true.

Explanation: Sound waves are a form of energy that propagates through a medium by causing particles to vibrate. The particles themselves do not travel the distance, but they transfer the energy.

Explanation: Assertion (A) is true because sound propagates as a wave of compressions and rarefactions. Reason (R) is false because particles in the medium only oscillate about their mean positions and do not travel with the wave.

Explanation: A mechanical wave is defined by its requirement of a medium for propagation, which distinguishes it from electromagnetic waves. The other options are general properties of most waves.

Explanation: Frequency is defined as the number of oscillations or compressions passing a point per second. Therefore, increasing frequency directly increases the number of compressions per second.

Explanation: Frequency is the number of oscillations (or compressions) per unit time. Frequency = Number of compressions / Time = 20 / 4s = 5 Hz.