Say whether following sentences are true or false :
(I) For propagation of sound medium is necessary.
(II) Unit of frequency of sound is Hertz.
(III) Loudness of sound is expressed in decibel unit.
An

Question

Say whether following sentences are true or false :
(I) For propagation of sound medium is necessary.
(II) Unit of frequency of sound is Hertz.
(III) Loudness of sound is expressed in decibel unit.
Answer options :

UnlockIAS · Accepted Answer

Correct — option (1), all three statements are true. The stem asks the candidate to judge each printed statement as true or false — the words 'true' and 'false' are set in bold in the paper — and note that this question labels its statements with Roman numerals (I) to (III), and prints only three of them rather than the four the rest of the paper uses. Statement (I) is true: sound is a mechanical wave, a travelling disturbance of compressions and rarefactions that needs particles of matter to pass the disturbance along, so it cannot cross a vacuum. The classic demonstration is the bell jar, where an electric bell is set ringing inside a jar and the sound fades to nothing as the air is pumped out even though the hammer can still be seen striking. This is the property that distinguishes sound from light and radio waves, which are electromagnetic and travel through empty space. Statement (II) is true: frequency is the number of complete vibrations per second, and its SI unit is the hertz, named after Heinrich Hertz, one hertz being one cycle per second. Frequency is what the ear interprets as pitch, and the human audible range runs from about 20 hertz to about 20,000 hertz, with infrasound below and ultrasound above. Statement (III) is true: loudness is expressed in decibels, a unit that is one-tenth of a bel and is named after Alexander Graham Bell. The decibel scale is logarithmic rather than linear, so every increase of ten decibels represents a tenfold increase in sound intensity, which is why the enormous range the ear can handle compresses into a scale running from zero at the threshold of hearing to about 120 decibels at the threshold of pain. With no false statement among the three, the choice that affirms all of them is the answer.

Why the other options are wrong — (b) This accepts statements (I) and (II) but rejects statement (III), the use of the decibel for loudness — the one statement of the three that a candidate is most likely to doubt, because loudness is a sensation while the decibel measures a physical intensity level. The doubt is worth understanding but does not make the statement false: the decibel is the standard unit in which loudness is reported, it is the unit in which noise standards and hearing damage thresholds are written, and school and public usage are unambiguous on the point. Rejecting it also leaves the question with no false statement to justify the rejection.  (c) This accepts only statement (II) and so rejects both the need for a medium and the decibel as the unit of loudness. The first rejection is the more serious, because the requirement of a material medium is the defining property of a mechanical wave and the reason space is silent; a candidate who doubts it may be confusing sound with the electromagnetic waves used to carry radio and television signals, which do travel through vacuum and which are what actually cross the space between a spacecraft and the earth. Both rejected statements are correct as printed.  (d) This declares all three statements false, which would require sound to travel through a vacuum, frequency to be measured in something other than the hertz, and loudness to be expressed in some unit other than the decibel — three separate errors, each contradicting a settled definition. An option of this shape is worth a moment's thought only when the statements share a common false premise that would bring them all down together; here they are independent facts about three different aspects of sound, and each stands on its own.

Concept: Sound is a longitudinal mechanical wave: the particles of the medium oscillate back and forth along the direction in which the disturbance travels, producing regions of compression and rarefaction that move outward while the particles themselves stay near their original positions. Because the disturbance is handed from particle to particle, a material medium is indispensable, and the speed of sound rises as the medium becomes stiffer and its particles more closely packed — roughly 340 metres per second in air at ordinary temperature, about four times that in water, and faster still in steel. Three properties of the wave map onto three sensations. Frequency, measured in hertz, is perceived as pitch, and the audible band runs from about 20 to about 20,000 hertz. Amplitude determines the energy carried and is perceived as loudness, which is reported on the decibel scale; because the ear responds to an enormous range of intensities, that scale is logarithmic, with each ten-decibel step corresponding to a tenfold change in intensity. Waveform or harmonic content is perceived as quality or timbre, and is what distinguishes the same note played on two different instruments. The decibel scale also underpins environmental regulation, since ambient noise standards and permissible exposure limits are all written in decibels.

State PCS 2024 Q55 — Science & Technology PYQ with Answer

State PCS 2024 Q55 — Science & Technology PYQ with Answer