Sound and Waves: Properties, Speed and PYQs

The correct answer is B, 600 m. The speed of a wave is the product of its frequency and its wavelength, so the speed here is 50 hertz multiplied by 4 metres, that is 200 metres per second. Distance is speed multiplied by time, so in 3 seconds the wave covers 200 multiplied by 3, which is 600 metres. Always find the speed first and only then apply the time.

Option A, 100 m, would follow from dividing rather than multiplying somewhere in the working. Option C, 200 m, is the distance covered in one second and is the answer to a different question; it is the speed, not the answer asked for. Option D, 300 m, corresponds to no correct step at all and is included because it resembles the rounded speed of sound in air, which is irrelevant here since the speed is given indirectly by the data.

The correct answer is C, Pitch. Pitch is the characteristic of a sound by which the brain judges it as shrill or flat, and it is decided by the frequency of the wave. A higher frequency is heard as a higher pitch, which is why a woman's voice or a whistle sounds shriller than a drum or a man's voice. The physical quantity is frequency; the sensation produced in the brain is pitch.

Option A, wavelength, is the distance between two successive compressions and is a measurement of the wave in space, not a sensation. Option B, wave velocity, depends on the medium and its temperature and is the same for a shrill and a flat sound in the same air. Option D, oscillation, simply means one complete to and fro vibration; counting oscillations per second gives frequency, but the word itself names no property that the brain interprets.

The correct answer is D, 2 s. Convert the units first: 1 kilohertz is 1000 hertz and 50 centimetres is 0.5 metre. The speed is frequency multiplied by wavelength, that is 1000 multiplied by 0.5, which gives 500 metres per second. Time equals distance divided by speed, so 1000 metres divided by 500 metres per second gives 2 seconds.

Option A, 5 s, would follow from using a speed of 200 metres per second. Option B, 4 s, corresponds to a speed of 250 metres per second, which arises if the wavelength is halved by mistake. Option C, 3 s, is close to the time sound actually takes to cover a kilometre in ordinary air, about 340 metres per second, and is the trap for candidates who use the remembered speed of sound instead of the speed calculated from the data given.

The correct answer is B, the amplitude of the sound waves. Amplitude is the maximum displacement of a particle of the medium from its rest position, and the energy carried by a wave increases with the square of the amplitude. A larger amplitude therefore delivers more energy to the ear and is heard as a louder sound. Loudness is measured in decibel.

Option A, the velocity of the wave, is fixed by the medium and its temperature; a loud and a soft sound in the same room travel at exactly the same speed. Option C, frequency, decides pitch, that is whether the sound is shrill or flat, and not how loud it is; turning up the volume of a ringtone does not change its frequency. Option D combines two quantities that both govern properties other than loudness, so it is wrong for the same reasons.

The correct answer is B, light travels much faster than sound. The flash and the clap are produced at the same instant by the same discharge. Light covers about three lakh kilometres in one second, so it reaches the observer almost immediately, while sound in air covers only about 340 metres in a second and takes roughly three seconds for each kilometre. The gap between seeing and hearing is therefore a direct measure of distance.

Option A reverses the true order and would make thunder arrive first, which never happens. Option C is wrong because equal speeds would make the flash and the clap arrive together, with no gap at all. Option D confuses brightness with speed; the flash is indeed very bright, but intensity has nothing to do with why it arrives earlier, and even a faint flash would still be seen before the sound was heard.

The correct answer is B, longitudinal mechanical waves. In a sound wave the particles of the medium vibrate back and forth along the same line in which the wave is travelling, producing regions where they crowd together, called compressions, and regions where they spread apart, called rarefactions. Because the vibration of particles is essential, sound is a mechanical wave and needs a material medium.

Option A is wrong on both counts: sound is not electromagnetic and its particles do not vibrate at right angles to the direction of travel. Option C describes a special case; sound can travel as a transverse wave inside a solid, but ordinary sound in air, water and solids is longitudinal, and the option wrongly restricts sound to solids. Option D describes electromagnetic waves such as light, radio waves and X-rays, which do travel through a vacuum, unlike sound.

The correct answer is C, vacuum. A vacuum contains no particles, and a mechanical wave such as sound can travel only by making particles vibrate and hand the disturbance on to their neighbours. The bell jar experiment shows this clearly: an electric bell ringing inside a glass jar grows fainter and finally inaudible as the air is pumped out, although the hammer can still be seen striking the gong.

Option A, air, carries sound at about 340 metres per second at room temperature and is the medium in which we normally hear. Option B, water, carries it faster, at roughly 1500 metres per second, which is why whales and SONAR can communicate and search over long distances at sea. Option D, iron, is a solid with tightly packed particles and carries sound fastest of the three, at several thousand metres per second.

The correct answer is B, 20 Hz to 20,000 Hz. This is the band of frequencies to which a healthy human ear responds, and the upper limit falls with age. Below the lower limit the sound is called infrasonic and is produced by elephants, whales and earth movements before an earthquake; above the upper limit it is called ultrasonic and is produced and heard by bats, dolphins and porpoises.

Option A, 2 Hz to 200 Hz, lies almost entirely in the infrasonic band at its lower end and cuts off ordinary speech and music at its upper end. Option C, 200 Hz to 2,00,000 Hz, wrongly extends deep into the ultrasonic range, which no human can hear. Option D, 20 Hz to 2,000 Hz, has the right starting point but an upper limit ten times too low, which would leave out most of the higher notes of music that people hear easily.

The correct answer is D, steel. Sound travels fastest in solids because their particles are packed closely and are strongly bound, so a disturbance is passed on to the next particle almost at once. In steel the speed is about five thousand metres per second, in water about fifteen hundred and in air only about three hundred and forty. The order to remember is solid, then liquid, then gas.

Option A, vacuum, carries no sound at all, since there are no particles to vibrate. Option B, air, is the slowest of the three real media listed because its particles are far apart and take longer to pass the disturbance on. Option C, water, is faster than air but far slower than steel. This is also why a rail track carries the sound of an approaching train long before the sound reaches a listener through the air.

The correct answer is B, reflection of ultrasonic waves. SONAR stands for Sound Navigation and Ranging. A transmitter on the ship sends short ultrasonic pulses into the water, the pulses are reflected from the sea bed, a submarine, an iceberg or a shoal of fish, and a detector records the returning echo. Since the pulse travels down and back, the depth equals the speed of sound in water multiplied by the time taken and divided by two.

Option A is wrong because light is absorbed within a short distance in sea water and is useless for sounding the deep ocean. Option C is wrong for a similar reason, as radio waves are strongly attenuated in salt water, which is why submarines must surface or use very low frequencies to communicate. Option D names the wrong band and the wrong phenomenon: infrasound is below the audible range and interference is not how ranging is done.

The correct answer is B, about 17 metres. The sensation of a sound persists in the human brain for roughly one tenth of a second, so a reflected sound must arrive at least that much after the original to be heard as a separate echo. In that tenth of a second sound in air covers about 34 metres, but the sound must travel to the surface and back, so the surface itself must be at least half of that away, roughly 17 metres.

Option A, 1.7 metres, is ten times too small and would give a reflection arriving in about one hundredth of a second, far too soon to be heard separately. Option C, 34 metres, is the total path length travelled by the sound rather than the distance to the wall, and is the most common error in this question. Option D, 170 metres, is ten times too large; an echo is heard well before the surface is that far away.