The amplitude of sound waves is measured in the units of
- (a)pressure
- (b)distance
- (c)time
- (d)speed
Correct — A, pressure. Sound travels through a fluid as a longitudinal wave of compressions and rarefactions, so what oscillates at a point is the local pressure. The amplitude of a sound wave is the largest departure of that pressure from the undisturbed value, and it is quoted in pressure units — pascals, or newtons per square metre. This is also what a microphone or a sound-level meter actually senses, which is why loudness is expressed on a decibel scale referred to a standard pressure.
- (b)distance — Air particles do move a tiny distance about their rest positions, but the quantity measured and quoted as the amplitude of a sound wave is the pressure variation, not that displacement.
- (c)time — Time units describe the time period of the wave, the seconds taken for one complete oscillation, which is the reciprocal of frequency and has nothing to do with amplitude.
- (d)speed — Speed is how fast the wave travels through the medium, about 343 metres per second in air at ordinary room temperature; it is fixed by the medium and is independent of how loud the sound is.
A sound wave in air is a travelling pressure disturbance. Its amplitude fixes the loudness, its frequency fixes the pitch, and its speed is set by the medium rather than by the source. Because the oscillating quantity is pressure, the amplitude of the wave is a pressure and is measured in pascals.
Each wrong option is a genuine property of a wave attached to the wrong term. Sort the vocabulary once and the item becomes automatic — amplitude with loudness, frequency with pitch, time period with the duration of one oscillation, and speed with the medium. Note that decibels are not a unit of amplitude in themselves but a logarithmic comparison of the sound pressure with a fixed reference pressure.
- Sound in a fluid is a longitudinal pressure wave, so its amplitude is a pressure, measured in pascals.
- The loudness of a sound rises with the amplitude of the wave, while its pitch rises with frequency.
- The decibel scale compares sound pressure with a reference value of 20 micropascals, taken as the threshold of hearing.
- The speed of sound in air is about 343 metres per second at ordinary room temperature and does not depend on loudness.
Only amplitude is a pressure quantity, so the answer is (a).
- Picking 'distance' by thinking of the displacement of air particles rather than the pressure that is actually measured.
- Treating the decibel as a unit of amplitude — it is a logarithmic ratio referred to a fixed pressure.
A vocabulary-and-units item; match each wave property to its physical quantity and the answer follows.
A noise level of 100 decibels would correspond to
- (a) just audible sound
- (b) ordinary conversation
- (c) sound from a noisy street
- (d) noise from a machine shop
Answer(d) noise from a machine shop
The decibel scale that item uses is built on the very pressure amplitude this question asks about, compared against a fixed reference pressure.
The loudness of sound depends upon the
- (a) velocity of sound waves in the medium.
- (b) amplitude of the sound waves.
- (c) frequency of the sound waves.
- (d) frequency and velocity of the sound waves.
Answer(b) amplitude of the sound waves.
The same amplitude, tested from the loudness side — that item establishes what amplitude does, this one asks what it is measured in.
- practice — not a real PYQ
The loudness of a sound wave is determined mainly by its
- (a)frequency
- (b)amplitude
- (c)wavelength
- (d)speed
Answer(b) amplitude — frequency determines pitch instead.
- practice — not a real PYQ
Sound waves travelling through air are
- (a)transverse waves
- (b)longitudinal pressure waves
- (c)electromagnetic waves
- (d)stationary waves
Answer(b) longitudinal pressure waves — air particles oscillate along the direction of travel, changing the local pressure.