Which one of the following cannot be the unit of frequency of a sound wave?
- (a)dB
- (b)s⁻¹
- (c)Hz
- (d)min⁻¹
Correct — A, dB (decibel). Frequency is the number of oscillations per unit time, so any unit of frequency must have the dimensions of 'per unit time' — which is exactly what hertz (Hz), s⁻¹ and min⁻¹ all are (1 Hz is one cycle per second, i.e. s⁻¹, and min⁻¹ is cycles per minute). The decibel is a logarithmic unit that measures the intensity level or loudness of a sound relative to a reference; it carries no 'per time' meaning, so it cannot be a unit of frequency.
- (b)s⁻¹ — s⁻¹ means 'per second', which is precisely the definition of the hertz (1 Hz = 1 s⁻¹), so it is a perfectly valid unit of frequency.
- (c)Hz — The hertz is the SI unit of frequency (one cycle per second), so it is a correct frequency unit and cannot be the odd one out.
- (d)min⁻¹ — min⁻¹ means 'per minute'; it is an unusual but genuine measure of frequency (cycles per minute, as on a tachometer), so it does count as a frequency unit.
Frequency is how many complete oscillations a wave makes each second, so any unit of frequency must have dimensions of 1/time (inverse time). The hertz is the SI unit, defined as one cycle per second (1 Hz = 1 s⁻¹). Loudness, by contrast, is measured on the decibel scale — a logarithmic comparison of sound intensity to a fixed reference — which has nothing to do with how often the wave oscillates.
The trap is that all four look like plausible 'sound' units. Sorting them by dimension settles it: Hz, s⁻¹ and min⁻¹ are all reciprocal-time (frequency), while dB is a dimensionless intensity ratio. Recognising that the decibel measures loudness, not pitch, is the key.
- 1 hertz (Hz) = 1 cycle per second = 1 s⁻¹; frequency always has dimensions of inverse time.
- The decibel (dB) is a logarithmic unit of sound intensity level (loudness), not of frequency.
- min⁻¹ (per minute) is a valid, if uncommon, unit of frequency — for example revolutions per minute.
- For sound, frequency (Hz) governs pitch while amplitude/intensity (dB) governs loudness.
- Treating the decibel as a 'sound unit' and assuming it measures frequency — it measures loudness.
- Not recognising that Hz and s⁻¹ are the same unit.
Asked as 'which is/ is not a unit of frequency' or by pairing a physical quantity (pitch, loudness) with its correct unit.
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
Same concept — the decibel measures loudness/sound-intensity level (here ~100 dB = a machine shop), confirming that dB is a loudness unit and not a unit of frequency.
When the same note is played on a sitar and a flute, the sound produced can be distinguished from each other because of the difference in
- (a) pitch, loudness and quality
- (b) pitch and loudness
- (c) quality only
- (d) loudness only
Answer(c) quality only
Related — tests the characteristics of sound (pitch/frequency, loudness, quality); pitch is set by frequency in Hz while loudness and quality are separate properties, the same distinctions behind identifying dB as a non-frequency unit.
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.
Same concept — separates loudness (amplitude, measured in dB) from frequency, the exact distinction that makes the decibel a non-frequency unit.
- practice — not a real PYQ
Which physical quantity of a sound wave is measured in decibels?
- (a)Frequency
- (b)Wavelength
- (c)Loudness (intensity level)
- (d)Speed
Answer(c) Loudness (intensity level) — the decibel is a logarithmic measure of sound intensity, not frequency.
- practice — not a real PYQ
The SI unit of frequency, the hertz (Hz), is equivalent to
- (a)m/s
- (b)s⁻¹
- (c)kg·m/s²
- (d)m
Answer(b) s⁻¹ — one hertz is one cycle per second, i.e. 1 s⁻¹.