CDS (II) 2022 General Knowledge · Question 26 of 120

Correct — D, Frequency. Pitch is the ear's reading of how many pressure pulses reach it per second. A wave of 512 vibrations per second is heard as the note above middle C; raise the rate and the note climbs, drop it and the note falls. That is the whole of the relationship, and it holds regardless of how hard the source was struck. Strike a tuning fork gently and then hard and the two sounds differ in volume but not in note, because the fork's prongs still swing to and fro the same number of times each second; only the width of the swing has changed. The three properties a sound wave carries are separable in exactly this way — frequency is heard as pitch, amplitude is heard as loudness, and the mix of harmonics riding on the fundamental is heard as quality or timbre, which is what lets you tell a flute from a sitar playing the same note at the same volume.

• (a)Speed — Speed is a property of the medium, not of the note. Sound travels at roughly 343 m/s in air at 20 degrees Celsius, about 1,500 m/s in water and about 5,000 m/s in steel, and a soprano singing in a swimming pool is not heard as a bass. What changes with the medium is the wavelength, because the wave must satisfy v = f x wavelength while the source keeps the frequency fixed.
• (b)Loudness — Loudness is the perception that goes with amplitude, and it is the property this question keeps separate from pitch. Turning a radio up does not raise the tune.
• (c)Amplitude — Amplitude is the physical quantity behind loudness — the maximum displacement of the particles from their rest position. The intensity a listener receives goes as the square of the amplitude, and none of that touches the number of vibrations per second.

A sound wave is a longitudinal wave: the particles of the medium oscillate back and forth along the direction the wave travels, making alternating compressions and rarefactions. Three measurable quantities describe it. Frequency, in hertz, is the number of complete oscillations per second; amplitude is the size of the displacement; and the waveform is the shape one cycle traces out. The ear maps them onto pitch, loudness and quality respectively. The three are independent — you can change any one and leave the other two alone.

The item is a straight recall check with one honest trap in it. Options (b) and (c) are the same idea wearing two hats, loudness being the sensation and amplitude the measurement, so a student who half-remembers 'amplitude is involved somewhere' has two ways to go wrong. Option (a) catches anyone who confuses the speed of the wave with the rate of vibration. The safe habit is to run the tuning-fork test in your head: hit it softly, hit it hard, and ask which of the four listed quantities has actually changed. Note that the stem's phrasing, 'which one of its following characteristics', is the paper's own awkwardness and is left as printed. In the human range, roughly 20 Hz to 20,000 Hz, the whole of what we call music sits in the frequency dimension of that answer.

• Pitch is determined by frequency; loudness by amplitude; quality or timbre by the waveform, i.e. the mix of harmonics.
• Frequency is set by the vibrating source and does not change when the sound passes from one medium into another; the wavelength changes instead, because v = f x wavelength.
• The audible range for a healthy young human ear is about 20 Hz to 20,000 Hz; below it is infrasound, above it ultrasound.
• Sound travels at about 343 m/s in air at 20 degrees Celsius, faster in water and faster still in solids, because the speed depends on the elasticity and density of the medium.
• The intensity of a sound wave is proportional to the square of its amplitude.

• Treating loudness and amplitude as two different answers when they are the same physical fact described twice.
• Believing that a sound gets higher-pitched when it enters water; the speed and wavelength change, the frequency does not.
• Confusing pitch with volume in everyday speech, where a 'high' voice and a 'loud' voice are often used interchangeably.

Almost always as a one-line match between a perceived property and the physical quantity behind it, sometimes flipped into a statement pair about pitch and loudness.

• practice — not a real PYQ

  The characteristic of a musical sound that lets a listener distinguish a flute from a violin playing the same note equally loudly is its

  • (a)pitch
  • (b)loudness
  • (c)quality
  • (d)wavelength

  Answer(c) quality — also called timbre; it is set by the pattern of harmonics riding on the fundamental frequency.

• practice — not a real PYQ

  When a sound wave passes from air into water, which one of the following remains unchanged?

  • (a)Speed
  • (b)Wavelength
  • (c)Frequency
  • (d)Intensity

  Answer(c) Frequency — it is fixed by the vibrating source; the speed rises in water and the wavelength stretches to match, since v = f x wavelength.