CDS (I) 2020 General Knowledge · Question 7 of 120

Correct — A, frequency. Pitch is how the brain interprets the frequency of a sound — NCERT puts it in exactly those words, and adds that the faster the source vibrates, the higher the frequency and the higher the pitch. A high-pitched sound is one whose compressions and rarefactions arrive at your ear more often per second. The other three quantities on the list describe the same wave but answer different questions: amplitude and intensity govern how loud it seems, and wavelength is fixed once frequency is fixed, because in a given medium the speed of sound is set and wavelength is speed divided by frequency.

• (b)amplitude — Amplitude sets loudness, not pitch. A loud note and a soft note of the same frequency have the same pitch and different amplitudes.
• (c)wavelength — Tempting, because in one fixed medium wavelength and frequency track each other. But wavelength changes when the sound moves into a different medium while the pitch you hear does not, so frequency is the property that owns pitch.
• (d)intensity — Intensity is sound energy crossing unit area per second. Like amplitude it belongs to loudness — it is why a sound fades as you walk away from its source, without the note changing.

A sound wave has three characteristics a listener can tell apart. Loudness follows amplitude and intensity. Pitch follows frequency. Quality, or timbre, is what lets you distinguish a violin from a flute playing the same note at the same loudness, and it comes from the mixture of frequencies present. A sound of a single frequency is called a tone; a pleasant mixture of frequencies is a note.

This is one of NCERT's own in-text questions — the chapter asks which wave property determines loudness and which determines pitch — so the paper is checking a definition rather than a derivation. Wavelength is the option that costs marks, because for a fixed medium it is a perfect stand-in for frequency and students remember the relation v equals frequency times wavelength. The discriminating case is a sound crossing from air into water: its speed and wavelength both jump, its frequency does not, and neither does its pitch. Frequency is the property the source imposes; wavelength is what the medium makes of it.

• NCERT defines pitch as the brain's interpretation of the frequency of an emitted sound.
• Loudness is governed by amplitude; a louder sound of the same frequency has a larger amplitude.
• For a wave, speed equals frequency times wavelength, so wavelength follows once the medium and the frequency are fixed.
• Quality or timbre distinguishes two sounds of the same pitch and loudness.
• The audible range for humans runs roughly from 20 Hz to 20,000 Hz; above it is ultrasound, below it infrasound.

• Choosing wavelength, which mirrors frequency only while the medium stays the same.
• Mixing loudness with pitch — a shout and a whisper can carry the same note.
• Assuming a higher-pitched sound travels faster; in a given medium all frequencies travel at the same speed.

As a direct property-to-characteristic item, sometimes turned round to ask which property determines loudness instead.

• practice — not a real PYQ

  Two sounds of the same frequency are produced, one loud and one soft. The two differ mainly in

  • (a)pitch
  • (b)amplitude
  • (c)speed
  • (d)wavelength

  Answer(b) amplitude — loudness follows amplitude, while pitch, speed and wavelength are all fixed once the frequency and the medium are fixed.

• practice — not a real PYQ

  A sound wave passes from air into water. Which one of the following does not change?

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

  Answer(c) Frequency — it is set by the source, so it carries across the boundary while speed and wavelength both change.