NCERT Solutions for Class 9th Science Chapter 10 Let us experiment (demonstration activity) — Activity 10.7

Book page 194 Updated on2026-09-08

Q1.

Compare the musical notes by taking the ratio of each frequency with respect to the 'Sa'. Do you observe any pattern?

Answer

Yes — the frequencies are not spaced equally, but their ratios to Sa are simple fractions, and the upper Sa comes out at exactly twice the lower Sa.

Note	Typical measured frequency	Ratio to Sa	Simple fraction
Sa	240 Hz	1.00	1
Re	270 Hz	1.13	9/8
Ga	300 Hz	1.25	5/4
Ma	320 Hz	1.33	4/3
Pa	360 Hz	1.50	3/2
Dha	400 Hz	1.67	5/3
Ni	450 Hz	1.88	15/8
Sa (upper)	480 Hz	2.00	2

Ratio for Pa = 360 Hz ÷ 240 Hz = 1.5 = 3/2
Ratio for upper Sa = 480 Hz ÷ 240 Hz = 2.0

The pattern: frequency rises from Sa to the upper Sa, and each step is a simple whole-number ratio. Because the upper Sa is exactly double the lower Sa, the two ends of the scale form an octave — an interval between two notes where one has twice the fundamental frequency of the other (for example 200 Hz and 400 Hz). This is why the two Sa's sound like 'the same note, higher up'.

Tip: your own numbers will not match the table exactly, because the pitch you start on depends on your voice or on the app's setting. What must come out right are the ratios, especially the 2 : 1 for the octave.

Q2.

If both voice and mobile-generated notes are used, compare their frequencies for the same musical notes.

Answer

For the same named note the two frequencies come out close but usually not identical, and the ratios between notes still follow the same pattern.

The app produces an almost pure single frequency, so its reading is steady and sharp on the spectrum.
Your voice gives a fundamental frequency plus overtones, so the spectrum shows one strong low peak with several weaker higher peaks. The fundamental is the one to compare.
The voice reading also wobbles by a few hertz because it is hard to hold a note perfectly steady.

Why the two can differ and still be 'the same note': what our ear judges as a note is fixed mainly by the fundamental frequency. If you sing Sa at 245 Hz while the app plays 240 Hz, the note is recognisably the same, and Pa will still be about 1.5 times your own Sa. What differs is the timbre — the pattern and strength of the overtones — which is exactly why a flute, an ektara and a human voice playing the same note at the same loudness still sound completely different.

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