2022 A/L Physics Past Paper · Question 03 (English Medium)

What the quality of a sound depends on

The quality of sound produced by a musical instrument depends on the

Answer and explanation

Answer: (5) existence of overtones in the sound.

What the question asks

Find what the quality of a musical sound depends on.

What each letter means:

$f$ ( $\text{Hz}$ ): the frequency of the sound.

$A$ ( $\text{m}$ ): the amplitude of the sound wave.

$I$ ( $\text{W m}^{-2}$ ): the intensity of the sound.

$\lambda$ ( $\text{m}$ ): the wavelength of the sound.

Quality of a sound means whether it is just one pure sound note or whether it is a mixture of one note with its overtones/harmonics.

Pitch of a sound means how a particular sound note is sensed by the human ear and depends on the frequency of the sound wave.

Loudness at a certain location means the level at which the intensity of sound exists at that location.

Intensity (I) ∝ A²

For example, think of a violin and a piano playing the same note.

Both notes have the same frequency $f$ , so they have the same pitch.

Play both just as loudly. Then the amplitude $A$ and the intensity $I$ are the same too.

You can still tell which one is the violin and which one is the piano.

Each instrument adds its own overtones to the note, in its own mixture. That mixture is the quality.

$I \propto A^2$

In words: a bigger amplitude gives a bigger intensity, so a louder sound.

So $f$ , $A$ and $I$ cannot tell the two instruments apart. Only the overtones can.

The book tells each part.

The frequency decides the pitch.

The amplitude and the intensity decide the loudness.

The quality is the mixture of a note with its overtones.

So the quality depends on the overtones that are present.

The frequency decides the pitch, not the quality.

The amplitude decides the loudness, not the quality.

The intensity decides the loudness, not the quality.

The wavelength goes with the frequency. It tells the pitch, not the quality.

Quality means whether a sound is one pure note or a note mixed with its overtones.

So the quality depends on the existence of overtones.

The answer is (5).

Remember three sound words and what each one depends on.

Loudness goes with amplitude and intensity.

Pitch goes with frequency.

Quality goes with overtones.

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