3.7 Qualities of sound

- Audible sound or human response to sound: Whenever we talk about audible sound, what matters is how we perceive it.

- This is purely a subjective attribute of sound waves.

- Major qualities of sound that are of our interest are

(i) Pitch,

(ii) Timbre or quality and

(iii) Loudness.

(i) Pitch:

- This aspect refers to sharpness or shrillness of the sound.

- If the frequency of sound is increased, what we perceive is the increase in the pitch or we feel the sound to be sharper.

- Tone refers to the single frequency of that wave while a note may contain one or more than one tones.

- We use the words high pitch or high tones if frequency is higher.

- As sharpness is a subjective term, sentences like “sound of double frequency is doubly sharp” make no sense.

- Also, a high pitch sound need not be louder.

- Tones of guitar are sharper than that of a base guitar, sound of tabla is sharper than that of a dagga, (in general) female sound is sharper than that of a male sound and so on.

- For a sound amplifier (or equaliser) when we raise the treble knob (or treble Button), high frequencies are boosted and if we raise bass knob, low requencies are boosted.

(ii) Timbre (sound quality):

- During telephonic conversation with a friend, (mostly) you are able to know who is speaking at the other end even if you are not told about who is speaking.

- Quite often we say, “Couldn’t you recognise the voice?” The sound quality in this context is called timbre.

- Same song played on a guitar, a violin, a harmonium or a piano feel significantly different and we can easily identify that instrument.

- Quality of sound of any sound instrument (including our vocal organ) depends upon the mixture of tones and overtones in the sound generated by that instrument.

- Even our own sound quality during morning (after we get up) and in the evening is different.

- It is drastically affected if we are suffering from cold or cough.

- Concept of overtones will be discussed during XIIth standard.

(iii) Loudness:

- Intensity of a wave is a measurable quantity which is proportional to square of the amplitude (I A2) and is measured in the (SI) unit of W/m^2.

- Human perception of intensity of sound is loudness.

- Obviously, if intensity is more, loudness is more.

- The human response to intensity is not linear, i.e., a sound of double intensity is louder but not doubly loud.

- This is also valid for brightness of light.

- In both cases, the response is approximately logarithmic.

- Using this property, the loudness (and brightness) can be measured.