Euphonism

Phonaesthetics (from the Greek: φωνή phōnē, "voice-sound"; and αἰσθητική aisthētikē, "aesthetics") is a branch of phonetics concerned with "the possible connection between sound sequences and meaning", according to Raymond Hickey. Linguist David Crystal defines phonaesthetics as "a term sometimes used in linguistics to refer to the study of the aesthetic properties of sound". According to Crystal:

Examples include the implication of smallness in the close vowels of such words as teeny weeny, and the unpleasant associations of the consonant cluster sl- in such words as slime, slug, and slush.

Euphony is used for effects which are pleasant, rhythmical and harmonious. An example of euphony is the poem Some Sweet Day.

Some day Love shall claim his own
Some day Right ascend his throne,
Some day hidden Truth be known;
Some day—some sweet day.

Cacophony consists of harsh, often discordant sounds. These sounds are often meaningless and jumbled together. A discordant series of harsh, unpleasant sounds helps to convey disorder. This is often furthered by the combined effect of the meaning and the difficulty of pronunciation. For example:

My stick fingers click with a snicker
And, chuckling, they knuckle the keys;
Light-footed, my steel feelers flicker
And pluck from these keys melodies.

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