What characterizes unpleasant sound?

Answer 1

The big picture: "unpleasant" is a completely subjective concept, and what one person considers divine music will be considered impossible to listen to from another's point of view. There has been a long enough history of music so that even among highly 'tonal' systems and techniques there will be widely varying opinions as to what musics include 'pleasant' sound. Ultra modern musics employ literally any auditory stimulus, sometimes randomly, from silence to any conceivable combination of frequencies, amplitude and attack/decay. Is such music made of sound that is "pleasant"? It probably is to those who create it, and to those who understand enough to uncover the intended patterns, if indeed there are any such patterns at all. Atonal or serial music as developed by people like Shoenberg is often played on standard instruments that are in perfect tune, and to initiates or the open minded the music is exquisitely structured and the sounds are very beautiful. Ultimately, what is unpleasant is a matter of taste.

In the context of standard Western musics, some of the general ideas related to pleasantness of sound have to do with operational definitions of tone and noise, and with the standard methods of harmony and counterpoint.

Pure sine wave sounds are interesting in some ways, but a steady diet of nothing but sine wave tones would become sickening in short order. Instruments, including the voice and many percussion instruments, produce tones that are readily identifiable as one of the twelve tones in the diatonic scale. Each instrument produces tones of varying complexity, sometimes including subtle differences in complexity along the instrument's full range. This complexity has a lot to do with the colors and textures we perceive in music. But when tones become so complex that they begin to resemble white noise (analogous to white light resulting from a mix of light from all the primary colors) then in some contexts they would be picked up by most as unpleasant. Even with some faint identity of 'tonal' base in such sounds, combining them into useful harmony would be problematic or impossible.

In terms of the standard 'rules' of harmony, purists would be able to detect many of the forbidden structures and modulations, and to their ears those breaches of the rules are immediately heard unpleasant, even while technically less well informed listener may find the music perfectly charming. Many find stand-alone tritones and minor seconds grating and unpleasant. But playing any kind of music without ever technically including a tritone in a chord structure is all but impossible, and minor seconds are often heartbreakingly powerful, beautiful and essential in some harmonic progressions.

I find this utterly incomprehensible, but some people (and there is no arguing taste) find pianos that are out of tune to be more pleasant to play, because the wavering of the notes resembles vibrato to them. This brings us again to the bottom line: pleasantness is subjective, and largely a matter of taste.

Answer 2

An unpleasant sound could be called any number of things, including annoying.

Answer 3

Bagpipes

Nails against a black board

Violins (if the player doesn't know what their doing)

Charcoal

Answer 4

farts,screams, and toilets flushing