Works of category D have been given various names over time, such as “visual music”, “videomusic”, “computer music video”, "electroacoustic audio-visual music", or simply “audiovisual composition/art”.
In particular, “visual music” (e.g. Scriabin, Stan Brakhage, Norman MacLaren, James Whitney, Jordan Belson, Oskar Fischinger) often refers to a tradition that seeks to represent sound through images and vice versa in a semi-scientific way, with 1:1 correspondences and parametric relationships (e.g. between color and pitch, size and volume), while "videomusic” (a term coined by Jean Piché and Tom Sherman around 1987-89) considers these equivalences flawed, arguing that the ways in which we see and hear are fundamentally different, with time being the only thing that sound and (moving) image have in common.
Works in category D, particularly videomusic, are often influenced by the tradition of electroacoustic music and its subset of acousmatic music [1], and can be described as “organized sound and image”, following Varese’s definition of music as “organized sound”.
The addition of moving images to acousmatic music can be seen by some as a subversion of acousmatic tradition, polluting sound with visual accompaniment. This concern is supported by the natural tendency of vision to dominate other senses (“visual dominance effect” or “Colavita effect”) and is reflected in mainstream cinema, where music mostly serves as an emotional support to images and storytelling.
On the other hand, acousmatic music offers an opportunity. With the invention of the loudspeaker, where the mechanics of the emission of sound are not visible, and with music composed specifically for the loudspeaker, we encounter a remarkable case of music that does not have by design any visual element, as opposed to music meant to have some (e.g. musicians playing an instrument) but occasionally reproduced with none (e.g. music broadcast on the radio), or music meant to accompany other activities (e.g. dancing). This distinctiveness allows for the enrichment of acousmatic music with visuals not traditionally linked to music listening, developing the acousmatic experience into something more, not something less.
As suggested by composer Andrew Lewis, audiovisual compositions pose the question: if we could see beyond the acousmatic veil of the loudspeakers, what might we see?
[1] The term "acousmatic" comes from the Greek akousmatikoi (hearers) and refers to Pythagoras' students listening to his lessons (allegedly) delivered from behind a veil, for their better concentration. This term was first used by composer Pierre Schaeffer in the 1960s to conceptualise a form of ("reduced") listening focused on the content of sound rather than its cause, but it is often used just to refer to music composed specifically to be reproduced on loudspeakers, without any human performance.