Synesthesia is indeed a neurodivergent perception. Some people "see" sound in the sense that sound invokes a perception of e.g. colours, but that doesn't meant they can literally "see" sound.That’s impossible. Sound is a varying pressure wave traveling through a medium (air in our case). Sound waves do not magically change the chemical composition of the air they travel through, so there is literally no difference to smell or taste. Synaesthetes therefore (completely literally!) cannot not sense sound with their nose or tongue (smell or taste sound), they just have the perception/experience of smelling sound when their auditory system is stimulated, without actually/literally smelling it.
“synaesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway.” (Wikipedia) - Synaesthesia is a perceptual error, specifically NOT literally smelling, tasting or seeing sound.
However, maybe there’s some synaesthete audiophiles out there who can be suckered with a nice $1,000 INM (In Nose Monitor), $2,000 for stereo INMs (one for each nostril, immersive smell!) or $2,500 for the fully balanced, factory burnt-in version? lol
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In order to "see" sound (or rather see shock waves in air) we need help from other equipment, such as Schlieren optics.
One of my favourite musicians has synesthesia since a car accident when she was 16. To her some sounds give a colour perception, and the songs manifest themselves in her head, pretty much involuntary. Her music is unique. In her own words: "I don't write these songs; these songs write me".