Information must still be gettable when a sensory channel is missing

What it is

Turn the display off. Is “ticket reissued” still in the world? If that fact lives only as a green badge, it is not — for anyone who can only hear or feel right now, it was never emitted. Perceivable asks whether the same fact is encoded in a sensory channel the person actually has. It does not ask whether a control can be fired, or whether firing it makes sense.

This is routinely collapsed into “remember alt text.” Alt text is one repair when visual encoding is absent. Colour-only legends, icon-only status, audio CAPTCHAs, station announcements with a speaker and no captions fail the same way: the meaning is welded to a single medium.

Why it happens

Sensory channels are not backup copies of one picture. They are different physical carriers. Vision rides photons and spatial layout; hearing rides sound in time; touch rides contact and vibration. Write the meaning into only one carrier and the others contain nothing — not a worse copy, never transmitted.

The second layer is who chooses the channel. The body and the situation choose; the product does not get to write “has vision, sits in a quiet lit room” as the world’s default. Glare, a silent carriage, glasses taken off, turn a so-called typical user into a missing-channel user for a while. Perceivable holds when equivalent information exists across channels, not when a charitable side door is bolted onto a visual original.

Studying it

Use channel deprivation, not satisfaction. Run the same task with the screen covered, with audio off, and on a greyscale print, and ask whether the fact is still reportable. Measure contrast on the pixels actually composited (including translucency). Hear images by name only, through a screen reader. Toggle captions on a clip.

Independent variables: encoding medium (colour only / shape only / sound only / multi-channel), environment (glare, noise, silence). Dependent variables: whether the target fact can be reported, whether the report matches the visual meaning, which deprivation condition produces the miss.

WCAG-EM sampling can tick whether non-text content, captions, or contrast have counterparts. A tick on a sampled page is not the same as “the sentence that decides the next step still exists after one channel is removed.” Automation can see an alt attribute; it cannot hear whether the trend in the chart arrived.

Where it stops holding

Decorative marks carry no fact; removing a channel should not invent meaning, and they should not be spoken as noise. Alternatives for live material lag — live captions and interpreting arrive late, so perceivable becomes “perceivable after a delay,” and delay can kill the fact’s usefulness. Forcing ambient sound or background video into every channel buries the real message. Smell and vestibular sense barely have on-screen equivalents; POUR does not cover every sense just because the word “sensory” appears.

Applying it

• For every fact someone must know, provide an encoding that does not depend on one sense: equivalent text for images, shape or words with colour, captions or a visible state with sound.
• Status that decides the next step — CAPTCHA, errors, stock, “reissued” — must not live in colour alone or sound alone.
• Compute contrast on composited pixels, not on the opaque swatch in the file.
• How to check: cover the screen and listen through the image names and states; mute the speakers and read captions as standalone sentences; print greyscale and ask someone new “which row is the warning.” Each missing fact is still welded to one channel.

Related

• Same group: J1.01.2 Every function must be triggerable from an input method the person can use · J1.01.3 Presentation is not the same as being able to understand the information and the action · J1.01.4 Content has to remain parsable by the assistive technologies people actually use
• Nearby: J2.04 Alternative text · J4.01 Captions · J5.01 Screen readers
• Search terms: perceivable · POUR · sensory channel