Sonification

A sonified chart plays the series instead of describing it: each value becomes a tone, usually with pitch carrying the value and time carrying the axis, so twelve months of revenue take about three seconds to hear. What that conveys is shape. A steady climb sounds like a climb, a spike sounds like a spike, a plateau sounds like nothing changing, and a reader who has heard the sweep once knows where in the series to ask a question. It is the same trick a heart monitor plays, and it works for the same reason: hearing is very good at change over time.

What it conveys badly is values. Pitch discrimination is coarse, absolute pitch is rare, and nobody listens to a tone and reports 41.8 percent. This is why sonification is an addition rather than a replacement. The reader who wants the number needs a data table or a long description; the reader who wants to know whether the line went up needs three seconds of audio rather than a paragraph read out one figure at a time. Serve both, from the same data, and let the reader pick.

Design decisions that matter more than the synthesis. Map value to pitch rather than to volume, because loudness is heard as importance and a small value should not sound like a mistake. Keep the mapping monotonic, so higher is always higher, and say what the mapping is before the sweep plays, in the same way a visual chart labels its axis. Mark the axis audibly if the series is long, with a click per month or a change of timbre per quarter, since a reader cannot count fifty tones any more than they can count fifty gridlines. Give a play control with a real accessible name, let the reader replay and step point by point, and never start the audio by itself: unrequested sound is exactly what audio control exists to undo.

The vocabulary is still settling. Audio chart and audio graph usually mean a chart component with sonification built in, sonification means the technique in general, and the phrase data table alternative describes the role it plays rather than the sound it makes. Whichever word you use, the alt text rule underneath does not change: the chart still needs a short text equivalent, because a reader browsing a page of figures is not going to play all of them.