Function durationToSeconds

durationToSeconds(startBeat: number, lengthBeats: number, map: TempoMap): number
Duration in seconds of a span of beats.

Equal to the difference of two beatsToSeconds calls — the integral is additive, so that reading is exact — but integrated over the span directly, so a short note late in a long piece does not lose precision to the subtraction of two large elapsed times.
Parameters
- startBeat: number
  Where the span begins, in quarter-note beats; a pickup starts before the map's first event and is measured like any other span.
- lengthBeats: number
  How long the span lasts, in quarter-note beats.
- map: TempoMap
  The tempo map.
Returns number
The span's duration in seconds, never negative.
Throws
If the map is malformed or the length is negative.
Example
```
import { durationToSeconds } from '@libraz/libcantus';
const map = [{ startBeat: 0, bpm: 120 }, { startBeat: 4, bpm: 60 }];
durationToSeconds(2, 4, map); // 3 = 2 beats at 120 + 2 beats at 60
```
- Defined in src/core/tempo/index.ts:260