How LC480 was made
LC480 is modeled on the Lexicon 480L. A reverb of that kind is thousands of reflections, and a decay time is one number. We went after the reflections, checking them against recordings of real units.
Right decay, wrong reverb
An early version of LC480 gave Large Hall a decay of 2.34 seconds. The real unit's is 2.38. On a spec sheet, that is a match.
Then we laid its waveform over the recording and scored the fit from 0 to 1, where 1 is the same shape. It scored 0.67: the right length of reverb with the wrong reflections inside it. Today's version has 2.38 seconds and 0.996. From then on, a good decay time proved nothing on its own.
How a space dies away
A long reverb does not fade evenly. In Large Church the low end rings for almost six seconds and the top for about two, so the sound darkens as it goes. LC480 stays within a percent of two separate sets of recordings of that program, in every octave from 125 Hz to 8 kHz.
The last gap
The last gap was at the very top, in the shape of each single reflection. Closing it took more than 10,000 comparisons with the recordings, and it moved the median match across one set from 0.90 to 0.99.
What came out
The chart shows four factory programs, the recording in blue and LC480 on top in orange. Across the halls, rooms, plates and wild spaces of that set, the median match is 0.99 and mid-range decay times differ by 0.02 percent.
