Motivation / Problem

The effect volume slider has barely any effect on perceived volume until down into the low values.

Description

This is because perceived audio loudness is non-linear, but our code treats it as such,

This is approximately documented here: https://www.dr-lex.be/info-stuff/volumecontrols.html

This change introduces x^3 scaling rather than the suggested x^4 or x^5 scaling, as this "feels" fine and whereas the latter curves seem excessive. This is just a game, not professional audio mixing software, after all. It also feels similar to how the music volume slider already works, at least for the soft-synth on Windows.

And additional change to increase the permitted sample range values to avoid clipping is included, but the volume slider change doesn't have any bearing on that.

Limitations

Existing volume adjustment will probably be too quiet now, however the level can easily be adjusted up.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker