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## Copyright (C) 2024 John Donoghue <john.donoghue@ieee.org>
##
## This program is free software: you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation, either version 3 of the License, or
## (at your option) any later version.
##
## This program is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with this program. If not, see
## <https://www.gnu.org/licenses/>.
## -*- texinfo -*-
## @deftypefn {} {@var{sone} =} phon2sone (@var{phon})
## @deftypefnx {} {@var{sone} =} phon2sone (@var{phon}, @var{standard})
## Convert from phon to sone
##
## @subsubheading Inputs
## @var{phon} - Loudness level in phon
##
## @var{standard} - Standard to use in conversion. Options are
## 'ISO 532-1' or 'ISO 532-2'.
##
## 'ISO 532-1' is used if no standard is provided.
##
## @subsubheading Outputs
## @var{sone} - Loudness level in sone
##
## @subsubheading Examples
## Convert 100 phon to sone
## @example
## @code{sone = phon2sone(100)}
## @end example
##
## @subsubheading References
## International Organization for Standardization., @cite{ISO 532-1 Acoustics – Methods for calculating loudness – Part 1: Zwicker method.}
##
## International Organization for Standardization., @cite{ISO 532-2 Acoustics – Methods for calculating loudness – Part 2: Moore-Glasberg method.}
##
## https://sengpielaudio.com/calculatorSonephon.htm
##
## @seealso {sone2phon}
## @end deftypefn
function sone = phon2sone(phon, standard='ISO 532-1')
if nargin < 1
show_usage()
endif
if ! (isnumeric(phon) && isreal(phon))
error ("Expected phon as a real scalar or vector");
endif
if ! ischar(standard)
error ("Expected standard as a string");
endif
standard = lower(standard);
if standard != 'iso 532-1' && standard != 'iso 532-2'
error ("Uknown standard specified - expected 'iso 532-1' or 'iso 532-2'");
endif
if standard == 'iso 532-1'
idx1 = find(phon<40);
idx2 = find(phon>=40);
sone(idx1) = (phon(idx1) / 40) .^ 2.642;
sone(idx2) = 2 .^ ((phon(idx2) - 40) / 10);
else
lookup_tbl = [
0.0 0.001;
2.2 0.004;
4.0 0.008;
5.0 0.010;
7.5 0.019;
10.0 0.031;
15.0 0.073;
20.0 0.146;
25.0 0.26;
30.0 0.43;
35.0 0.67;
40.0 1.00;
45.0 1.46;
50.0 2.09;
55.0 2.96;
60.0 4.14;
65.0 5.77;
70.0 8.04;
75.0 11.2;
80.0 15.8;
85.0 22.7;
90.0 32.9;
95.0 47.7;
100.0 69.6;
105.0 102.0;
110.0 151.0;
115.0 225.0;
120.0 337.6;
];
idx1 = find(phon<0.0);
idx2 = find(phon>=0 & phon<120);
idx3 = find(phon>=120);
sone(idx1) = 0.0;
sone(idx2) = interp1(lookup_tbl(:,1), lookup_tbl(:,2), phon(idx2), 'linear');
sone(idx3) = 337.6;
endif
endfunction
%!error phon2sone()
%!error phon2sone('a')
%!error phon2sone([100], 'invalid')
%!assert(phon2sone(80), 16, 100*eps)
%!assert(phon2sone(80, 'ISO 532-1'), 16, 100*eps)
%!assert(phon2sone(80, 'ISO 532-2'), 15.8, 100*eps)
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