class:: AmpCompA
summary:: Basic psychoacoustic amplitude compensation (ANSI A-weighting curve).
related:: Classes/AmpComp
categories::  UGens>Analysis>Amplitude


Description::

Higher frequencies are normally perceived as louder, which AmpCompA
compensates. Following the measurements by Fletcher and Munson, the
ANSI standard describes a function for loudness vs. frequency.

Note that this curve is only valid for standardized amplitude.
footnote::
Function freq → dB,
derived from http://www.beis.de/Elektronik/AudioMeasure/WeightingFilters.html
and modified to map freq → amp.
code::
(
var k =  3.5041384e16;
var c1 = 424.31867740601;
var c2 = 11589.093052022;
var c3 = 544440.67046057;
var c4 = 148698928.24309;
f = {|f|
var r = squared(f);
var m1 = pow(r,4);
var n1 = squared(c1 + r);
var n2 = c2 + r;
var n3 = c3 + r;
var n4 = squared(c4 + r);
var level = k * m1 / (n1 * n2 * n3 * n4);
sqrt(level)
};
)
::

::


For a simpler but more flexible curve, see  link::Classes/AmpComp::

classmethods::

method::ar, kr, ir

argument::freq
Input frequency value. For freq == root, the output is rootAmp.

argument::root
Root freq relative to which the curve is calculated (usually lowest freq).

argument::minAmp
Amplitude at the minimum point of the curve (around 2512 Hz).

argument::rootAmp
Amplitude at the root frequency.

discussion::
Apart from code::freq::, the values are not modulatable

Examples::

code::

// compare a sine without compensation

{ SinOsc.ar(MouseX.kr(300, 15000, 1)) * 0.1 }.play;

// with one that uses amplitude compensation
(
{
	var freq;
	freq = MouseX.kr(300, 15000, 1);
	SinOsc.ar(freq) * 0.3 * AmpCompA.kr(freq)
}.play;
)


// adjust the minimum and root amp
// (in this way one can flatten out the curve for higher amplitudes)

(
{
	var freq;
	freq = MouseX.kr(300, 18000, 1);
	Formant.ar(300, freq, 20, 0.1) * AmpCompA.kr(freq, 300, 0.6, 0.3)
}.play;
)

// the curve:

{ AmpCompA.ar(Line.ar(48, 120, 1).midicps, 48.midicps) }.plot(1.0);

// freqs:

{ AmpCompA.ar(Line.ar(0, 20000, 1)) }.plot(1.0);

// compare with AmpComp (exponential decay)

{ AmpComp.ar(Line.ar(48, 120, 1).midicps, 48.midicps) }.plot(1.0);

// freqs:

{ AmpComp.ar(Line.ar(40, 20000, 1), 40) }.plot(1.0);



// amplitude compensation in frequency modulation (using Fletscher-Munson curve)
(
{
	var freq;
	freq = MouseX.kr(300, 15000, 1);
	freq = freq * SinOsc.ar(MouseY.kr(3, 200, 1), 0, 0.5, 1);
	SinOsc.ar(freq) * 0.1 * AmpCompA.ar(freq, 300)
}.play;
)

// amplitude compensation in frequency modulation (using AmpComp exponential decay)
(
{
	var freq;
	freq = MouseX.kr(300, 15000, 1);
	freq = freq * SinOsc.ar(MouseY.kr(3, 200, 1), 0, 0.5, 1);
	SinOsc.ar(freq) * 0.1 * AmpComp.ar(freq, 300)
}.play;
)


// without amplitude compensation
(
{
	var freq;
	freq = MouseX.kr(300, 15000, 1);
	freq = freq * SinOsc.ar(MouseY.kr(3, 200, 1), 0, 0.5, 1);
	SinOsc.ar(freq) * 0.1
}.play;
)





[1] Function freq -> dB,
	derived from http://www.beis.de/Elektronik/AudioMeasure/WeightingFilters.html
	and modified to map freq -> amp

(
var k =  3.5041384e16;
var c1 = 424.31867740601;
var c2 = 11589.093052022;
var c3 = 544440.67046057;
var c4 = 148698928.24309;
f = {|f|
  var r = squared(f);
  var m1 = pow(r,4);
  var n1 = squared(c1 + r);
  var n2 = c2 + r;
  var n3 = c3 + r;
  var n4 = squared(c4 + r);
  var level = k * m1 / (n1 * n2 * n3 * n4);
  sqrt(level)
 };
)

::