AmpCompA ANSI A-weighting curve

basic psychoacoustic amplitude compensation

superclass: UGen

Higher frequencies are normally perceived as louder, which AmpCompA compensates. 

Following the measurings by Fletcher and Munson, the ANSI standard describes 

a function for loudness vs. frequency. 

Note that this curve is only valid for standardized amplitude. [1]

For a simpler but more flexible curve, see AmpComp

*ar(freq, root, minAmp, rootAmp)

*kr(freq, root, minAmp, rootAmp)

*ir(freq, root, minAmp, rootAmp)

freq input frequency value. For freq == root, the output is rootAmp. (default freq 0 Hz)

root root freq relative to which the curve is calculated (usually lowest freq) (default 0 Hz)

default value: C (60.midicps)

minAmp amplitude at the minimum point of the curve (around 2512 Hz) (default -10dB)

rootAmp amplitude at the root frequency. (default 1)

apart from freq, the values are not modulatable

// 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)

 };

)