function stereo_demask(f) {
        /* setup stereo demasking thresholds */
        /* formula reverse enginerred from plot in paper */
        var arg = freq2bark(f);
        arg = (Math.min(arg, 15.5) / 15.5);

        return Math.pow(10.0,
            1.25 * (1 - Math.cos(Math.PI * arg)) - 2.5);
    }
NOTE: the bitrate reduction from the inter-channel masking effect is low
compared to the chance of getting annyoing artefacts. L3psycho_anal_vbr
does not use this feature. (Robert 071216)