gain(gain, time) {
    var t = time || 0;
    if (typeof gain === 'number') {
      this.biquad.gain.value = gain;
      this.biquad.gain.cancelScheduledValues(this.ac.currentTime + 0.01 + t);
      this.biquad.gain.linearRampToValueAtTime(
        gain,
        this.ac.currentTime + 0.02 + t
      );
    } else if (gain) {
      gain.connect(this.biquad.gain);
    }
    return this.biquad.gain.value;
  }
Controls the gain attribute of a Biquad Filter.
This is distinctly different from .amp() which is inherited from p5.Effect
.amp() controls the volume via the output gain node
p5.Filter.gain() controls the gain parameter of a Biquad Filter node.

@method gain
@param  {Number} gain
@return {Number} Returns the current or updated gain value