code: function Envelope (atk, dcy, aGrad, dGrad) {
    this._atk = (atk || 0) * 44000
    this._dcy = (dcy || 1) * 44000
    this.aGrad = aGrad || 2
    this.dGrad = dGrad || aGrad || 2
    this._total = this._atk + this._dcy
    this.t = null
  }
docs: attack and decay in seconds (or beats ?)