code: func pianoAt(i int, freq float64) float64 {
  	// Create piano-like waves with multiple sin waves.
  	amp := []float64{1.0, 0.8, 0.6, 0.4, 0.2}
  	x := []float64{4.0, 2.0, 1.0, 0.5, 0.25}
  	v := 0.0
  	for j := 0; j < len(amp); j++ {
  		// Decay
  		a := amp[j] * math.Exp(-5*float64(i)*freq/baseFreq/(x[j]*sampleRate))
  		v += a * math.Sin(2.0*math.Pi*float64(i)*freq*float64(j+1)/sampleRate)
  	}
  	return v / 5.0
  }
docs: // pianoAt returns an i-th sample of piano with the given frequency.