#199 Correct dB Scale

[Griffin Cerva]

Please correct me if I'm wrong, but I believe there is a mistake with the dB scale calculation. Values below 0 dBFS are only half of what they should be (for example -6 dBFS as shown in Audacity is only shown as -3 dBFS in Sonic Visualizer). For reference, the decibel drop for a voltage should be equal to 20log10(V2/V1). The current implementation suggests that 10log10(V2/V1) is being used instead.