How can I get 2 signal differences in logarithmic scale?

Hello all! I have a problem, hope you can help me! I have 2 signals in logarithmic scale, their power levels are different. The plot is here: <https://www.dropbox.com/s/17dkngq0a4dqzgc/plot.bmp>

I need to subtract this 2 signals and get their power level differences in dB, for example, for 1K level difference is appx. 30 dB. All results I need to plot in log scale as well. I used 10^(db/20) to get them into power scale, but do not know how to get them back to dB scale. So, which command to choose to subtract them in log-scale??

Thank you in advance!