I've been saving spectrograms as png files.  They're 60 pixels per second horizontally and 5 bands per half-step vertically.

Each pixel is a 24-bit number.  This covers 10*log10(2^24) dB=72.24dB, which is pretty good.  I guess I could use the alpha bit to kick it up to a 32 bit number (96dB) but I like the fact that it's human readable: