this may sound straightforward, but could someone please explain the units on the y axis of the waveform spectrums - perhaps these should be more clearly marked on the programme, I am using Audacity ® 1.3.6 (Unicode). What are the units?, how is the waveform displayed? how is this calculated? should there be a 0 value on the logarithmic decibel scale? Please stick to layman’s terms and respond quickly!!! 
The waveforms represent the motion or vibration of the air during sound. It’s a picture of your sound performance.
The default presentation is in percent, zero to 100 (or 1 on the scale). It’s handy to view the waveform like that and it’s totally logical, but it’s not how your ear works. Your ear perceives half loudness or volume at about 12%.
The other display is more correctly dBFS, dB Full Scale, not just dB. Zero dBFS is the point that the digital signal is so loud that it can no longer increase. It’s a mathematically precise place. So yes, the two zeros don’t line up at all. dB is handy because that’s how your ear works. -18dB is about half volume, and some people (young women) can still hear things at -60. That’s 0.1%.
dB works best if you make the waves really tall. Grab the display by the bottom and pull. You can also split them. Make the top one taller than the bottom.
Koz
Thank you so much!