Thanks for replies on "what scale to use". For viewing the details of the sound in the higher frequencies the difference is not big. But it might help to stretch the band around the 20kHz. A lot of sound there what looks like noise. Stretching it (by use a logaritmic scale) + treshold (-3 dB point of the high-pass filter would be a good starter) like @wmxz says would probably help to get a better spread on the screen. Just my two cents from the field I am more familiar with (musical sound engineering).