The software

Available from :

Spectrum Analyzer with Waterfall Display and real-time audio processing

This program started as a simple FFT program running under DOS a long time ago, but it is now a specialized audio analyzer, filter, frequency converter, hum filter, data logger etc (see history). You can download it from this site. Or look into the manual (in HTML format), even though the manual included in the archive will be more up-to-date. Furthermore, the same manual has occasionally been converted into a single PDF (SpecLab_Manual.pdf), but any attempt to create a common index and table of contents for this PDF, using OpenOffice (with proper page numbers instead of the hyperlinks) has failed miserably – see note in the preface of the PDF document.
If you are looking for a short description in German language, look here. The revision history is here. How to use SpecLab with SDR-IQ is described here; how to use SpecLab with PERSEUS (without an annoying virtual audio cable) is here, and how to use SpecLab with software-defined radios supporting ExtIO (instead of a soundcard), like FiFi-SDR, RTL-SDR, FunCube, SDRplay RSP, etc etc, is described here.