I know that some of you are familiar with the Spectrum Lab application that you can download for free at:
http://www.qsl.net/dl4yhf/spectra1.htmlAttached is a screen capture from Spectrum Lab ... in use to produce a 3D "spectrogram" (spectrum v. time) of the audio output of one of my receivers*.
*I was using my SDR-1000 set to synchronous AM mode, with 8kHz of bandwidth (+/-4kHz), and with the received signal centered in the passband; but, of course, it doesn't matter what receiver, or other audio source I use to produce the audio signal whose spectrum is displayed.
The 2D (power spectral density v. audio frequency) spectrum is painted every 300ms... and the total time shown is 140 seconds... where the current time is 0 seconds.
The amplitude is shown in colors.. where I have adjusted the coloring so that the background audio noise level is blue. You can see in the box at the lower left how the color (as currently adjusted) varies with the relative power spectral density in dB.
As you can see, the +/-4kHz brick wall i.f. filtering of the SDR-1000 is limiting the audio spectrum of the receiver's output to 4kHz. You can also see that the vocal tract of the person whose signal I was listening to has some natural frequencies where the power spectrum tends to peak up.