Both tones where produced with Bb harps (2-draw, F = 349Hz = 3.49 on the x-axes), at the same pressures and 40 cm distance from a condensor microphone with no cover plates in order to make it more comparable.
You can hear and see the difference!
Any tone consists of the fundamental frequency plus harmonious and disharmonious overtones.
There are much more overtones produced with stainless steel reeds (see range 1600-4000 Hz). This is one of the reasons for the fuller (or more bluesy) sound of stainless steel reeds!
Fig.1: Power spectra: 2-draw (fundamental frequency of is F=349 Hz, mean of ca. 1/3 second, see envelopes below)
Sound examples:
small:Session Bb vs. 1847 Bb, 2-draw, no covers
small:Session Session Bb vs. 1847 Bb, no covers
Envelopes:
Fig. 2: Signal on the left: Brass-reeds (blue in the upper panel), signal on the right: stainless steel reeds (red in the upper panel)