Well stated, Bill. If one hits a thin face iron such as those of Wishon Golf 771, which I own, there is a distinct ping sound. Some do not like that sound. It is just like a driver that has a ping to it. Wishon irons are not hollow, and the thin face is not damped. When I hit a hollow body iron with an injected material, such as my S-2’s, the sound is quite different. PXG indicated or implied their injected polymer adds distance. This is utter torro waste. If that were the case, every driver company would be putting polymers in their heads. As Bill honestly and factually indicates, the filler is about sound and feel (less vibration or resonance) as a result of its dampening properties. I make DIY loudspeakers. If one does not dampen the enclosure (cabinet), the speaker will resonate (ring) like a bell.