As far as I know, the North Safety Valve - which did have a mechanical operation with a small diaphragm closing off air passage, is no longer available anywhere. That is the only product that I am aware of which had a mechanical operation.

All of the other products that make claims about level selective or non-linear attenuation, are using some form of acoustic impedance change that is related to the overall SPL. Most of these function with small-orifice acoustic physics. This is similar to how a fluid-filled automobile shock absorber works. The acoustic particle velocity increases with increasing SPL. This velocity increase makes it more difficult for air molecules to travel back and forth through the small orifice, increasing the impedance of the orifice. Just like with an old oil-filled shock absorber, moving the ends of the shock absorber slowly allows one to compress it or expand it with typical arm strength, but attempt to move the ends together or apart quickly and all of a sudden it feels like it's a solid, unmovable cylinder.

Also, most of the non-linear attenuating devices do not have a very flat attenuation curve.

-Scott

My Songs, photos, writing,etc.
My Sound Cloud