REL Carbon Special Subwoofers or the REL G1 Gibraltar Reference Subwoo

b-db

Very similar to what happens when you toss a large rock into calm water. The first wave is sent out, and smaller and smaller waves follow. An no, manufactures will not tell you their speakers often double. You might want to do a little research on this. 

s_r-a,   I'm a hack, so I just googled, 'how is subwoofer speed measured?' What turned up was subwoofer distortion, performance, phase, and port length, nothing on measuring a subwoofers speed. Can you suggest another search?

If I were a subwoofer manufacturer whose sub measured faster than brand W, publishing the measurements or their attributes would seem to be valuable marketing.

Could what you call doubling be a result of a drivers back wave? Your rock in the water analogy seems appropriate for reflex and open baffle subwoofers. Most people can easily hear the difference between a ported and a sealed sub.

Pressure created by the back wave within a sealed cabinet subwoofer appears to have a noticeably different reaction. Sealed subwoofers with increasingly more robust driver motors, servo limiting and those using proprietary equalization have lowered measured distortion dramatically.

This link seemed an interesting primer albeit from a manufacturer:

https://jlaudio.zendesk.com/hc/en-us/articles/204374170-Isobaric-Enclosure-Types

m-db

No manufacture will admit that their sub woofers double....I am retired from business as a small focused dealer. Have nothing to sell. If you want to know what bass should sound like google Wilson Benesch torus...there is so much technology there.