Background: The standard by which Dynamic Headroom is measured has been set by IHF which was replaced by the EIA, and is now the CEA. This text was found on the Audio Engineering Society web site, which I have in high estime. The link is
http://www.aes.org/e-lib/browse.cfm?elib=4898
So it appears that the standard is 20ms pulses. (May be even only 1 pulse) Knowing how manufacturers go after appearances first, I think we can assume that this is how they measure their DH.
In another article by Meyer Sound at
http://www.meyersound.com/support/papers/amp_power.htm, it appears that even from a temporary high power audio signal, that 20ms just doesn't cut it:
When we think about it, how much does a loud movie sound like an explosion lasts, may be 1-2 seconds. 20ms represents ONE cycle of a 50Hz sine wave, which would be probably be unrecognizable as a sound.
I found other research finding anywhere between 80ms-300ms, but never under 80ms. So it appears that the believers of DH are victims of deception with the 20ms measurement, may be as much as people who do not believe in DH. Hey, misery loves company