How did they measure -20 db sound? Current world record for quietest room is Microsoft anechoic chamber that was measured at  -20 db.How did they measure it? I mean,lowest noise measurement microphone I found is 1 inch special low noise G.R.A.S condenser that goes down to -2db.
What kind or microphone or measurement technique can possibly be used to measure such extremly weak sounds?
 A: Interesting question! Having visited the previous Guinness Book of World Records "quietest place on earth" at Orfield Labs, I have a personal interest in the answer as well.
To start, there's an article on the Brüel & Kjær website, which describes the instruments and technique used.

The measurement method was specified by Guinness, and used a two-microphone coherent power measurement technique with two Type 4955 low-noise microphones.


It's worth immediately noting that the B&K Type 4955 mics have a broadband inherent noise of 6.5 dBA, far greater than the -20.3 dBA of the room. This is what makes your question an interesting one.

So how did they do it?  
With two very precisely in-phase microphones pointed at each other, the pressure at both microphone surfaces is known simultaneously, which allows a basic description of the pressure gradient between the microphones.
Brüel & Kjær describes the calculation of intensity from this (pg. 10):

The   pressure   gradient   signal   must   now   be   integrated   to   give   the   particle   velocity.   The   estimate   of   particle   velocity   is   made   at   a   position   in   the   acoustic   centre   of   the   probe, between   the   two   microphones.   The   pressure   is   also   approximated   at   this   point   by   taking   the   average   pressure   of  the    two microphones.    The    pressure    and    particle    velocity    signals    are    then    multiplied    together    and    time    averaging    gives the intensity. 


So far, we just have a description of the technique - and we can see it's still limited by the inherent noise of the mic.
What I suspect the researchers did was to apply signal processing techniques to their measurement akin to those described in a freely-available paper by Ellingsona et al 2015.  (In fact, one of the coauthors is an employee of Brüel & Kjær and the measurement technique is described using the exact same microphone and data logger used in the Microsoft anechoic chamber tests.)

The dual microphone, cross-correlation technique is based upon the premise that signals of interest will be highly correlated between the two microphones, while noise components will be independent and uncorrelated.

Note that this technique requires an unusually precise measurement of pressure change: ±0.05 dB.  Most ANSI Type 0 (laboratory standard reference) sound level meters have a tolerance of ±0.40 dB, which explains why they didn't just use an "out of the box" type intensity probe setup.
