Getting very low noise out of a microphone is hard as you might expect. My Earthworks M30 is rated to 20 dB SPL A-weighted. Not my area of expertise, but the times I have had a mic capable of sub-20 dBA SPL, it was expensive and required special power supply and preamp. This article/advert presents one with 6.5 dBA SPL noise floor that is based upon a standard piezo capsule plus some internal filtering. Does not target audio but interesting to some. The A weighted spec is common but of course not a wideband measurement. Still a pretty neat idea...
http://www.edn.com/electronics-prod...43d6&elqaid=30821&elqat=1&elqCampaignId=26962
http://www.edn.com/electronics-prod...43d6&elqaid=30821&elqat=1&elqCampaignId=26962