From the following webpage:

http://www.aerokurier.rotor.com/akNachr/aeNewsE.htm


Antares too quiet
Lange Flugzeugbau faces an unusual hurdle with the
type certification of its electric glider, Antares.
The required noise level measurement according to Chapter
X was not possible. The noise emissions from the 20-meter
self-launching glider are so low that it can't be heard
above the ambient sound of the surrounding environment.
But the measurements are mandatory for the EASA certification.
Even a modified measuring process failed to produce
data that could be converted to values suitable for
compliance with Chapter X. It has simply never happened
befo An aircraft that is actually too quiet.