Extended range measurement mics and pre-amps
 

I am looking for suggestions for measurement microphones that could measure well into the 40-50Khz range, and pre-amps to go with.

Any diy projects out there for that purpose ?

TiA!

A preamp that has 50k bandwidth is not that hard to find, good luck finding a microphone capsule to go with it though.
Basically your choices are a M50 from Earthworks, DPA (formerly B&K) 4004 or a Sanken CO100k- all of them cost between $1500.00 and $2000.00.
I own an Earthworks M30, which costs 1/3 of the others and don't miss the extra ultrasonics when working with loudspeakers and systems.
A Behringer is good to about 16k, then it starts rolling off. Most capsules you'll find for DIY will be comparable. Otherwise Behringer and other Chinese based manufacturers would be selling $100.00 measurement mics that go to 50k.
Why do you need a 50k mic?