Help quack sexing ducklings

[blufangirl]

I am new to this they are about 4 weeks now. They have started making sounds sort of like quacks than peeps. What am I looking for quack sexing?

 

[Miss Lydia]

http://www.majesticwaterfowl.org/artquacks.htm

Duck quack loudly, drakes have a raspy quack sound like they have chest cold