Recording ultrasonic sound

I looked up at bat detector, a device that converts ultrasonic signals into audible sounds.
Most bats produce high frequency calls which are often higher than a human ear can pick up. A bat detector picks up the frequencies and then converts the sound into a lower, audible frequency. I was thinking if I could get a bat detector, I use it to capture the ultrasonic sounds.While bat detector could pick up these high frequencies, it would be interesting to see how it will pick up the faintly heard, to human ears - sounds of the industrialised areas/city or even at isolated/quiet areas.
The bat detector I want to use is this - http://www.magenta2000.co.uk/acatalog/B ... _Bat4.html
I'm thinking of hooking up the bat detector to a digital recorder and a binaural microphone and capture the sound through those equipments.

I would greatly appreciate your suggestions/opinions on this matter.