What kind of Ultrasonic Transducer is capable of 85db at 25khz-40khz using 5vdc and is 1cm diameter?
The device you linked is a buzzer not a transducer- you give it a voltage and it produces a 2300Hz beep. It has a circuit inside.
You want a transducer that has enough output at high frequency--  to get a loud enough sound will probably require a tuned cavity resonant at the operating frequency or a lot of drive power.  Unfortunately, most of the dedicated inexpensive ultrasonic transducers are mechanically tuned for 40kHz and the SPL will be considerably lower at 30kHz (Kobitone typical datasheet spec below).

If you are enterprising and have access to a few tools (such as a lathe) you might consider modifying (enlarging) the cavity of a 40kHz type to resonate at the lower frequency. The design equations are easily found online.