Building a simple microphone jammer
Markus, let's walk through the circuit, then talk about whether it &quot;will work&quot; to safely jam a microphone. Answering your questions:

The 555 output at pin 3 is a square wave. It won't be perfectly symmetric because the capacitor C1 charges through R1+R2+R4 (178k ohms) and discharges through R4 (100k ohms), but close enough.
The Scope and Scopeground pins are just a way for you to monitor the frequency of the 555 square wave output.
The G13599 transformer is easily found by Googling. It's $3.99 at Electronic Goldmine. They even provide a datasheet from which you'll see it's a step-up transformer to convert 3V to 200VAC for strobe lights.
The UST-40T transducer has a max input voltage of 20V. You would probably blow it out instantly from a 200V signal from the transformer output.
How could you make it work? -- you could try connecting 10 ultrasonic transducers in series, so each one only sees a 20V signal. This is risky, because if they are not matched, the voltage won't divide equally, and you could blow them all. Instead, just leave out the transformer, connecting the transducerpower pin where pin 1 of the transformer is currently.
The UST-40T is a piezo crystal and yes, they can be easily blown. If you want higher power, you could try a hi-fi tweeter (here's one for less than 10 bucks). Or, adding a horn to provide a smoother transition between the transducer and the air will make it more efficient, as well as directing the sound.

Now, let's talk about safety and efficacy. Even though you can't &quot;hear&quot; ultrasound, it can still damage your ears, and it's even worse for your pets who can sense high frequencies. Be careful!