Just wanted to offer my input. With the 4x C-cell alkaline batteries installed in the siren, the small round grey button acts as a kind of test button—if you press it once, it emits a short (loud) beep.
I purchased this universal power adapter: http://www.amazon.com/dp/B000Z31G3M/ref=sr_ph_1?m=ATVPDKIKX0DER&ie=UTF8&qid=1437487590&sr=1&keywords=hosa+universal+1200ma
With the adapter set on 6V, I measure its output at 5.68V. With It set on 7.5V I measure it at 7.64V. It's rated for 1200mA, so the 600mA draw of the siren should be no problem. One of the tips fits snugly and seems correct. I installed the tip with positive polarity to match the diagram on the siren.
When the the adapter is plugged in, the grey button doesn't work as a test button anymore. Nothing happens when you press it. Furthermore, if you try to use the 10 second test feature on the IRIS website, nothing happens. (The Iris website continues to see the device, but the siren won't sound).
To make matters worse, I think leaving the adapter connected actually drained the alkaline batteries, so I need to go buy new ones right now. If anyone has any input I'd be happy to give it a shot, but it seems to me that the siren does not work on wall power. Wish it did.