As far as I know I have tried every bike alarm on the market. They actualy wear out or don't work too well when new. They are all made as cheaply as possible. They are made too cheap. I got a couple of years out of one and a month or two out of another. Those where my favorites. I only used them in addition to a lock, or in a couple of places where I could see the bike at a restauraunt. And had loaded panniers that I wanted left alone. Or where someone was near the bike all the time and it was a low crime tourist area on an island.
You can put your hand over the loudspeaker or a towel or a jacket. You can just hit many of them with a rock.
You have to use a regular bike lock anyway.
They will all stop working even if they seem great when new.
Other people don't care anyway. They have been triggered by people trying to move my bike, no one cares.
Your bike will be lighter and easier to lock, if you just take your money for an alarm, and throw it out the window.
The best was the Cy-curity. I put it in my panniers and it has a remote like a car alarm. It worked great because you can't see it and can't reach it to silence it. It drained the batteries if I left them in the thing and then stopped working in a month or two. It's dead, and in the big pile of alarms in the cellar.