I have owned two different, ancient, American-made versions of this sort of fog horn in the distant past.
Both sounded VERY loud and very expressive, depending on how you cranked.
A Zound horn works on the same principle: that of a striking reed.
The horn of both my ancient rotary fog horns was un-screwable and blow-able by mouth.
They sounded like
this one no doubt sounds.
But they are too large.
I have just ordered
this plastic whistle for under six US dollars and will review it, someday, here.
Have broken the "wordless" rule of this thread because, if having a plastic (tooth-friendly) whistle in the mouth when in dicey traffic situations makes for instant reaction to deathly car threats, then it is good to be obnoxiously heard, as quickly and surely as possible.
There no faster reaction time available than by exhaling though the mouth instead of the nose.
There is no faster or more natural reaction afforded, than by exhaling through the mouth instead of the nose. Faster than any button press is likely to be, because you are not likely to ride with your finger on your bike horn's button. But even if you do: an exhale through the mouth is more likely to be faster than the mental operation of pressing a button to indirectly make a noise. Think: we use our mouth to make a noise, HEY!