Any whistle should be ok in an emergency, its the whistle patterns that communicate help, danger etc.
If you need to whistle for long distance, perhaps a hand held air horn.
Also good skill is being able to whistle with just your lips, as well as louder with your index and thumb fingers depressed on your curled tongue. That weighs nothing and loud!
Although in an emergency, assume you broke your good hand and cant whistle, my REI pack has a sternum strap chest buckle, that is orange, slim and doubles as a whistle.
Google search for: plastic buckle whistle
I could not find it on the REI website, but I have seen it in the store.
I typically think of the morse code SOS ... --- ...
But in whistle code, i keep it to three short bursts to signal distress
I searched bpl here and found a previous thread on whistles that may be of interest:
I lifted this from wiki for what its worth
"Mountain distress signals
The recognised mountain distress signals are based on groups of three, or six in the UK and the European Alps. A distress signal can be 3 fires or piles of rocks in a triangle, three blasts on a whistle, or three flashes of a light, in succession followed by a one minute pause and repeated until a response is received. Three blasts or flashes is the appropriate response.
In the Alps, the recommended way to signal distress is the Alpine distress signal: give six signals within a minute, then pause for a minute, repeating this until rescue arrives. A signal may be anything visual (waving clothes or lights, use of a signal mirror) or audible (shouts, whistles, etc.). The rescuers acknowledge with three signals per minute.
In practice either signal pattern is likely to be recognised in most popular mountainous areas as nearby climbing teams are likely to include Europeans or North Americans."