Alex,

Thanks for posting the loudness curves; visuals always help.

And 3.5kHz is indeed the area we are best at hearing. At the risk of offending anyone's beliefs...
... one common explantion for this is that mankind, being social by nature and for survival, has gotten exceptionally good at recognizing each other's voices.

In any event, it is in fact this frequency area that is most critical in determing "clarity" of the human voice. We simply hear best in the area that is most important to us. Design or chance, that's pretty cool.

So if I were to look for the "perfect" whistle... that is indeed the frequency range I would look for. To Susan's point, all of us are better at hearing in this range. All that changes is how much better (or worse) depending on volume, age, gender, etc.

Hope this helps,