Looking at the frequency of the Fox 40's, the reason why they don't seem to be very effective to me is more apparent. That narrow frequency is in a range that I've lost due to hearing damage (turbine engines, et al).

One aspect to these tests that is missed is modulation of the whistles/tones. The sound may need to overcome background noises in the same frequency (like my tinnitus) and one way to do that might be to modulate the tones.