And the final thing to consider is what the generated frequency of the AC unit is. Some frequency's are more sensitive to human hearing than others, so even if there is a 8db difference they could both be perceived as being just as loud as each other.
The stuff I've read suggests that 3 dB is identified by most folks as "one notch" louder or quieter. THat's different from barely noticeable. There reportedly are folks who can hear a 0.5 dB frequency response difference between two speakers, if the difference is over a broad enough range of frequencies.
I never said "barely noticeable", I said "readily detected".