When listening to music with an average of 85 dB, any 20 kHz sound will not be 85 dB. Rather is 40 to 60 dB down from there.
Voodoless has already said everything necessary in the second post. Nevertheless here is a graphical version of it for easier understanding.
Normally one would set the said 85dB sound pressure level around 1kHz. Then the sound pressure level when listening to "normal" music at 20kHz would be lowered by 35dB on average.
Source:
Long-term Average Spectrum in Popular Music and its Relation to the Level of the Percussion (with additional content by me)
In addition, every tweeter or widerange driver shows extreme beaming at 20kHz and even slight off-axis listening further lowers the sound pressure level of the sound arriving at the ear considerably.
Even with on-axis listening, the natural head movement when hearing music will ensure that the ear only temporarily captures the sound optimally at 20kHz (very likely even only a fraction optimal).
Source
for a possible car audio application yesterday.
In the car, the head movement is even more pronounced (depends of course also on your skill level

) and it is very unlikely that you will hear the full range driver exactly on axis.