Complex sentence: However, the human ear is quite insensitive to acoustics below 20Hz and as sound frequency goes below 20Hz down to 0Hz, the sound pressure level needs to increase in order to remain audible to the human ear.
Split into simpler sentences: However, the human ear is quite insensitive to acoustics below 20Hz, until the sound pressure level is above a certain level. As sound frequency goes below 20Hz down to 0Hz, the sound pressure level needs to increase in order to be audible to the human ear.