Measuring only AC Magnetic fields (40Hz - 130 Hz range) with an arduino and a Hall effect sensor
Measuring only AC Magnetic fields (40Hz - 130 Hz range) with an
  arduino and a US1881 Hall effect sensor


You can't because the US1881 won't work; it is a latching device with a digital on/off output and severe hysterisis. It' won't give a linear output: -