The A-curve is a side bandpass philtre centred at 2.5 kHz with ~20 dB attenuation at 100 Hz, and ~10 dB attenuation at 20 kHz. Therefore, it tends to heavily roll off the low end, with a more modest effect on high frequencies. It is essentially the inverse of the 30-phon (or 30 dB-SPL) equal-loudness curve of a Fletcher-Munson.