A high pass filter (also known as a
low cut filter) allows all frequencies
above its cutoff point to pass through
from filter input to filter output without
attenuation, whereas frequencies
below the cutoff are attenuated. The
cutoff point is defined as that frequency
where the signal has dropped
3 dB relative to the flat or bandpass
region. Below the cutoff point, the filter
will exhibit increasingly more attenuation
(in dB) as the frequency goes
lower and lower. The rate at which this
attenuation occurs is defined in dB per
octave. Standard high pass filters are
available in 6 dB/octave increments,
since each 6 dB represents one filter
pole (one filter circuit element). Thus,
6 dB, 12 dB, 18 dB and 24 dB per
octave HP filters are common