This image shows the effect of a 3rd order (18dB/octave) high-pass filter with a turnover
at 100Hz (green line), and a low shelf cut of -12dB, also set to 100Hz (blue line) on a
white noise signal (red line).
As you can see, the high pass filter's slope
just goes on down, providing ever-more attenuation as the frequency gets lower, where as
the low shelf looks just like a shelf -- it reduces the signal to a set level below the
turnover frequency, but no more.
Shelf equalisers can also boost the signal,
whereas a filter can only cut.
Technical Editor, Sound On Sound