Garrett AT Pro Owner's Manual page 16

Audio Target Response Comparison: Standard vs Pro Mode
Shallow coin
Deep coin
Notice the differences in Standard binary audio versus PRO audio in the two
target scenarios above. The binary audio gives a solid, consistent beep for
both the shallow and the deep coin. In contrast, PRO Mode's proportional
audio provides a stronger signal for the shallow coin and a softer signal for
the deeper coin. The actual profile, or signature, of the target's response is
heard, thereby providing more information.
16 Garrett Metal Detectors