Garrett AT Pro Owner's Manual page 28

Refer to the illustrations below regarding the use of the Iron
Audio feature:
IRON AUDIO OFF: Normal division of low, mid and high tones.
IRON AUDIO OFF: With the Iron Discrimination set to 20, all targets below
20 are silent.
IRON AUDIO ON: Targets below 20 are now heard as a low tone and targets
above 20 will produce a mid or high tone.
When Iron Audio is on in the PRO Mode, iron target will not
only be heard, but they will produce an even more distinctive
response with multiple tones. For example, a nail will produce
several fast low tones as the searchcoil passes over. A flat
28 Garrett Metal Detectors