Garrett AT Pro Owner's Manual page 16

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 targets 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
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iron object like a bottle cap or steel washer will produce a very
distinctive Low-High-Low response.
Tip for using Iron Audio: In areas with high concentrations
of iron, it is recommended to switch off Iron Audio. Otherwise,
it may produce far too many signals. Then, if a target is
detected that has a questionable or inconsistent response,
switch on Iron Audio to check if it is iron.
To fully appreciate the additional information offered by
the Iron Audio feature, conduct the following experiment.
Start with the AT Pro in Standard (STD) Zero Mode and pass
the searchcoil over a bottle cap which is lying flat on the
ground. Note that the target response is consistent with the
audio of a good target.
Then, switch the detector into PRO Zero Mode and pass
the searchcoil over the bottle cap again. Note the subtle
low tones at the beginning and end of the target response,
indicating a questionable target that might be made of
iron. Finally, set IRON DISC to 35, switch on Iron Audio
and pass the coil completely over this target again. The
distinctive Low-High-Low response now indicates a target that
is unmistakably iron.
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AT Pro™