Garrett AT Pro Owner's Manual page 46

at a distance of 8 to 10 cm. Note that the bottle cap's flat
surface gives a Digital Target ID reading in the 75–85 range
and produces the high tone of a "good" target.
Next, switch to PRO Mode and pass the bottle cap across
the coil again and notice the different audio. The clean sound
heard in STD has been replaced with a mixed, chirping tone
that has subtle low tones at the beginning and end, indicating
a possible junk target. Pass a conductive, coin-sized target
across the coil and note its clean high tone in comparison to
that of the bottle cap.
Finally, remain in PRO Mode and set IRON DISC to 35
and switch on the Iron Audio feature. Pass the bottle cap
completely across the coil again and note the high-tone chirp
that is flanked by distinctive low tones on either side. Again,
use a conductive, coin-sized target to compare its signature
to that of a bottle cap in this PRO Mode Iron Audio setting.
The bottle cap makes a response that is unmistakably iron.
Final tip: Record the results of your bench tests and refer
to them when hunting in the field. Knowledge of the PRO
Mode and the Iron Audio feature can reduce the amount of
trash targets that are dug.
46 Garrett Metal Detectors