Did an air test today and discovered that each BAR has its own audio pitch in SUM mode. Haven't been using the SUM mode since I got the machine. During todays air test I noticed that the IRON and FOIL bars give the lowest pitch, 5 cent bar gives a higher tone pitch, round tab bar is higher, square tab bar is higher yet, and so on. Also noticed that a wheaty gives a higher tone pitch than a zinc penny in SUM mode. Quarters have a higher pitch than dimes. Took me 9 months to learn that each bar has a distinct audio tone. Test was done using foil, gold rings, nickle, pulltabs, and coins in discriminate mode and switching to SUM mode on each target.