Amplifier is not shown. Square with lamps is front view of target mounting board, made of heavy wood or metal, and lamps show position of openings which may be different sizes as 2 to 4 inches.
To right of target mounting board is side view of .125 sheet metal ( the vertical line) a little larger than speaker ( the lines with 4 ohm resistor) size that will be mounted with silicone rubber bead around speaker outer cone to back of sheet metal.
First, make sure of range used and test metal strength. That pellets will not seriously dent or penetrate the metal. Just make "ding" sound on impact. Probably best if sheet metal shop shears metal a little larger than speaker sizes. Next, silicon rubber speaker cone to back of metal sheet and mount behind openings on target mount board.
Wire speaker electrical to 9 to 12 volt amplifier line-in or mp3 input to hear amplified ping when disk over speaker is hit. Or design own amplifier using EC.
Sudden Impact Transducer is another name for this speaker set up.
