HELP - Connect a piezo sensor board to audio-jack

Other Hardware Sensors
1

Hi,

I’ll like to connect the “Grove - Piezo Vibration Sensor” to an audio-jack, but I have some doubts about the connections.

"Grove - Piezo Vibration Sensor" connections : OUT; VCC; GND

OUT -> audio-jack (+)
? -> audio-jack (-)

VCC -> arduino (5V)
GND -> arduino (gnd)

How I make the ? (gnd) connection to the audio-jack (-) ?
(from a gnd of the arduino?)

http://www.seeedstudio.com/depot/grove-piezo-vibration-sensor-p-1411.html?cPath=25_26
http://www.seeedstudio.com/wiki/Grove-_Piezo_Vibration_Sensor

thanks,
Carlos Esteves