No. Firm pressure with either surface of the Cardio III produces diaphragm effect. Light pressure apparently produces bell effect. Most people take off the peds diaphragm to make a traditional bell. Any bell will act like a diaphragm when pressed with firm pressure, though.
To get the peds diaphragm off, remove the snap-on thin rim. Then the diaphragm comes off easily. Then you can replace the rim. Do not try to take off the diaphragm without removing the rim, as this is how you will damage the diaphragm.