Passive Radiators
Passive Radiator (PR) is usually used in audio systems when the usage of sound port is inappropriate because of any reasons. You can buy passive radiators of different sizes. For the Peppy player I selected PR which is close in size to the speaker. The speaker is 4" and radiator is 3.5". The plastic grill for this radiator should be bought separately. I had to drill 4 holes in the radiator to mount the plastic grill.
Technically the PR has the same construction as speaker but it doesn't have the electrical part. So nothing should be connected to the PR.
PR also adds some completed look to the back side of the system. The following image shows how it looks like after attaching the plastic grill.
