I heard on the news that the voter participation (in the recent election) was at an all-time low.
Explanation:
When you are looking to place parenthesis in a sentence, you look for a dependent clause: part of a sentence that cannot work on its own and relies on the dependent clause to work; it is also usually background information.
If you just removed the part I put parenthesis around, the rest of the sentence would still make sense.
