I've never seen Sting with foam under his strings and his electric bass playing sounds nothing like a double bass. You can clearly hear the difference when he plays double bass on his recordings.
- Steve
He used a piece of foam with his Spector. That's the only bass I've ever seen him use foam. If we're talking about Police albums, I find it difficult to tell which is a fretless P and which is double bass (IIRC Synchronicity has 2 basses on every track)