Depends on the plugs. Some do what you describe, others do a more thorough blocking. The foam inserts are great on long flights - the jet noises are diminished, but voices aren't. (You can suddenly hear conversations one or two rows away, when people think they have complete privacy.
) Silicone ones tend to block out much more of the noise in general though - they're great for sleeping in a noisy environment, where people may be having conversations around you.