Hit each with hammer. Plalastic probably shows no greater density variation than metal. Listen to sound. Put head under water, listen to sound in water. Sound is louder in water because water is denser than air.
Plastic is a broad term covering a wide range of properties.
Protection isn't exactly the issue here. The original writer was making a harp. Presumably that means they want the best harp they can reasonably make. I try to make sure that each harp I make is the best I could do at the time. That means not cutting corners on my materials and methods.