Materials don't have any natural frequency. They have properties like mass, elasticity, etc. Natural frequency is determined by the shape into which the material is formed, how it is attached to other objects or otherwise damped, and the abovementioned mechanical properties. Consider that you can make a whole belltower of bells, playing different notes, out of the same metal.
To be specific, the fiber board that speaker enclosures are usually made of can ring like a bell, or be completely dead, depending on the bracing and damping material included in the design.
To be specific, the fiber board that speaker enclosures are usually made of can ring like a bell, or be completely dead, depending on the bracing and damping material included in the design.