The dust cap is a gently curved dome mounted either in concave or convex orientation over the central hole of most loudspeaker

Loudspeaker

A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

 diaphragm

Diaphragm (acoustics)

In the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be...

s. It protects the inner mechanics (such as the pole pieces and the voice coil

Voice coil

A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...

) from small particles (which can cause rubs) and other contamination. Dust caps can also contribute structural integrity to the voice coil assembly or the cone

Diaphragm (acoustics)

In the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be...

.

In some loudspeaker designs dust caps can also be part of the acoustic design of the driver by radiating high frequency energy or suppressing it. Typically the dust cap is made of the same material as the cone. In some tweeter

Tweeter

A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...

designs, the dome is in fact the only sound radiating surface and so it performs both roles.