A crystal earpiece is a type of piezoelectric earphone, producing sound by using a piezoelectric crystal, a material that changes its shape when electricity
Piezoelectricity (/ˌpiːzoʊ-, ˌpiːtsoʊ-, paɪˌiːzoʊ-/, US: /piˌeɪzoʊ-, piˌeɪtsoʊ-/) is the electric charge that accumulates in certain solid materials—such
discovered to exhibit piezoelectricity. This property led to its extensive use in crystal phonograph cartridges, microphones and earpieces during the post-World