STAX in-the-ear earspeaker SR-003MK2 as well as SRS-002 features an open-air type structure that inevitably cause sound leakage, and therefore they are sometimes not suitable for outdoor use until now. Also, since their housing part is large and heavy compared with usual earphones, it is hard to obtain the stable fit feeling. The CES-A1 (CC-A1+ET-A1) has been developed in order to clear these problems.
