Like most other transformer products, audio transformers consist of two or more magnetically coupled coils of wire, usually wound around a magnetic core. They are distinguished from other transformer types by being adapted and characterized for use at frequencies across all or a significant portion of the human audible range spanning roughly 20 Hz to 20 kHz, and are used for isolation and transformation of information in analog format, as contrasted with transformers designed for transmission of information encoded using a digital signaling protocol. Examples include output transformers for vacuum tube amplifiers and voice coupling transformers for traditional telecom applications.