Headphones do not have microphones, such as the Telex instructional Explorer series. They are available with or without volume controls with either six or eight headphone jacks. The listening stations include two expansion connectors for daisy-chain connections to additional junction boxes.
All audio sources have an output load capacity, called output impedance.
Headphones are also rated in ohms, and when connecting a headphone to an amplifier, it is important that the ohms rating of the headphone be equal to or greater than the ohms rating of the amplifier. For example, if the amplifier is rated for eight ohms, the headphones should be at least eight ohms. Headphone impedance is dramatically reduced when connecting multiple headphones. The audio source must have an impedance of 37.5 ohms or less. This formula only works when all headphones are of the same impedance.