²⁸It is relatively easy to build an apparatus that makes HART tone signals audible: simply connect a small audio speaker to the low-impedance side of an audio transformer (8 ohms) and then connect the high-impedance side of that transformer (typically 1000 ohms) to the HART signal source through a coupling capacitor (a few microfarads is sufficient). When HART communications are taking place, you can hear the FSK tones reproduced by the speaker, which sound something like the noises made by a fax machine as it communicates over a telephone line.