Narrowband modem

In telecommunication, a narrowband modem is a modem whose modulated output signal has an essential frequency spectrum that is limited to that which can be wholly contained within, and faithfully transmitted through, a voice channel with a nominal 4-kHz bandwidth.

Note: High frequency (HF) modems are limited to operation over a voice channel with a nominal 3-kHz bandwidth.

Source: from Federal Standard 1037C and from MIL-STD-188