Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A voice frequency (VF) or voice band is one of the frequencies, within part of the audio range, that is used for the transmission of speech. In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz. It is for this reason that the ultra low frequency band of the electromagnetic spectrum between 300 and 3000 Hz is also referred to as voice frequency (despite the fact that this is electromagnetic energy, not acoustic energy). The bandwidth allocated for a single voice-frequency transmission channel is usually 4 kHz, including guard bands, allowing a sampling rate of 8 kHz to be used as the basis of the pulse code modulation system used for the digital PSTN.