Hiss consists of random signals concentrated mostly in the higher audible frequencies. Because of its random nature, hiss has no musical pitch, having a sound like ocean surf, but steady and continuous. Virtually all electronic devices contribute some hiss to music, although professional-level equipment keeps the noise to very low levels. Magnetic tape recordings are the most notorious source of hiss, which comes from the fine iron oxide particles that make up the recording medium. Radio interference is another source of hiss, as the jumble of radio and television signals bleeds into sensitive electronic recording equipment, especially in urban areas.
A noise gate is a device or program that eliminates hiss in the gaps between songs on a recording or during soundless moments in a song. To use a noise gate to remove hiss, a technician sends an audio recording to the gate's input. The gate blocks the source audio if the loudness falls below a threshold level. During the quiet moments, the gate stops the hiss between tracks, making them completely silent. When the music plays, the gate passes the whole signal, including the hiss, which can be less noticeable during musical passages.
A dynamic filter selectively reduces high audio frequencies, but does so only during the "decay" parts of a sound. Dynamic filtering works well for recordings of acoustic music, as the high frequencies of acoustic instruments tend to decay more rapidly than the bass frequencies; as the treble parts die off, hiss becomes more noticeable. Because hiss is predominately high frequencies, a careful adjusting of the filter can substantially reduce hiss.
Spectral noise gating is a type of hiss reduction software that uses a two-part technique. During the first part, a mathematical process called Fast Fourier Transform analyses a brief sample recording containing only hiss and determines all the frequencies that make up the noise. In the second part, the software processes the audio file, subtracting the hiss frequencies from the audio recording. Because the process targets specific hiss frequencies, it has less negative impact on the brightness and quality of the music.