Where Does Tape Hiss Come from?

Even though there’s already a number of ways invented to reduce it to near inaudibility, but can you still remember when last time you asked: “Where does tape hiss come from?”

By: Ringo Bones

Believe it or not, all forms of electronic devices generate noise largely because a noise voltage is generated by the random state of movement of free electrons since they tend to dodge to and fro from atom to atom in a random manner. Thus all current-carrying components can contribute to the overall noise signal but those at the front-end of the channel of high amplification devices (like phono head preamplifiers and tape head amplifiers) are the most critical.

The noise produced by the electrons whizzing about within the electronics – including the power-supply hum of a well designed cassette tape deck – however is negligible compared with the noise produced by the tape passing over the replay head. In most situations, the noise level of the main power amplifier electronics is approximately 15dB below that of the combined noise generated by the cassette tape deck’s front-end preamplifier and tape hiss. Thus it is the tape noise which is the most troublesome in audio systems built around hi-fi cassette tape decks.

Tape noise or hiss – whether from analog based tape systems like open-reel tapes, 8-Tracks, cassettes or even Sony’s famed Elcaset – primarily results from the lack of homogeneity of the metal coating or other magnetic medium used. Tape hiss actually has a frequency that starts from 500-Hz, the annoying level, and then extends within 2,000 to 3,000-Hz, the unbearable level. Even though tape noise steadily remains constant with increasing frequency, the hiss that irritates most hi-fi enthusiasts the most mainly lie within the 500-Hz to 3,000-Hz part of the audible spectrum thanks to the Fletcher-Munson Equal Loudness Contour response of the human hearing that makes the 2,000 to 3,000-Hz region the nexus of human hearing audibility.

At present technology, the ferrous particles – or other “advanced” magnetic formulations – can never be distributed absolutely uniformly throughout the coating and the resulting aggregation of the discrete magnetic particles create their own discrete magnetic fields which during replay manifest itself as a noise electromotive force or EMF at the playback head and thus be amplified along with the desired audio signal. And by the way, noise is also produced from the mild irregularities in the traction of the tape towards the record head pole pieces during recording – which explains why during much of the 1980s, the heyday of the cassette tape, manufacturers made increasingly elaborate shell structures and internal mechanisms of their flagship blank cassette tapes as a way to further reduce tape hiss.