Mains hum

From Wikipedia, the free encyclopedia

Spectrum of mains hum at 60 Hz

Electric hum, mains hum, or power line hum is an audible oscillation at the frequency of the mains alternating current, which is usually 50 Hz or 60 Hz, depending on the local electric utility configuration (see Mains electricity). The sound often has heavy harmonic content.

The most common cause of electric hum is magnetostriction, wherein ferromagnetic materials change shape minutely when exposed to magnetic fields. Magnetostrictive electric hum is most often noticed around large transformers. The magnetrostriction in a transformer iron core is caused by the strength of the magnetic flux. The flux is created by the voltage imposed on a conductive winding wrapped around the magnetic core. The flux density, and thus the "hum" intensity is proportional to the applied voltage. The current flow through a transformer winding has negligible influence on hum. Because the magnetic flux density is strongest twice every electrical cycle, the fundamental "hum" frequency will be twice the electrical frequency. Additional harmonics above 100 Hz or 120 Hz will be caused by the non-linear behavior of most common magnetic materials.

In the realm of sound reinforcement (as in public address systems and loudspeakers), electric hum is often caused by induction. This hum is generated by oscillating electric currents induced in sensitive (high gain or high impedance) audio circuitry by the alternating electromagnetic fields emanating from nearby mains-powered devices like power transformers. The audible aspect of this sort of electric hum is produced by amplifiers and loudspeakers.

It is often the case that electric hum at a venue is picked up via a ground loop. In this situation, an amplifier and a mixing desk are typically at some distance from one another. The chassis of both items are grounded via the mains earth pin, and are also connected along a different pathway via the shield conductor of a shielded cable. As these two pathways do not run alongside each other, an electrical circuit in the shape of a loop is formed. One gets the same situation occurring between musical instrument amplifiers on the stage and the mixing desk. To fix this, stage equipment often has a "ground lift" switch which breaks the loop. Another solution is to connect the source and destination through a 1:1 isolation transformer, called variously audio humbucker or iso coil. Another — extremely dangerous — option is to break contact with the ground wire by using an AC ground lift adapter or by breaking the earth pin off the power plug used at the mixing desk. Depending on the design and outlay of the audio equipment, lethal voltages between the (now isolated) ground at the mixing desk and earth ground can then develop.

The other major source of hum in audio equipment is shared impedances; when a heavy current is flowing through a conductor (a ground trace) that a small-signal device is also connected to. All practical conductors will have a finite, if small, resistance, and the small resistance present means that devices using different points on the conductor as a ground reference will be at slightly different potentials. This hum is usually at the second harmonic of the power line frequency (100 Hz or 120 Hz), since the heavy ground currents are from AC to DC converters that rectify the mains waveform. See also ground loop.

Assuming a tempered scale with A=440 Hz, a 60 Hz tone is almost exactly halfway between A♯ and B two octaves below Middle C, and a 50 Hz tone is between G and G♯ two octaves below Middle C, but slightly sharper than the quarter-tone. These notes fall within the range of a 4-string bass guitar.