Mains hum

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Mains hum (also known as mains hum ) refers to an undesired oscillation that originates from the electrical mains voltage . The hum frequency in Europe is 50 or 100  Hz .

The term mains hum comes from sound engineering when this oscillation can be heard as an interference signal on the loudspeaker as a low tone. In general, mains hum is used to denote not only the oscillation coming from the mains voltage but also its superimposition over an electrical useful signal if it leads to a noticeable corruption of the useful signal. This becomes more apparent the lower the electrical signal voltage .

Typical 50 Hz hum

Mechanical vibration

Mechanically caused mains hum occurs in power transformers , for example, due to magnetostriction in the iron core . The change in length and thus the oscillation of the core is not the result of inadequate mechanical fixation, but rather a quantum mechanical effect . This vibration immediately creates an audible sound. It occurs with twice the mains frequency , since the mechanical forces act in the same direction on parts of the core or the coil with every half oscillation. The volume of these noises can only be muffled by sound-insulating and structural additions.

Electric vibration

Due to the electromagnetic field in the room , electrical alternating voltage is coupled into every electronic device . The mains hum is particularly unpleasantly audible, for example, when the input terminal of an amplifier with a loudspeaker output is touched with a finger. Like every piece of electrical wiring, humans act as antennas in the 50 Hz field; the high-impedance amplifier input picks up the weak signal and generates a hum in the loudspeaker.

Electronic devices are operated with DC voltage , which is usually generated from AC voltage via a power supply unit . Depending on the type of rectification , ripple voltage occurs with the mains frequency or twice that.

Reasons for the mains hum can be inadequately smoothed supply voltage with residual ripple , poorly shielded cables and unsuitable routing of the ground cable (among other things as a hum loop ).

The mains hum also has a disruptive effect in all types of high-resistance circuits. For example, the mains hum can change or cover the curve of the useful signal and hinder the measurement with the oscilloscope . The measurement of weak signals that are recorded directly on the human body can also be disturbed, as is the case with EKG or EEG measurements. In order to avoid mains hum, complex precautions are necessary in terms of circuitry .

See also

Web links

Wiktionary: Netzhummen  - explanations of meanings, word origins, synonyms, translations


  • Joachim Franz: EMC, fail-safe construction of electronic circuits . Teubner, Stuttgart Leipzig Wiesbaden 2002, ISBN 3-519-00397-X .