Mains hum

Hum (also hum ) denotes an unwanted vibration resulting from the electrical mains voltage. The ripple frequency is in Europe, 50 or 100 Hz

The term comes from the hum sound when this vibration is heard as a low pitched sound as false signal to the speaker. In general we denote by mains hum next to the herkommenden from the mains voltage oscillation and their superimposition on an electrical information signal when it leads to an appreciable distortion of the desired signal. This all the more evident in appearance, the lower the electrical signal voltage.

Mechanical vibration

Mechanical conditional hum occurs, for example in power transformers by the magnetostriction in the iron core. The change in length and thus oscillation of the core is not caused by insufficient mechanical fixation, but a quantum mechanical effect. This oscillation generates an audible tone immediately. It occurs with the double line frequency as the mechanical forces at each half-wave in the same direction on portions of the core or the coil. These noises can be attenuated only by sound-absorbing and constructive additions in the volume.

Electric vibration

By the electromagnetic field in the space alternating electric voltage coupled to one in every electronic device. Particularly unpleasant heard the hum, for example, if an amplifier with speaker output, the input terminal is touched with a finger. Man, like every piece of electrical conduit acts as an antenna in the 50 - Hz field; the high-impedance amplifier input takes on the weak signal and produces a humming sound in the speaker.

Electronic devices are operated with DC voltage, which is usually generated by an AC adapter from AC power. Depending on the type of rectification arises ripple voltage at the mains frequency or its double.

Reasons for the hum can be a ripple in poorly shielded cables and improperly laying the ground line ( including as an earth loop ) in inadequately smoothed supply voltage.

Disturbing the hum from further acts in all kinds of high-impedance circuits. Example, the hum change or cover the curve of Nutzsgnals and hinder the measurement with the oscilloscope. Likewise, the measurement of weak signals is disturbed, which are received directly by the human body, as is the case with the ECG or EEG measurements. To avoid AC hum occasionally elaborate precautions are necessary circuitry.

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