Noise - Narrowband noise

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As we can deduce from its name, this kind of noise takes up a limited frequency band. Sources of this kind of noise include:

16.2.1. HVAC

(Heating Ventilation Air Conditioning)

Heating systems, air conditioners and ventilators all introduce buzzing noises, in other words undesired frequencies. These can deteriorate the signal both because they are picked up by microphones during recording, and because they induce an extra frequency to the electric current that powers the equipment. The latter circumstance comes about because these kind of electrical appliances don't have a constant current absorption, as do televisions for example, but rather, have an inconstant absorption which causes undesired frequencies. Appliances that turns on and off during recording or sound reproduction may also introduce evident unwanted "clicking noises".



16.2.2. Electromagnetic emissions

These emissions are generated by mobile phones, televisions, blow-dryers and other home appliances which, fortunately, are a rare sight in a recording studio, although we can be almost sure to find a computer. They are also generated by electric cables, more specifically by power wires [Power wires ] : the higher the ampere level (the amount of electric current) flowing through the cable, the greater the electromagnetic emission in close proximity will be. This is because close to each conductor through which an electric current passes there is a magnetic field that is directly proportional to the current itself. Vice versa, when a conductor is immersed in a magnetic field, a current is induced [Inductor ] . The value of this current depends on the intensity of the magnetic field itself. The induction phenomenon becomes stronger if the conductor that generates it is wrapped up in a spiral shape, hereby creating an inductor, in the fullest sense, at the centre of which a strong magnetic field flows. Therefore it is best to avoid bundles of knotty power cables, which, rather, should to always be fully extended. It's a good rule not to superimpose the power wires with signal wires [Signal wires ] so that the power wire doesn't interfere (via induction) harming the weaker audio signal.



16.2.3. Interferences

Two adjacent tracks on a mixer will interfere with each other as a result of the magnetic fields generated by the electric currents flowing through the channels. The same is true with analogue multi-track recorders, where the magnetic particles of one track interfere with those next to it. Two conductors can undergo capacitive interferences when they can be assimilated to the plates of a condenser, accumulating a charge within. This has an obvious effect in the case of microphone leads in which the microphone's resistance couples up with the inducted capacity, creating an RC circuit which works as a low-pass filter [ Low-pass filters and high-pass filters ] depriving the audio signal of its high frequencies. This situation is illustrated in the following diagram:

Noise - Capacitive coupling of two conductors

Capacitive coupling of two conductors

If the two conductors through which this phenomenon occurs are displaced, the distance between the condenser's two plates changes, therefore altering the capacity's value and hence generating an unwanted current.



16.2.4. Vibrations

These especially occur during live events in which people constantly walk up and down the stage, which is often made of wood, and which truly wonderfully transmit the vibrations to the mike stands, which then transmit them to the mixers, which in turn send them to the mixers and so on...










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