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Post Index » STAGE TECHNIC » Pro Sound


Media College (Chương 1 : Âm Thanh) (5,6) - Page 2 PDF Print E-mail
Written by tuyenphuc   
Monday, 15 March 2010 17:15
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Audio Equalization

Equalization, or EQ for short, means boosting or reducing (attenuating) the levels of different frequencies in a signal.

The most basic type of equalization familiar to most people is the treble/bass control on home audio equipment. The treble control adjusts high frequencies, the bass control adjusts low frequencies. This is adequate for very rudimentary adjustments — it only provides two controls for the entire frequency spectrum, so each control adjusts a fairly wide range of frequencies.

Advanced equalization systems provide a fine level of frequency control. The key is to be able to adjust a narrower range of frequencies without affecting neighbouring frequencies.

Equalization is most commonly used to correct signals which sound unnatural. For example, if a sound was recorded in a room which accentuates high frequencies, an equalizer can reduce those frequencies to a more normal level. Equalization can also be used for applications such as making sounds more intelligible and reducing feedback.

There are several common types of equalization, described below.

Shelving EQ

In shelving equalization, all frequencies above or below a certain point are boosted or attenuated the same amount. This creates a "shelf" in the frequency spectrum.

Bell EQ

Bell equalization boosts or attenuates a range of frequencies centred around a certain point. The specified point is affected the most, frequencies further from the point are affected less.

Graphic EQ

Graphic equalizers provide a very intuitive way to work — separate slider controls for different frequencies are laid out in a way which represents the frequency spectrum. Each slider adjusts one frequency band so the more sliders you have, the more control.

A graphic equalizer is, as the name implies, an equalizer which uses a graphical layout to represent the changes made. It uses a series of sliders (usually vertical) which correspond to a set of fixed frequency bands. You raise or lower each slider to boost or lower (attenuate) the level of that frequency band.

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Graphic equalizers are commonly referred to by the number of bands (e.g. 51-band, 31-band) or by the frequency separation of each band expressed in octaves (e.g. 2/3 octave, 1/3 octave, 1/6 octave).

Parametric EQ

Parametric equalizers use bell equalization, usually with knobs for different frequencies, but have the significant advantage of being able to select which frequency is being adjusted. Parametrics are found on sound mixing consoles and some amplifier units (guitar amps, small PA amps, etc).

Parametric Equalizers

The word parametric means something which has one or more parameters on which the outcome depends. When applied to audio equalization, this means equalization which depends on parameters such as centre frequency, bandwidth and amplitude. The user is able to adjust these parameters to determine exactly how the equalization is applied.

Centre Frequency

The most important feature of a parametric equaliser is that it allows you to select which frequency to adjust. For example, instead of having a simple mid-range adjustment which boosts or reduces a pre-set range of frequencies, you can specify exactly which mid-range frequency to boost or reduce. This gives you great flexibility and accuracy.

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The illustration on the right shows parametric controls for upper-mid-range frequencies. These controls work together — the brown knob determines which frequency is to be adjusted (0.6KHz to 10KHz) and the green knob makes the adjustment (-15dB to +15dB).

Note that although you select a specific frequency, the actual adjustment will apply to frequencies above and below this frequency as well. This is why it is called the centre frequency — it is the frequency at the centre of the adjustment.

Example of use: Let's say you have a feedback problem somewhere in the 5KHz range but you aren't sure of the exact frequency. Turn the green knob right down, then slowly rotate the brown knob through the frequency range. As you do so, you will hear the selected frequencies being reduced. When you reach the frequency which is causing the feedback, the feedback will be reduced.

Bandwidth Control (Q)

As noted above, adjustments are made to a range of frequencies around the centre frequency. The bandwidth control determines how far above and below the centre frequency the adjustment will affect, i.e. the width or spread of frequencies.

A narrow bandwidth adjustment is very specific, useful for accurately removing or accentuating a specific frequency. This would be helpful in the feedback situation described above — once you have identified the offending frequency, reduce the bandwidth so you are adjusting the smallest range possible while still eliminating the feedback.

A broader bandwidth affects more frequencies, useful for adjusting a wider range such as the upper frequencies in a voice. Broader adjustments tend to sound more natural.

Note: Bandwidth controls are not available on all parametric equalizers.

Amplitude

The is the level of adjustment, measured in decibels (dB).



 

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