In music production we are pretty lucky to have so many tools available to us and in the area of equalisation we are actually spoilt for choice. Understanding which equaliser to use for any given task is the first battle in selecting the right equaliser for the task in hand. There are so many types of equaliser topologies that it would be impossible to list them all here and offer detailed explanations as to how they work. However, I have made sure to include the most common types.
These types of EQs have the distinction of being extremely simple in design and, more importantly, they cannot boost frequencies, only cut. The way they work is actually very much to do with perception. By cutting, for example, low frequencies (bass), they make the mid and high frequencies sound ‘louder’.
Passive EQs do have their uses. Although they are inflexible, they can perform reduction (cut or attenuation) tasks reasonably well. However, by their very nature, passive EQs, or filters, have to then have the signal boosted to compensate for the cut. This, in itself, introduces noise into the signal path, the noise coming from the amp used to boost the signal.
In terms of circuitry passive equalisers place the equalisation circuits either before or after a fixed-gain amplifier — in which case the amp makes up for the inherent loss in the EQ circuit, effectively boosting the frequency range(s) that haven’t been cut.
Because of the limitations of passive EQs, most EQs are built around active filter circuits which use frequency selective components, together with a low noise amplifier.
Fixed Frequency EQ
Pretty self explanatory, this EQ allows cut/boost of one or more frequencies. There are no additional controls over the usual components, like bandwidth, Q, etc.
A peaking EQ is an EQ which boosts a specific band of frequencies.
Whereas a shelving filter has a shelf like curve, this filter has a bell shaped curve. The Q setting determines the width of the bell, while boost or cut determines the height or depth of the bell.
A low-pass shelving filter passes all frequencies below its cut-off frequency, but attenuates all frequencies above its cut-off frequency. Similarly, a high-pass filter passes all frequencies above its cut-off frequency, but affects all frequencies below its cut-off frequency.
This is the simplest type of active EQ. This EQ can shape response in a number of ways: boost/cut low frequencies, boost/cut high frequencies.
It is also common for the filter slope to be 6 dB per octave. This allows for a gentler effect. The shape is shelf like, so the boost or cut is progressive over a range. Filters do not have a no-effect at a frequency and then instantly jump and suddenly reappear at the next frequency. They have to get there somehow. The way, and by how much, they get there is called the gradient or slope. In the case of the shelving filter, the most common slope is 6 dB gain change per octave (doubling of the frequency). It takes time for the filter to attenuate frequencies, in proportion to the distance from the cut-off point. This is the slope.
Shelving filters are generally designed to apply equal gain changes beyond the shelving frequency and have controls for selecting the shelf, cut and boost.
A graphic equalizer is simply a set of filters, each with a fixed centre frequency that cannot be changed.
The only control you have is the amount of boost cut or in each frequency band. This boost or cut is most often controlled with sliders. The sliders are a graphic representation of the frequency response, hence the name ‘graphic’ equalizer. The more frequency bands you have, the more control and accuracy you have over the frequency response.
A graphic equalizer uses a set of band-pass filters that are designed to completely isolate certain frequency bands.
A filter that passes frequencies between two limits is known as a band-pass filter.
This is a great filter. It attenuates frequencies below and above the cut-off and leaves the frequencies at the cut-off. It is, in effect, a low-pass and a hi-pass together. The cool thing about this filter is that you can eliminate the lower and higher frequencies and be left with a band of frequencies that you can process in isolation.
Notch Filter – also know as Band Reject Filter
This is the exact opposite of the band-pass filter. It allows frequencies below and above the cut-off and attenuates the frequencies around the cut-off point.
Invented by George Massenberg, this filter is one of the most common used today.
This filter controls three parameters, frequency, bandwidth and gain. You select the range of frequencies you want to boost or cut, you select the width of that range and use the gain to boost or cut the frequencies, within the selected bandwidth, by a selected amount.
The frequencies not in the bandwidth are not altered. If you widen the bandwidth to the limit of the upper and lower frequencies ranges then this is called shelving. Most parametric filters have shelving parameters.
This is just another form of parametric EQ but without the bandwidth control.
This is very similar to a band-pass filter, but with variable center frequency, and no control over the width of the filter response (Q).
Another variation on the graphic EQ. This EQ provides control over the center frequency of each band.
Also regarded as a cross between the graphic and parametric EQs, this EQ offers multiple parametric peaking filters, where the gain of each is provided on a slider much like the graphic EQ.
In the video I cover all of the most common types of filters and explain how each type works. I show you how we can create different EQ responses by using the various filter types. Active and passive EQ types are covered in detail and I show you how to create the various filter types using digital equalisers.
Plugins used in this video:
Topics covered in this video are:
- Active, Passive, Graphic, Parametric, Fixed and Peaking Eqs
- Differences between Active and Passive
- How to use Graphic and Peaking EQs
- What are Shelves and Slopes
- Bandwidth (Q) and Resonance
- Matching Slopes
- Linear versus Minimum Phase
- Full Range EQ
- Tips and Tricks