Ultimate Guide to Filtering
There are a variety of different filter types, so it’s important to understand what each one does and when to use it. Liam O’Mullane walks you through each kind, explains their general application, and shows you how they can be used as creative effec
Audio filters of some description can be found on various pieces of everyday studio equipment – outboard gear, synthesizers, plug-ins, mixing desks, microphones and monitors all often feature some form of filtering designed to perform one task or another. As an audio engineer, the reasons for using a filter in the studio can be divided into two main categories, with each fulfilling one or more specific tasks that straddle both creative and corrective territories.
The first category is purely functional tasks, such as when you’re trying to eliminate unwanted frequencies from a sound source (a low-cut switch on a microphone or preamp is a good example of this kind of filtering). They are designed to remove low frequencies that contain very little musical information, but if left intact will eat up precious headroom in your mix. Rogue frequencies in this region can also cause undesirable reactions if you’re using a compressor on the signal as it reacts to this powerful bottom end instead of the more relevant information present. By contrast, an audio interface employs a high-cut filter to eliminate frequencies above its operating range. This happens before the A/D conversion stage to avoid aliasing problems. There are plenty more examples like this where filters feature due to technical necessity.
The second category of filter usage is creative. Whether they’re used with static settings to shape each sound in a mix or manipulated musically as an expressive tool, filters are present in most forms of production, whether they’re obvious to the ear or discrete enough to go unnoticed. In this feature we’ll explain each filter’s function and variables and suggest good practice regardless of the genre you’re working in. So, let’s look at how filters work and explore everyday filters typically used in music production.
The basics
We’ve already noted that a filter can reject certain frequencies in a sound. The frequencies rejected are dictated by the filter’s cut-off frequency, referred to in short as cut-off. This can be fixed and set by the manufacturer or it can be user-defined. The user can set and leave the cut-off frequency or move it around while a sound plays, turning it into a creative tool. A low-cut filter on a microphone has a fixed cut-off and is typically set to around 100Hz. This frequency is chosen in order to reject thud and rumble noises from below the cut-off while, at the same time, not detracting too much from the useful frequencies in the source sound.
But even though frequencies below 100Hz are being rejected, you won’t get 100 per cent rejection at 100Hz – all filters have a gradual downwards slope from the cut-off point until they reach full attenuation. The angle of this slope is referred to as the filter roll-off, measured in decibels per octave (6, 12, 18, 24, 48 and 96dB are common values). In the case of a mic’s low-cut filter, it is an octave below the cut-off point where the decibels per octave measurement is made. So, with a roll-off of 6dB per octave, an octave below 100Hz is 5Hz, meaning there will be 6dBs reduction at 50Hz. The attenuation doubles per octave thereafter, which makes this filter slope unsuitable for this task as it won’t attenuate enough of the low end. You are more likely to see slopes of around 18dB per octave being used for this application.
Another user-definable aspect of a filter’s behaviour is the Resonance control. This creates feedback around the cut-off frequency and is essentially a volume control for this region of frequencies. If a low Resonance is selected the volume around the cut-off is similar or lower than other frequencies passing through. As you increase the Resonance, frequencies around the cut-off will increase in volume. If you keep raising it you will eventually start to hear it sing, with a discernible tone at the cut-off frequency. If you’ve heard a screaming 303 bass line, this is created by using high amounts of Resonance on a filter with a rhythmically moving cut-off point. Roll-off curve and resonance are both elements that can differ in character from one filter to the next, and these factors contribute to how a filter will sound.
The usual suspects
We’ve already talked about the low-cut filter found on many microphones, and the same basic behaviour of this filter is exploited in other elements of production. But to add to the confusion, when referring to its use as a mixing tool it’s called a high-pass. Same filter, same function, but the cut-off frequency isn’t fixed, it’s variable.
When used as a mixing tool, the cut-off frequency can be positioned to attenuate the lowest frequencies of a bass guitar track, for example. Using a shallow roll-off slope will let you adjust the balance of low-end energy and midrange musical information. Using a steep roll-off will enable you to completely reject the very lowest sub information. This information can’t be heard over most speaker systems anyway, so its removal allows you to raise the level of bass overall in the mix. This isn’t a hard-and-fast rule, but it’s a consideration when mixing.
Another commonly used high-pass filter technique involves rolling off the low-to-midrange frequencies from a drum kit’s overheads, or a drum loop that’s layered with other drum sounds. This trick enables you to keep busy high-end cymbal information while creating space in your mix for other sounds. Slowly raise the cut-off while the rest of the drums play and you’ll notice how the overall drum kit sound becomes tighter and more defined. This technique can seriously tidy up your mixes if applied to other sound sources as well. Anything that doesn’t need excessive bottom end in the context of an overall mix can have some form of low-end trimming. Everything but your kick and bass instrument will definitely benefit from this treatment.
The other most popular filter used in audio production is the low-pass. It does exactly what its name suggests: all frequencies lower than the cut-off point will pass through, those above it will be rejected. This filter tends to get noticed only when used in an obvious, extreme way. Nearly all electronic genres will make use of low-pass filters in a creative way and they’re excellent tools to aid a song’s arrangement. The most common use is to introduce a sound slowly as you raise the cut-off point over time. This will start with a muffled, party-next-door-type of effect, then the listener is slowly guided into a new section of a track as the cut-off is raised, making the sound clear and recognisable. A good example of this technique in action can be heard in the intro to Daft Punk’s hit Around The World.
A somewhat less well-known low-pass filter technique used in the digital age involves re-creating the general high-end loss that occurred when working with analogue equipment and tape in the ‘old days’. The lack of signal degradation in the digital age can result in brittle, harsh top end if not addressed in the mix. If you add a low-pass filter to each sound in your mix you can decide how much top end is allowed through from each element. Just like removing the bottom end, limiting the amount of high-end activity will also benefit the overall sound of your mix.
This technique is key to getting more dynamics in the top end. Try using a shallow roll-off curve for a natural sound, or a steeper slope to give a sharper audible bracket to seriously restrict a sound’s frequency range. This bracketing can also be applied to a low-pass filter added into this processing chain, creating a top and bottom limit that enables you to decide how much frequency bandwidth is allocated to each sound. This technique can give certain sounds a unique, narrow bandwidth, which is good for a tight, minimal yet techy-sounding mix.
Second in line
The next most common choices for audio production are band-pass and notch filters. A band-pass lets a band of frequencies pass around the cut-off position. The filter rejects frequencies to either side of this band. This is similar to the bracketing effect just mentioned and the resonance amount controls how wide this band is. The higher the resonance, the narrower it becomes; the lower the resonance, the wider it will be.
The band-pass sweep is popular in electronic music as none of the passing frequencies passes continuously as you move the cut-off up and down. You can hear it used on synths, giving those midrange wobble bass sounds. It’s also good for creating swoosh noises (apply a sweep across white noise). A telephone effect can also be created using the right width and cut-off positions, but a much more tuned result can be achieved by using the bracketing system with low- and high-pass filters because you get more control over either side of the band from each filter, enabling you to mould the width and shape of the frequency bracket.
The third notch
A notch filter is the reverse of a band-pass: a band around the cut-off area is rejected while the surrounding area above and below this point can pass through unaffected. The width is again variable via the Resonance control. Although this type of filter can be used for practical tasks like rejecting unpleasant frequencies or reducing feedback in a live situation, a parametric EQ tends to be more practical as it too can perform these tasks but it will have at least a few more parametrics to hand for notching out neighbouring frequencies too.
However, a notch filter can be a useful creative tool, capable of generating phaser-like effects (move the cut-off up and down with a high Resonance setting). An advantage of using a filter for this type of effect rather than an actual phaser is the ability to rhythmically control the sweeping sound with the cut-off control. Try it on bass or lead parts.
Filter talk
Formant filters are based around the vowels used in speech. This means that by moving a formant filter you can create A, U, E, I and O sounds from any sound source. A formant preset consists of pre-determined peaks that move in a specific way when the cut-off position is changed. Some filters have even more elaborate presets that can pass through a few vowels as you sweep the cut-off. When used with restraint they can add rich timbre changes to any sound source. When used in a heavy-handed manner they are capable of creating bass or lead sounds that can resemble Tibetan monk chants, old-school computer speech or dinosaur-like growls.
Vowel filters have recently enjoyed a huge resurgence thanks to their ability to supply bass sounds for dubstep genres. But you don’t have to be into dubstep to find a good use for them in your own work. They are really extended versions of the classic wah wah pedal, which has been happily embraced by devotees of numerous contrasting genres over the years. Try them on a fuzzy lead guitar sound, or use them with small movements on a bass guitar for more subtle changes. They might be a little strong for treating a main drum sound, but a vowel filter on percussion parts can create a very unique texture, which will in turn help them to stand out in your mix.
Brushing up
Our last filter type is the potentially drastic-sounding comb filter. This creates a large number of notches across the frequency spectrum which when looked at on a spectral analyser resemble the teeth on a hair comb, hence the name. We hear natural occurrences of comb filtering in everyday life when a sound reflects back on itself from surrounding objects. As different frequencies bounce back at different times, phase cancellation and enforcement will create peaks and troughs across the frequency spectrum. In the studio, comb filtering creates the familiar sound of both flanger and phaser effects. In the same way as a notch filter, you can also sweep the cut-off to produce a controlled modulation-like effect. As there are a lot more frequencies being affected than a notch filter would affect, a comb filter sounds more complex, resulting in more drastic changes in tone.
Setting a fixed cut-off position on a comb filter can create a metallic resonance when processing a drum sound. Raising the amount of Resonance will then create an overlap that can be used to smear the sound’s definition and create more of an audio blur. It’s also a useful filter for pushing a sound further back in the mix while giving it a unique tone at the same time. This makes it easy to discern even though it’s being physically pushed further back in the mix. Experimenting with the cut-off position can boost the musical content in the sound as pleasant frequencies still pass through; on the other hand, it can block the musically pleasing frequencies, creating a cold, and dissonant effect.
Frequency control
You should now have an understanding of the many reasons and situations that demand you reach for a specific filter, such as situations in which you need to restrict the bandwidth allowance for each sound in your mix to better control the amount of frequency overlap allowed. Alternatively, it could be adjusting filter parameters to aid your song’s arrangement, or adding interest to a part by automating a notch or formant filter, then using the extreme tonal-changing properties of a comb filter to achieve a unique tonal balance. There are so many potential sounds to be made with these tools and big rewards can come from experimenting with different combinations of source and filter type. Happy exploration! MTM
This feature first appeared in Music Tech Magazine issue 102
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