Chances are that unless you are recording in a perfectly isolated studio in the middle of nowhere, you’ve come up against unwanted hums and noise. Background noise is around us at all times, and for the most part, we naturally tune it out.
Microphones however afford no such luxury, and those unruly noises can sometimes make or break a recording. This problem has been around since people started talking into mics, and so there are a variety of ways to salvage an otherwise unusable take. Today we will be focusing specifically on the art of filtering, what it is, how it works, and most importantly – how it may just save your future recordings.
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What is a Filter?
Before we get into the nitty-gritty of the subject, let’s first clarify what we mean by filtering. To cut a long story short, a filter is a frequency-dependent amplifier. While this may seem an incredibly simple principle, a filter’s importance in signal processing cannot be overstated.
Filters come in many forms, at their simplest they are a pass filter (either a low or high), rejecting all frequencies below or above a certain point. Put these together and you get a bandpass filter, allowing only a select range (or band) through. Notch filters are the opposite of a band, targeting only a small frequency range.
While this may seem like a lot of information to digest at once, I can assure you that you will be familiar with these different types, as together they may be more commonly referred to as a parametric equalizer. If you’ve ever mixed in the box, you’ll have interacted with this type of equalizer in some capacity, as they are the industry standard at this point.
This should demonstrate just how versatile filters are, and each has a specific use case – but let’s take a look at when you will get the most out of a high-pass filter.
When to Cut Lows
When podcasting, there are many different circumstances where a low cut may be used to great effect, and while there are no hard and fast rules, knowing specific use cases will help you make more informed decisions when processing audio.
If you are recording outside, next to a busy road, or have a washing machine on during recording, you may experience significant sound leaking into your bottom end. This is what’s known as mechanical transmission, and occurs mainly in the lower frequencies. Cutting these frequencies may help clear some of that muddiness, giving clarity to your mix.
If you or a guest are unfamiliar using a microphone, they may be unknowingly sat too close to the capsule, which results in a boost in the bottom end known as the proximity effect (for more on the correct mic distance check out our article on why ‘Singers & Podcasters Hold the Microphone so Close to Their Mouth’).
This proximity effect may result in boomy lows, but cutting a small portion of these frequencies can help reduce the effect and make for a more pleasant listening experience. Another cause of too much low end is a recording with two deep voices, in this instance cutting the lows of one may help the two sit better in relation.
Now you know a bit more about what a low cut is, and when you’d use one, let’s look at the controls you can expect to find on the device. While all devices will vary slightly in functionality, these are the controls you can expect to find on the majority of filters – regardless of whether you are using a plugin or outboard gear.
Most modern filters will have a frequency control, also known as cutoff, this determines at what frequency the filter kicks in. Being able to set the frequency allows for as much or as little low-end cut as required, which helps tailor the filter to your needs.
The resonance control is a relic of old-school filters. Old outboard filter resonance controls determined the slope of the filter (how fast level drops below cutoff), however the steeper the slope, the higher the resonance, resulting in a gain boost around the cutoff. Nowadays your slope is separate from resonance (typically with a 12dB and 24dB option) while resonance can be added to emulate the effect of a vintage filter.
Modern filters offer a plethora of additional controls, for example, an LFO to modulate the cut-off frequency, or an envelope to determine how fast the filter attacks, and releases. These all have their own advantages but are by no means necessary on typical signal processing.
Corrective vs Creative Filtering
While designed primarily as a corrective feature – reducing noise, taming unruly lows, etc. filters may also be used in creative ways. In podcasting especially, knowing your tools can help you sculpt a sound that will help you stand out from the crowd. Here are a few ways you can use a low cut creatively in your recordings.
If you are recording a drama or fiction podcast, you may find yourself playing multiple characters. Cutting the lows creatively can help with distinguishing between different speakers, removing the low end of a voice can completely alter its sonic characteristics, and without the lows to ground the sound, you can create a disembodied sounding voice.
This low-cut voice could also be used to emphasize highs. If you find that you read segments of the podcast in different tones of voice, cutting lows can help create a further distinction between sections. For example, people tend to raise the pitch of their voice when talking faster, so by high-passing in these sections, you can subtly change the tone of the voice to give the section a different feel.
Pre or post-recording filtering?
Sometimes lows can be cut right at the source. This is preferable as any low cuts prior to the recording will save you a task later down the line. The most common low cut you will find outside of the box is on microphones, most modern microphones have options for 40Hz – 80Hz low cuts, which can be useful if you know you will be grappling with these frequencies in post-production.
Due to their high signal output, an option for a low cut is most often found on condenser microphones. Microphones such as the Rode NT2-A, and Neumann U87 AI have options for low cuts at varying frequencies. That’s not to say no dynamics offer this option, but it is typically reserved for the high-end dynamic microphones, such as the Shure SM7B.
Filters play a vital role in signal processing. If used right, they can salvage an otherwise unusable recording, or make a good recording great. Despite their simple functionality, by now you should realize their significance in post-processing, and by reading this article you are hopefully well acquainted with how to use them like a pro.
But filters play a relatively small part in the art of frequency manipulation. By knowing when and where to employ equalization in your recordings, you can make a podcast shine. Good EQ can make all the difference in helping your show stand out from the crowd, but you need to know how to use it right. If you want to up your EQ skills, then our article on the ‘Best Equalizer Settings for Podcasts’ is worth a read.