I’ve just got back from nearly two weeks away from the blog, one of the articles which published while I was away One Often Overlooked But Essential Mix Plugin touched on the importance of filters. We felt this would benefit from a follow up article looking in a little more depth at the humble filter.

After the fader and pan pot, the next most fundamental tool in a mix is a simple high pass filter. You might know it as a low cut filter, it’s the same thing (though if you call it a low cut filter I’ll silently correct you…). The high pass filter is unusual in that much of the time its effect isn’t intended to be audible at all. It’s a form of EQ but unlike other equalisation it is often an exception to the principle that you can’t EQ frequencies which aren’t there. Much of the time you use an HPF just in case those frequencies are there. This brings me to my first issue when using filters. How should you set them?

Filters As Insurance

In this protective role the fact that most preamps and mixers offer a single on/off setting does hint that many sources will work with near identical settings. While they vary, a typical filter designed to exclude unwanted low frequencies will be somewhere around 80Hz. A typical slope would be 12dB/Oct, or sometimes 18dB/Oct.

These settings will usually exclude mic pops, stand-borne bumps and the like which tend to present themselves as very low frequency transients which if allowed through a playback system will eat headroom and get the drivers flapping and if allowed near the detector circuit of a compressor will dip the level of the whole spectrum. The choice of a corner frequency somewhere around 80Hz is a good setting to sit above the sub bass nasties these filters are so often employed to protect your audio from, and below the lowest frequencies we’d expect to find in speech, vocals and the majority of instruments. For example the fundamental of the lowest note on a guitar in standard tuning is around 80Hz.

If you’ve ever seen a chart of frequency ranges like the one above, the part which to me is useful is the lower range of the instruments, this can be enlightening when selecting a frequency for a high pass filter. There is content below the lowest fundamental in an instrument’s range and it can be useful, but it’s not the ‘note’ as such. For example mandolins have a surprising amount of low end in them, and cymbals too.

The choice of 12dB per octave is linked to the design of analogue filters and 12dB/Oct is a happy medium between steepness and complexity. These days a plugin can provide filter types which were impractical or just impossible in the analogue days but as they say, if it ain’t broke…

This role of using high pass filters as insurance against unexpected low frequencies is invaluable in live events and broadcast settings where a mic pop can be very distracting. This is probably where the often repeated “high pass everything” maxim comes from. I’ve even seen it on T shirts. However in the studio this isn’t necessary and to unthinkingly apply HPFs to every track apart from bass and kick drum is potentially going too far. Rather than filtering everything I keep an HPF as a Clip Effect preset in Pro Tools and apply it where necessary, and for really bad mic pops there’s always RX.

What About Using Filters In Other Ways?

Just because the previously discussed standard settings are so useful it doesn’t follow that there aren’t times that filters are used differently. There are of course times when you might want to ‘telephone voice’ a vocal by using both a high and low pass filter to bracket the sound and restrict it to a narrow part of the midrange. Some people high pass drum overheads pretty aggressively, to maybe 300Hz or even higher. High pass filters can be useful on delays and reverbs, though if these filters are supplied in the plugin they are often more useful. A low pass filter in the feedback loop of a delay makes the repeats progressively darker and the opposite effect using a high pass filter to make each repeat progressively lighter and thinner shouldn’t be overlooked. If your delay doesn’t have filters then you can create the effect using sends and mixer routing in your DAW with a single repeat delay.

High Pass Filters For More Bass?

A high pass filter removes bass but it can be an invaluable tool for managing low end and actually making things bassier. Big bass is about managing the low end energy, that’s a big subject but a brief example which sounds counter-intuitive makes the point. I’ll often high pass my kick drum and I’ll also frequently not high pass electric guitars. Superficially that might sound the wrong way round. I’ll explain.

A real kick drum doesn’t usually contain much energy below around 40Hz. Even if it does most playback systems won’t reproduce it well, but using a high pass filter it is possible to focus on the energy in that kick drum in a useful way, while also excluding subsonic stuff. Filtering out the extreme low end is something I often do when I work with an excellent drummer I know who has a tendency to ‘bury his foot’ meaning he leaves the beater in contact with the drum head. He also tends to bounce his right knee while he plays resulting in a pumping of the drum head, this is inaudible in the room but can cause subsonic ‘woofing’ on the kick mic. A high pass filter sorts this neatly.

However the added bonus on kick comes when the filter has a Q control. This results in a resonant peak just above, or in the case of a low pass filter, below, the corner frequency and this is immensely useful for focusing in on the useful bass energy and making the most of it. You can do this with a standard EQ using a high pass filter and a bell filter together but having them together like this means you can find the sweet spot really quickly. My favourite tool for this is McDSP’s excellent F202 but you could always try Brainworx’s free Subfilter which works in the same way.

So I high pass the bass drum. What about the lack of filters on guitars? This came from a conversation with William Wittman, who has strong views on the over-use of high pass filters. He prompted me to stop high pass filtering guitars, a habit I’d brought from live sound and he was right. They aren’t often necessary and regardless of where the fundamental might be, you can definitely lose something. Try it.

What About Phase?

While most people will understand what a high pass filter does to the frequency content, less talked about if the fact that filters affect the phase of the audio passing through them. Phase can be hard to get your head around but the important point here is that unlike polarity, which has a uniform effect on the entire spectrum, phase shifts can vary with frequency. Not all frequencies pass through a filter at the same speed and this causes a smearing in the time domain. This results in a phase rotation relative to the input and you can plot this on a graph.

Does this phase rotation make any difference? It depends. Phase becomes important when phase shifted sounds are heard together with sounds with phase which differs. Sound Radix addressed this in Auto Align Post 2 because the effect of phase rotation caused by high pass filters needed to be taken into account. Most of the time the effect is anywhere from subtle to negligible but its worth being aware that as a filter gets steeper, its phase behaviour becomes more dramatic. Check out the screen shots of the phase response of F202 at different slopes.

You might think that in these digital days we can just use a linear phase filter. This is true but there’s no such thing as a free lunch and a linear phase filter introduces significant latency and also introduces pre-ringing. A case of the cure being worse than the disease.

Can this phase shift be used to benefit your productions? It’s subtle but I’ve used it in the past with my BAE 1073mpf. This 2 channel preamp has nice, inductor-based high pass filters and using is as a hardware insert I’ve used it to add analogue colour courtesy of its transformers. In the context of that use I sometimes set the high pass filters to different frequencies when used on a stereo group as it can add width. Some of this is going to be due to the effect in the frequency domain but some of it is clearly due to phase. Listen to this example. The filters are set to 80Hz, then change so the right is set to 160 and the left stays at 80, then back to 80 on both sides and then back again to 80 and 160. In the screen grab blue is 80/80, red is 80/160. You’ll hear the width change along with the timbre. Interestingly this technique doesn’t really work for me using a plugin. Listen on headphones.

There’s a  lot to say about filters. This article has concentrated on high pass filters but low pass and band pass are also hugely useful. Filtering dynamics side chains, assigning the frequency and Q of a low pass filter to a controller to nail DJ style filter sweeps, there’s so much more to say. Do you have original or unusual uses for the humble filter?

Image: Alexyo.Netcom, CC BY-SA 3.0, via Wikimedia Commons