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High Pass/Low Pass Filters

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High Pass/Low Pass Filters

Postby CostaSaint » Tue Feb 05, 2013 11:56 am

Am I right in saying (using the chart below) that, for example, I should put a high pass filter on a bass track at 40Hz, a low pass filter at about 4Khz, a high pass filter on a kick drum at 50Hz and a low pass filter at about 8Khz etc etc?
And should this help get rid of some of the muddiness in my mix?
Thanks! ... -Chart.png
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Re: High Pass/Low Pass Filters

Postby Hugh Robjohns » Tue Feb 05, 2013 12:28 pm

Kind of...

It is certainly worthwhile removing unwanted low frequency rubbish which most mic and DI inputs pickup, but which doesn't contribute musically (in most cases) and just robs the playback system of power and headroom. So high-pass filtering is pretty much routine and very worthwhile.

As to the actual turn-over frequency, that depends both on the source and how that source sits in the mix. The fundamental of a four-string bass is theoretically 40Hz or so, as you say, but most of the bass sound in the mix is actually conveyed by the higher harmonics -- some of which may well be stronger in level than the fuindamental, actually.

So it could be, perhaps, that filtering at 80Hz, say -- and even though it will reduce the fundamental -- will help to let the kick drum come through more powerfully without being masked by a flabby bass. And the overall mix could well sound tighter and better as a result. Auditioned in isolation, the bass might sound less powerful than with the HPF set at 40Hz (or lower), but it's the sound of the full mix that counts!

Similarly, it is very common to roll off a lot of the bottom end from acoustic rhythm guitars,m making them sound decidely anaemic in isolation, but they sit much better in the mix without making things sound muddy and congested in the lower mid.

Low pass filtering is less important and less commonly done. You might do it if a source is noisy, and removing some of the high end hiss cleans things up usefully. You might also do it to reduce the harshness of a distortion effect. But as a general rule of thumb, it isn't usually necessary.

So, the answer is yes, high-pass filtering will help to reduce the muddiness in your mix -- but be prepared to set the filter surprisingly high and remember that it's the sound of the full mix that matters, not the individual filtered source!

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Re: High Pass/Low Pass Filters

Postby The Elf » Tue Feb 05, 2013 12:33 pm

It's not as simple as learning a few numbers and pressing the buttons. Often I will HPF bass guitar up to 80Hz or more, and sometimes I'll leave it be.

The answer, as always, is to use your ears, but a spectrum analyser can be helpful if you're struggling to hear what's going on in the subs. It's all about maximizing the frequencies that are useful to your mix (and so worth using up headroom to reproduce) and removing the frequencies that aren't.

My advice is to not get bogged down by those numbers and think about what you're trying to achieve with what you've been given.
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Re: High Pass/Low Pass Filters

Postby James Perrett » Wed Feb 06, 2013 11:25 am

After answering a query on reel to reels just before seeing this, a thought occured to me. I wonder if we worry more about high pass filtering things now that digital recordings have a much more extended low end compared to analogue tape. Tape gives you a natural high pass filter - especially if you are running at 30ips - which can help remove any really low rumble.

If I'm recording through my mixing desk I will often use the 80Hz filter built in to the desk on anything that isn't intended to have any low end.

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Re: High Pass/Low Pass Filters

Postby joelonsdale » Fri Feb 08, 2013 12:00 am

By all means low-cut everything until it sounds bad, then back off, in the context of the full mix. But muddiness is usually caused by the frequencies you actually WANT, competing for space in the more obvious frequency ranges: for instance, if you low cut everything except the kick and snare at 200hz,the kick and snare will both have plenty of space to be full and fat, but everything else will sound thin. A better way would be to try to choose sounds that don't rely on the the same frequency band to sound good in the first place! Very deep kick? Go for a bass with more mids. Very thick guitars? Choose a bass to sit under and possibly over them. Alternatively, or as well, and as an example, side chain a compressor from the kick to dip the bass each hit. No EQ required, but no build up of excess bass!
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