when to use High-pass / Low-pass filter

When they're needed.

ok, I already knew that stuff... I mean, how to know when those frequencies need to be filtered?

I use Hipass (low filtering) filters on nearly every track. It really helps the bass stay clean and not rumble.

I don't use Low pass filters although I'm sure others will believe in them as much as I believe in the High pass filters.

Only experience will teach you that. If we say "Use a high pass when there's too much low stuff, you'll ask us "When is there too much low stuff?". Next thing we'll stumble on is that you hear differently cause you have different monitors etc......
Read books and articles on the subject (Mixing and mastering) and make a zillion different mixes and mastered versions. Finally you have an idea of what needs to be done with the gear you use.

One thing to keep in mind is that there are other ways to "filter" than simply a high-pass/low-pass filter will necessarily offer.
I'm beginning to realize just how big this subject can be.

There are the more obvious things:
- if you want to get rid of things that aren't wanted: like mic rumble... filters are great for this type of thing
- you've got a bass line you want to hear "cleanly," but you have a patch or two from a synth that has stuff down there too... use a filter or eq, or tweak the patch (I prefer tweaking the patch, when possible... no sense in even playing something you're going to filter out anyway)
- you've got a lead that you want to hear "cleanly," but you have a patch or two from a synth that has stuff up there too... use a filter or eq, or tweak the patch

But there are some less obvious things as well (to me at least).

I tend to use EQ over filters, since many times an EQ will give you more precise control... and there might be times when you want to filter in the middle (band-pass).

Another thing you can do if you want to NOT filter all the time:
- side chaining... it's great for stuff like when you have a bass line competing with an electronic kick drum, and you want both, but don't want them to get "muddy" when they trigger at the same time.

My current track has a lot of layered/sweeping synths/basses that compete quite heaviliy with one another, but I want all of them.
For this type of thing, I'll likely do a bit of everything: tweak the patches a bit, add some modest EQ, add some panning (particularly for the muddy mids), and side chain the primary bass and the kick drum

It's song specific, and for me requires LOADS of listening in different environments. If it's muddy or grungy, and that's not what you're going for, clearly filtering/eq/side-chaining/patch editing is in order.
I'm documenting, for personal development, my process with this particular track, and will post the results on my blog once I get to the mixdown experimentation phase.
I'll post a link when I'm done, if you're interested. It might be useful, but I'm not a "pro" per say, so it would be good to get some feedback from others as well.

There are loads of books/websites about mixing/mastering.
I can't remember off-hand, and don't have time to look them up (I'm at work right now), but maybe someone could post some links/recommendations?

- zevo

Must. Resist. Smart Alec. Answer.
okee, think I did it.

Hi-pass and lo-pass are just another form of equalization. You may want to bone up on your EQ theory a little bit in prep.

Usually they're used when you only want a very specific part of the sound to come through, especially used in mixes where you want to keep the low-end from getting as cluttered as possible.

They're also used quite a bit as filters that can be swept to create those "swooshing" techno effects we all love. By moving the freq. of the pass, you can alter a sound in a fun way. William Orbit loves filters like that.

ORIGINAL: AndyW

ORIGINAL: DonaldDuck

I use Hipass (low filtering) filters on nearly every track. It really helps the bass stay clean and not rumble.

I never thought I would actually see myself say this but...I agree with Donald Duck.

+1

I keep cutting until I can hear the instrument changing in the mix, then back off just a little. Pretty much everything will have sonitus and a HP on it. It's important on low instruments as well as pretty much anything below 40Hz is USUALLY not musical and just adds mud. If you were mixing a scary movie, the keep those 40Hz rumbles for sure, but for music, you would usually want to cut them out.

One thing I swear by is putting a HP on drum overheads and room mics. Bass should really mainly be a mono source. It just gets muddy if it's in stereo. Now look at an overhead track, it's in stereo (usually). You leave that bass in and every kick and tom and snare is all boomy and everywhere and in your face and crappy. I will usually HP the room and overheads at least up to 100Hz as this REALLY cleans the drum mix out a lot.

Guitars, they can be tricky depending on your song. Most songs you will give the guitars justice if you HP them to something like 100Hz, or maybe a bit more, depending on where your bass fundamentals are sitting. If you are doing a big heavy rock song, however, you often want those big bassy powerfull chugging sounds of your palm mutes. You need that low end for that. But for the normal playing, you don't want as much. They are HARD to mix well. They sit VERY close to the bass. EQ envelopes or volume envelopes could be very useful here. With proper use, you may find that multiband compression on the low end may give you some good results.

when are we supposed to use High-pass and Low-pass filters? Thanks,

Your not going to like this, but your never suppose to use anything, you use it because you want to use it to achieve a certain sound or a certain mix. There are no rules of when to use this or that. They do exactly what there called. I suggest that you read up on mixing and mastering and tracking also. This will open your eyes to allot of stuff and youll better understand these thnigs your asking, cause its a huge topic
Cj

nobody can help?? please!!

Boris,

You're asking a dozen questions. Can we try solving one problem at a time?

I once recorded a female volcalist that was eating her mic during a live performance.
Proximity effect was way full.
Later, I had to cut LF all the way up to 500hz before she sounded good.
Very intimate. You could hear her breath, and the music was clean.

I know this is highly subjective and really depends on what instrument you are applying it to. But, regarding the high-pass, are most of you using just the first node or are you moving into the second as well?

Thanks!

I can see that low-pass and high pass-filtering differ in several ways from (to use your example) cutting a range of frequencies around a center frequency, and leaving frequencies on either side of that range unaffected. But is it really accurate to exclude HP and LP filtering from the definition of 'equalization'?

Is the idea of EQing really, or only, to get a "flat" frequency response? Wouldn't that depend on what you were EQing, and why you were EQing it?

It seems to me that EQing is changing the relative balance of frequency components of a given audio signal. As for the nature of that change -- whether it's a smooth tapering on the low end, or on the high end, or creating one or more dips and/or peaks at points across the audible range, and whether it's to correctively produce a "flat" and "natural" sound or to destructively produce an unnatural tonal balance of the source material -- isn't it all EQ? Or am I using the term too casually, or with too little regard for EQ's original historical function, as opposed to its common use as a creative tool (as an "effect" if you will) for sculpting sound... similar to how compression can be approached with a greater or lesser regard for its "correct" purpose?

In any case, I like CJ's answer:

...your never suppose to use anything, you use it because you want to use it to achieve a certain sound or a certain mix. There are no rules of when to use this or that.

FWIW, it's the rare track of mine that doesn't use a high pass filter. In fact, I wish I could set the built in track EQ to default to HPF mode.

Ideally, you should always avoid using EQ unless you are going for an "effect". In my early analog days we tried to avoid using EQ by making changes to the source instrument, be it vocal, guitar etc. It is all about mic placement, mic type (coloration etc), and the room environment. Today, many engineers rely too heavily on EQ to fix it in the mix. The more you learn about tracking, mic placement etc, the less you have to rely on using EQ in the mix. That is why you should plan your mix during the recording/tracking process. Always think about how, what you are recording, is going to fit in the mix.

ORIGINAL: boris.dante
... is it possible to remove those noises caused by puffs on the mic during recording session by such EQ filters?

As with many things, prevention is better than cure. Try backing the singer away from the mic, or move the mic up higher to reduce plosives. Good singing technic can obviously help as well.

If you have plosives in your recording and need to fix them, often a multiband compressor is a better solution than EQ. With a multiband comp, you can attenuate the desired freq's only when they go over a certain threshold, which may help you preserve your desired sound thoughout the track.

If the plosives are in low enough freqs where you don't mind killing the freqs entirely (below 150hz), the EQing them out is the simplest way.