# Filter rules of thumb?



## t6902wf (Nov 14, 2008)

Rules of thumb

I have read many threads and most of the setup guide but I don’t think I came across this.

What are the rules of thumb for filters? 

My thoughts based on what I have read are.

Less filters are better then more.

Both gain and cut reduce headroom, I read that cut reduces headroom but I don’t understand why.

Are their guidelines about the width of a filter?

Are their guidelines about the amount of cut or lift?

Are some frequencies more or less amicable to being Eq’d.

Any wisdom (or a link to where the wisdom is) would be appreciated.


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## Wayne A. Pflughaupt (Apr 13, 2006)

There is no consensus for “rules of thumb” because there are as many thoughts on this subject as there are people giving them. Here’s mine.



> Less filters are better then more.


Generally, yes – use no more than necessary to get the job done. With a powerful tool like the Room EQ Wizard program, and an equalizer like the Behringer Feedback Destroyer, its easy for a novice to get overly enthusiastic and try to eliminate every little nook and cranny in the response graph (can’t blame them, I’m no novice and but initially I did too). Often they’d later report back that they liked the way things sounded better using less aggressive equalizing. Of course, not everyone reaches this conclusion, which is why it’s best to experiment and decide for yourself.

It’s typically only when equalizing subwoofers that the use of aggressive equalization is employed or debated. When it comes to equalizing the main channels I haven’t seen anyone recommend that approach.




> Both gain and cut reduce headroom, I read that cut reduces headroom but I don’t understand why.


”Headroom” typically refers to _reserve amplifier power._ If you use a boosted EQ filter at a certain frequency, and if the downstream amplifier runs out of headroom, it will first clip in that frequency range, since it is higher than everything else.

With cutting filters it’s a bit different, but the end result is the same. Let’s say the room is giving your subwoofer an 8 dB peak at 50 Hz. Since that’s the dominant frequency that you’re hearing, you will set your sub level (in relation to the main speakers) based on that loud peak. So, you use the equalizer to eliminate that peak. Now you will find that your subwoofer is not loud enough. So you turn it up. Now you’re driving it _harder_ than you were before, so it will run out of headroom and clip sooner.

The same roughly applies if the main speakers are equalized, only it can be more critical with them since there’s usually much less power available than what is typically used with subwoofers. An overall treble increase can cause the amp to clip at the high frequencies sooner than if no boost was employed. Likewise, if there is some response peak in the upper midrange or lower treble, for instance, you would probably find the strident sound irritating and not play the system as loud as you otherwise would. Eliminate that peak with an equalizer, and you might find yourself turning up the system louder than you previously did – i.e., running it harder.




> Are their guidelines about the width of a filter?


Only that ultra-narrow filters generally aren’t good, especially with the main speakers, unless absolutely called for.




> Are their guidelines about the amount of cut or lift?


You can generally get away with more severe boosts or cuts with subwoofers than with the main speakers. With the subs, the main issue is that you have enough headroom to accommodate the needed equalization. Also, if you have a ported sub, you shouldn’t boost below the tuning frequency of the port.

With the main speakers, and headroom issues aside, what you usually end up with is that severe boost or cuts just don’t sound good. With the main speakers, the objective would be _improving_ sound quality, not getting a picture-perfect graph.




> Are some frequencies more or less amicable to being Eq’d.


Not really, it’s just that subs and main speakers require a different approach.

Hope this helps.

Regards,
Wayne


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## t6902wf (Nov 14, 2008)

Wayne A. Pflughaupt said:


> There is no consensus for “rules of thumb” because there are as many thoughts on this subject as there are people giving them. Here’s mine.
> 
> 
> Generally, yes – use no more than necessary to get the job done. With a powerful tool like the Room EQ Wizard program, and an equalizer like the Behringer Feedback Destroyer, its easy for a novice to get overly enthusiastic and try to eliminate every little nook and cranny in the response graph (can’t blame them, I’m no novice and but initially I did too). Often they’d later report back that they liked the way things sounded better using less aggressive equalizing. Of course, not everyone reaches this conclusion, which is why it’s best to experiment and decide for yourself.
> ...


Wayne thanks for taking the time to respond.

I am EQing a sub only. Can you define how narrow filter is too narrow sub wise and what is an aggressive move gain wise.

I am using a FBQ2496.

I have not installed the 2496 yet but I am working on some filters to try.
I am not going for flat, my goal is generally heading "in the direction I want" with as little eq as I can get away with.

My most aggressive set is 4 filters.


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## Wayne A. Pflughaupt (Apr 13, 2006)

I’d say a filter generally shouldn’t be tighter than about 1/6-octave, but that’s “generally,” not necessarily rigid. But typically you won’t find any peaks or dips narrower than that.

As far as the amount of boost and cut, I’d say about 12 dB max. Typically if I see a graph that has a span of 15 dB or greater between the worst peaks and dips, I pronounce it “unequalizable.” If deviations are that severe, relocation is required. If that’s not an option, then you just have to be prepared to live with less-than-ideal response.

Oh yes, forgot to mention in the previous post, response dips that are nulls typically do not respond to equalization, so it’s a waste of headroom to try. But not every dip in response is a null. Typically nulls can be recognized as deep and narrow, but (again) not necessarily always. The “giveaway” that identifies them is if you try to boost them and response does not change, or changes little.

Regards,
Wayne


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