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Question about EQ for speakers

Roy_L

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I've been recently playing around with a software EQ (Peace equalizer APO).
My question is- can you get some "free" bass extension using the EQ? For example, I have a pair of big bookshelf speakers which naturally go down flat to about 47Hz. Below that, there is a steep slope downward (40Hz is about -4db, 30hz is about -6db etc.). What happens, in principle, if I try to make this slope less steep by giving a boost in the EQ to diminishing frequencies? Obviously I can't make the speakers produce sub-woofer style bass. But what happens if I make a 3-4db correction upwards to the range between 25db-45db for example? Did I just "steal" some free bass extension, or am I paying for it with distortion (I suppose)? Is there a rule of thumb about how much extra bass you can get away with without screwing something else up, or perhaps even mild bass boosts should be generally avoided?
 

abdo123

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How big is the woofer in the speaker, how far are you sitting from it, and is the speaker ported?
 
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Roy_L

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It's a pair of Dynaudio 20i. Rear ported, 7.1 inches woofer (18cm), sitting about 3 meters away. Speakers are close the wall behind them. According to the manufacturer, the speakers go down to 39Hz@ -3db, but I'm relying on Stereophile's measurements of the previous (similar) version.
Regardless, I'm also wondering about it in general.
 

abdo123

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Yeah i would not do that. I did a 10 dB boost for my subwoofers but they're sealed and they're both 12-inch (they end up working at -6dB because of mono sub-bass). at such a distance, with the port, with the size i would not bother.

Sounds like a recipe for disaster / distortion.
 

audio2920

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3 - 4dB is a pretty small number, if that's literally all you're after. With positive EQ you just need to make sure you have the headroom in the chain before the amp/speaker to cope with the added level. (Some domestic EQs put attenuation in automatically to avoid clipping, dunno about "Peace Equaliser")

Main trade off is available SPL before the speaker runs out of travel, hits the limiter, or runs out of amplifier (or ultimately, overheats the LF driver). Some active monitors have settings for exactly this - I.e. push f3 lower and have reduced SPL, or lift it to get more. Oh and yeah, distortion in various forms, but port noise may be the first thing you reveal.

Put it this way, if such EQ had been applied in the mastering of what you're listening to, you wouldn't even think about it. If you were after like, 20dB, that might be a bit more of a challenge! Even that should be doable given low enough listening levels. Or big enough speakers and short enough listening distances, I guess :)
 
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Roy_L

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With 250 watts amplification and just 3 meters away, I'm nowhere near the amp's or speakers' limits, so I guess "stealing" a few extra db for the lower bass is fine... :)
My follow up question is how to do it correctly? Do I select a few frequencies to boost (suppose 3 db apart) or just apply a low shelf filter to everything below a certain frequency (like 45Hz)?
 

audio2920

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I would probably try a shelf first to avoid messing up the natural roll off shape, I think. Single bell correctly placed might be OK. In either case one goal is to avoid having gain above where it's needed (else you end up with that sound like you get on small/cheap speakers where the thing is tuned to give the impression they have good bass, when actually there's just a peak before the roll off.)

Frequency, Q and gain will all need refinement. To get it spot on you'd ideally measure the in-room response. But IMHO for a listening system (rather than content creation) while it's helpful to understand the science of what you're doing to some extent, ultimately, if you like how the EQ makes the system sound more without it, then it doesn't matter one iota. That's my 2 cents anyway.
 
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