# BFD - Parametric EQ below 20hz



## Zeitgeist

Is there any way to EQ frequencies lower than 20hz with a BFD?

It's been a while since I've used REW, but I just downloaded it again.

I believe that you can EQ lower than 20 by selecting a frequency of 20 and then a Bandwidth (BW/60) of 25 to get 11.8hz for example?

I know that you need to be careful with sub excursion.. I just want to EQ below 20hz a little bit to boost the response in a sealed sub cabinet.

Thanks!


----------



## Wayne A. Pflughaupt

The BFD can certainly reach below 20 Hz, but 11.8 Hz is about 3/4 octave below 20 Hz. It may be unrealistic trying to do anything worthwhile that far away from a filter's frequency center. A bandwidth of 25 @ 20 Hz and boosted 8 dB would lift 12 Hz no more than 2-3 dB. The same filter would require a very wide bandwidth of 60 to lift 12 Hz 6 dB. And you have to keep in mind how either filter would substantially affect things above 20 Hz. You would most likely need “counter-filters” to reverse the effect.

BTW, you can model filters in REW – just select the BFD as the equalizer of choice, open the EQ panel, and enter the parameters. REW will show the electronic response of the filter(s) on-screen.

Regards,
Wayne


----------



## Zeitgeist

Interesting.... I knew that there would be some difficulty in trying to do something far away from the freq center. 

So, if it's difficult to boost below 20 hz (And I think that the BFD lists it's freq range from 20hz+ by design..) with BFDs..

Is there a better way to EQ infrasonics?

I've seen some devices that let you pick a db boost and a frequency of say 15/16hz plus (and that's it)-- but is there anything else out there? I could be wrong too. I thought the CX2310 could boost below 20 - but double checked and it's just a low pass filter that can cut as low as 10hz.

I'm going to build a sealed 10cuft sealed amp with an Exodus Audio Maelstrom X2 (18") - and was curious about boosting below 20 hz a little bit.


----------



## mdrake

There is also another option to boost the low-end using a Linkwitz Transform circuit. 
http://sound.westhost.com/linkwitz-transform.htm

Matt


----------



## Wayne A. Pflughaupt

> Is there a better way to EQ infrasonics?
> 
> I've seen some devices that let you pick a db boost and a frequency of say 15/16hz plus (and that's it)-- but is there anything else out there? I could be wrong too. I thought the CX2310 could boost below 20 - but double checked and it's just a low pass filter that can cut as low as 10hz.


The Rane PE-17 and Symetrix 551 analog parametrics accommodate filter centers down to 10 Hz. Both are discontinued; you’d have to eBay them.

Regards,
Wayne


----------



## Zeitgeist

Cool. Thanks for your help!

It always seems like the good EQs have all been discontinued... with everything going totally digital!


----------



## Zeitgeist

Wayne A. Pflughaupt said:


> The BFD can certainly reach below 20 Hz, but 11.8 Hz is about 3/4 octave below 20 Hz. It may be unrealistic trying to do anything worthwhile that far away from a filter's frequency center. A bandwidth of 25 @ 20 Hz and boosted 8 dB would lift 12 Hz no more than 2-3 dB. The same filter would require a very wide bandwidth of 60 to lift 12 Hz 6 dB. And you have to keep in mind how either filter would substantially affect things above 20 Hz. You would most likely need “counter-filters” to reverse the effect.
> 
> BTW, you can model filters in REW – just select the BFD as the equalizer of choice, open the EQ panel, and enter the parameters. REW will show the electronic response of the filter(s) on-screen.
> 
> Regards,
> Wayne


Thanks for the recommendation to model in REW. I was having issues with REW on one machine (I assume Java version issue).. and I tried it on another one and had success.

I now understand the difficulty in EQing far away from the center with a BFD. I played with it quite a bit and you have to have a REALLY high gain, and then it's hard to create another filter that offsets it perfectly in the frequencies that you don't want to boost.

You only end up with 3-5db gain or so below 20.. I don't think I fully got it until I started modeling it.


----------



## Zeitgeist

......and I bought the Symetrix 551.

When EQing below 20hz I can SEE it in the woofers........ but definitely not hear it (DUHHH!).

I know you can feel below 20hz - but for some reason expected it to make more of an impact. I was testing with a sealed Tempest-X. I know that most subs roll off quite a bit below 20 - so even with EQing above 20hz you're not really doing a whole lot. I should probably play with the 8cuft sealed Maelstrom-X that I built - see what that looks like.

Fun to play with regardless though - for anyone interested in 20hz and below.

I guess next up would have to be a IB setup to play with.


----------



## Wayne A. Pflughaupt

Thanks for the update! I don’t have a “subsonic” subwoofer myself, but I’ve heard (felt) the difference between subs that could do 20 Hz vs. 18 Hz. That said, I’ve often wondered at what point ultra-low end output no longer made an audible or visceral difference.

Regards,
Wayne


----------



## lsiberian

I'd be worried about pushing the driver to hard down that low.


----------



## Zeitgeist

lsiberian said:


> I'd be worried about pushing the driver to hard down that low.


The subs I've tested with are in sealed enclosures - and I've just done short test tones.

I'm aware of the risks of excursion and heat..

With a Maelstrom and a Tempest - on each channel of an EP4000 - I don't really have the power to drive them to their limits anyway.


----------



## Zeitgeist

Wayne A. Pflughaupt said:


> Thanks for the update! I don’t have a “subsonic” subwoofer myself, but I’ve heard (felt) the difference between subs that could do 20 Hz vs. 18 Hz. That said, I’ve often wondered at what point ultra-low end output no longer made an audible or visceral difference.


That's good to know - because I was starting to think that there wasn't really much difference at all below 20hz! I just see the *amazing* amount of content that's on DVDs that are below 20hz. There is quite a bit.

The way that people with Infinite Baffle subs talk, some of which are flat down to 10 hz, I have to figure that some of that low content must be fairly noticeable. Or perhaps more accurately - only noticeable because of the fact that the IBs are so much louder at infrasonic frequencies. 

I should probably listen to a couple scenes with it boosted below 20 hz - to see if that makes a difference. Like a nerd I really just played with test tones and never even thought to play some actual content!

l think lsiberian has a good point though.. Probably should be careful because of the excursion required at such looooooow freqs.


----------



## Zeitgeist

Wayne A. Pflughaupt said:


> Thanks for the update! I don’t have a “subsonic” subwoofer myself, but I’ve heard (felt) the difference between subs that could do 20 Hz vs. 18 Hz. That said, I’ve often wondered at what point ultra-low end output no longer made an audible or visceral difference.
> 
> Regards,
> Wayne


Sooo.. I feel incredibly stupid. The other day I was hooking up a second EP4000.... and I looked at the DIP switches for my current one............. Could not believe it, but the low-cut filter was enabled!!!! It was cutting off at 30hz.

I never figured it out because I had a HSU sub that was putting out enough low freqs that the REW graphs look alright at low frequencies (lots of room gain apparently).

Since I have little experience with low freqs - I had no idea what I was looking for..

The "Irene" scene in Black Hawk Down is freaky now.

Now I can really play with the low freqs.......... onder:


----------



## lsiberian

Zeitgeist said:


> Sooo.. I feel incredibly stupid. The other day I was hooking up a second EP4000.... and I looked at the DIP switches for my current one............. Could not believe it, but the low-cut filter was enabled!!!! It was cutting off at 30hz.
> 
> I never figured it out because I had a HSU sub that was putting out enough low freqs that the REW graphs look alright at low frequencies (lots of room gain apparently).
> 
> Since I have little experience with low freqs - I had no idea what I was looking for..
> 
> The "Irene" scene in Black Hawk Down is freaky now.
> 
> Now I can really play with the low freqs.......... onder:


yeah it's amazing the stuff we miss. For a long time I didn't have my sub plugged into the right out on the receiver and wondered why my bass stunk. Only to find the rca cable hooked up to the aux in rather than the pre-out.


----------



## johnplayerson

When listening to 99 percent of music, there really is not much to hear, or any benefit to, processing below 20 hz. Most music has very little there, in the best of cases. 

With home theater however, you have lots of low bass being used, in the absence of higher bass notes, in scenes where 10 hz to 20 hz is slowly and constantly played and increased, to go along with something exciting supposedly about to happen in the movie. Because all this material is in the soundtrack there is very little need to process it.


----------

