BFD Boost Question (1 Viewer)

[MartyP]

I finally took the plunge and inserted a BFD in my system to help me with my subwoofer problems. I've had terrific results so far with smoothing out bass response by cutting several frequencies. I have a dip at 80 Hz that requires about a 5 dB boost. Will this boost adversely affect my sub (ACI Titan)with regards to dynamics, internal amp power requirements etc?

 

[Wayne A. Pflughaupt]

This post should help.

Basically any full-range sub equalizing will place additional demands on the driver and amplifier, so you have to have some headroom going in. Keep in mind, too, that some low spots are nulls and can’t be equalized. You can easily spot them – no matter how much you boost, the null gets the same reading.

Regards,
Wayne A. Pflughaupt

 

[MartyP]

Wayne:

Thanks. Yeah, that post was very helpful. I "lurk" around here alot and appreciate your insight...you were the first one to recommend to me EQing the sub in the first place.

 

[Richard Greene]

Wayne is correct that one can't EQ nulls.

However you don't have nulls, you have what I like to call "partial nulls" ... and in addition, it's very likely your ears are not located at the deepest portion (weakest bass) of any of these partial nulls.

A true null would have no output at all because it would be caused by a sound wave meeting an equally loud out-of-phase reflection. Obviously EQ is impossible.

A partial null is when a sound wave meets an out-of-phase reflection that is not as loud = partial cancellation.
EQ is possible but not recommended for several reasons:

- Partial nulls are not that noticeable -- when listening to music it's much easier to notice when a bass note is too loud than too weak. (Perhaps a bass guitar musician would easily notice a specific note was not loud enough, but not the average audiophile.)

- Partial Nulls are very narrow -- you'd practically need your head clamped in place so the EQ corrections worked as intended. Just lean to one side and the EQ corrections would be wrong. While your ears may be in a partial null, far larger portions of the listening room will have bass peaks at the same frequency. So using EQ to boost output for your ears will create even larger bass peaks at other listening positions.

- Extra amp power and driver XMAX needed

In a real listening room, averaging two measurements made eight inches apart at a real listening position
(8 inches apart represents left ear & right ear measurements) I have never measured a partial null
deeper than -6dB. Perhaps it's just a coincidence that none of the one-dozen listeners happened to be sitting in the deepest part of any partial null for subwoofer frequencies which were most likely deeper than 6dB.


If one plays a sine wave tone and walks around the room with a sound meter there will be a specific location where a partial null measures up to -20dB or more ! But that doesn't matter much if no one is sitting there.

 

[Nathan Stohler]

I was searching the forum, trying to find an answer to my question, and I thought I'd exhume this thread since it kind of relates to my problem.

I've been spending some time trying to EQ my speakers (my fiance is starting to get annoyed with my fiddling). First, I wanted to focus on the range below 160 Hz, using 1/6 octave steps. By changing the polarity on my sub from 0 degrees to 180, I made a big improvement.

So, I had a fairly flat curve from 25-160 Hz. Then, when I went to look at the rest of the frequency range, I found that my listening position had a big null at 200 Hz. Boosting at 200 Hz with the built-in EQ on my receiver didn't help (it only boosted the frequencies on either side of it -- 160 Hz and 250 Hz).

3 feet or so to the left, the SPL measures about 90 dB, but in my "sweet spot" (well, maybe not), it measures around 60 dB. How would one go about fixing this problem? Or maybe the null is so narrow that I shouldn't care? I suppose I could create some more test tones near 160 Hz to find out just how big this null is.

Here is my response curve (see subsequent post below). The blue line is polarity 0; the purple/pink line is polarity 180. I'm having trouble understanding how the phase change affected frequencies at the high end of the curve since I'm crossing over at 70 Hz:

[link deleted]

Thanks in advance.
--Nathan

 

[Wayne A. Pflughaupt]

Nathan,

The link is not happening, so I can’t comment on your response curves.

Nulls are usually pretty narrow – 1/16-octave or so. I wouldn’t worry about the 200Hz problem unless you are actually hearing something you don’t like. Moving the chair over a little may indeed solve that problem, but you may end with a similar problem at a different frequency.

It’s not unusual for the phase irregularities introduced by an electronic crossover to affect response an octave or more in either direction of the crossover frequency. The problem you're seeing at 200Hz is only 1-1/3 octave from the 70Hz crossover frequency.

Regards,
Wayne A. Pflughaupt

 

[Nathan Stohler]

Wayne,

It looks like I'm forbidden from linking my pictures on Geocities to other sites. Here is the hard link:

"http://www.geocities.com/floyd_1977/pictures/htf/cal1.jpg"

Thanks for your help. I was hoping you would respond, as you are very knowledgable about equalization and freq. response.

--Nathan

 

[Wayne A. Pflughaupt]

Nathan,

Yes, I’ve heard before that is the case with Geocities (unfortunately).

Notice that there is no drastic change in the response curve either way. The basic “shape” of the peaks and dips are the same, only the pink is a little higher in SPL level. So I can’t see where the phase switch did anything especially beneficial or detrimental.

Here’s a little something I’ll throw in for your edification. I noted in my last post that phase changes are typically seen an octave or so in each direction from the crossover frequency, and you can see that here. Notice that the difference you see with the pink line merges with the blue line at 31.5Hz, which is slightly more than an octave below 70Hz. At the other end, you can see between 143 and 160Hz that the two lines are on their way to merging there, too. Assuming they would be merged by the next marker, that would be an octave plus 1/3 above 70Hz.

Regards,
Wayne A. Pflughaupt

 

[Nathan Stohler]

Thanks again, Wayne.

I see what you're saying about the phase switch not really improving anything. I guess I perceived the change at 143 Hz to be an improvement, especially when you consider the response curve from another listening position (in quotes to avoid an automatically generated link):

"http://www.geocities.com/floyd_1977/pictures/htf/cal2.jpg"

I suppose the increased SPL (average of about 4 dB) could be seen as an improvement, since the sub doesn't need to work quite as hard at the same output level.

It looks like my room could benefit from a BFD; I'll put that on my list.

--Nathan