# BFD and Response Curve

Discussion in 'Home Theater Projects' started by DavidNu, Jan 13, 2004.

1. ### DavidNu Auditioning

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I'm about to receive my BFD in the next few days and I'm looking to tame a couple of peaks in my response curve. I wonder if anyone could suggest a couple of filters? I fairly understand how to setup the BFD, but I'm don't really understand the bandwidth settings. Here the figures below:

16hz 95.5db
18hz 100db
20hz 101.5db
22hz 97.5bd
25hz 93db
28hz 93db
31.5hz 93db
36hz 94.5db
40hz 95.5db
45hz 98db
50hz 96.5db
56hz 90.5db
63hz 82.5db
71hz 77.5db
80hz 78.5db
89hz 73.5db
100hz 66db
111hz 69db
125hz 67.5db
142.5hz 61.5db
160hz 52.5db

The above is measured with the sub only (no mains) and the SPL is compensated (not raw).

I guess I would look at tameing 20hz and 40-50hz. Could anyone suggest a couple of filters?

Many thanks

2. ### Wayne A. Pflughaupt Moderator Moderator

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You calculate bandwidth first by determining how wide the peak or valley is.

For instance, your 45Hz peak - notice that the readings in both sides of it (40hz and 50Hz) are about the same. Thus the peak resides between 40 and 50Hz. So how wide is the peak? Well, you probably already know that your test tones are at 1/6-octave intervals. So between 40-45Hz is 1/6-octave, and between 45-50Hz is another 1/6-octave. That means this peak is about 1/3-octave wide.

Once you determine the width of the peak or valley, you set the bandwidth for half that amount. So, you use a 1/6-octave filter to address a 1/3-octave peak or valley. In real-world usage, however, you will find that the bandwidth the filter actually affects will depend on the severity of the boost or cut. The BFD especially is notorious for affecting a wider area than the set bandwidth (it is a cheap equalizer after all), especially with boost or cut values of more than 6dB. So you will probably have to tweak the filter's width to make sure it doesn't cut a wider path than you want.

The peak at 20Hz is another situation, since it is asymetrical - that is, response falls more steeply on one side than the other. Same with the dip at 100Hz. This situation typically requires two or more filters to address, since the filters alter response in a symetrical fashion.

However, the lowest frequency setting you can adjust with the BFD is 20Hz. I'd start with a 1/6-octave filter there, too.

A few other things: Since you're going to equalize, there is no need for any "move the sub around the room and find the spot where you get the smoothest response" excercises. That's for people without an equalizer. Go ahead and put the sub in the best-available corner (the one with the longest uninturrupted wall length in both directions) to maximize output and extension. The BFD will take care of the response-smoothing.

Also, when you get the EQ in-hand and get serious with tweaking, take your readings with both the sub and L/R mains on.

Regards,
Wayne A. Pflughaupt

3. ### DavidNu Auditioning

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I already have the sub in the corner and it seems to get the best response there. Your explanation has helped a lot, so it's just a matter of waiting for the BFD and experimenting

By the way, I have already run test tones with the mains on, although seeing as I won't be using the BFD to eq the mains, what would be the use of this? Is it so you can further eq the sub once you know how the sub interacts with the mains, say around the xover mark, 80hz or higher?

Thanks

David

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