BFD Guide Revisited

I recently bought 2 x GRAMMAs for my subwoofer so I decided to start my BFD settings from scratch and document everything so beginners have some idea of what to do. I'll assume you already have a BFD and it's all setup so we'll go straight to the actual graphs and filters.

 

 

 

Taking Measurements For The Graph

The Sinewaves used are analogue so you have 2 choices really when it comes to taking measurements, Stereo or Pro Logic. The majority of users swear by Stereo and I used to but recent experiments in my listening room have altered my opinion. When in the Stereo mode I could get a nice flat response but still have issues with bass peaking during movies, it was only when I tried the readings in the THX Pro Logic mode that I discovered the cause. My center speaker was outputting a significant amount of bass from where my Axial modes excited the room at 50Hz and 56Hz respectively, in the Stereo mode I was only curing the effect of the front L&R. This is something each user will have to decide for themselves, in my scenario Pro Logic definitely works better.

On with the graphs then, I take the readings with the Radioshack Digital SPL in an upright position angled forwards slightly positioned where my head would normally be in the ideal listening position. I set a volume on the processor that generates a decibel reading in the region of 80~90 db with the sinewaves, in my case this was -23Db on the processor.

 

Below is my first readings with the BFD engaged but with no filters activated. Looking at the curve I can get an idea of what I want ideally, beyond 80Hz is out of my control.

For people interested in room behavior you can clearly see the 2 peaks are an exact doubling of the frequency, these are my Axial room modes that occur at 50 and 56 Hz respectively

My first 2 filters will be targeted at the dip at 31.5Hz and the peak at 50Hz. At this point I make an educated guess as to what bandwidth or Gain is required. Below is the result after my initial filters :

The first filter was almost spot on, the second filter is interpreted incorrectly on the graph, the 45 & 56 Hz measurements were much lower than the 50Hz reading so I now know that the 50Hz reading should be tackled on it's own

 

The peak at 50Hz actually covers 49, 50, 51 and 52 Hz then falls dramatically from 53 onwards so I need to cure this small zone with some hefty negative filters. After about 5 minutes of trial and error I settled on 2 filters, the first was at 50Hz with a bandwidth of only 2 and a negative filter of -15. The second filter was at 51Hz with a bandwidth of 2 again and a negative filter of -17. As you can imagine the intensity of this Axial mode is scary but these were the results after those 2 filters, note the slight effect it has on the curve preceding it.

 

Not bad eh, now it's time to look at the trough at 71Hz and the cause of much conjecture in the BFD world.......Boosts. Any boost you make to a frequency with the BFD will cut into the overall output of the sub, the problem is how much can you get away with before it becomes noticeable to your ears. In my humble opinion you can safely use boosts of up to +4db once or twice and get away with it as long as your sub/subs have significant wattage (SVS, Servo 15, CHT-15, HGS Range). Looking at the trough at 71Hz I know a huge positive gain would be needed so I went along with it for the purposes of showing just what a boost can do. Bear in mind I have 1250 watts of power to play with so lesser subs would see greater losses.

 

 

That's a +13Db Gain used there so what has it done, well look closely at the pink line and how it travels then scroll up and look at the pink line before I activated a +13 Gain, note the size of the gap between the blue and pink lines, that is the reduced output after a +13db Gain. In my opinion that huge boost has hardly affected the overall output, certainly not that I could hear anyway. The fact that I could not hear a difference with the huge Gain in place was the reason I've deactivated it, why have an unnecessary boost of that size was my thinking but if anyone out there is scared of using a positive Gain I hope I've alleviated your concerns to a degree.

 

The Results With Movies & Music

In a word, TIGHT. The tightness and accuracy of installing the GRAMMAS is unreal to me and I'm used to the tight bass from my sub. Couple that with the numerous room treatments and the BFD and you get a hugely impressive sound that brings a smile to my face on a constant basis. Many people believe room treatments and BFDs should be exclusive, to me they are a perfect couple............a symbiosis :-)

 

Phil J