Is My Null Fixed?

[DIYer]

As to why it made an improvement at all, well if his mains were somehow creating sound that low (where the null is) and he removed the sub's output down there, that could explain why the null disappeared.

That was my thought as well. Removing cancellation by (partially) removing the second source.

Of course the switch could have been installed upside down also; thus, reversing the "ON" and "OFF" positions.

Zing, on the back, does it say where it's "Made in"?

 

[Zing]

Of course the switch could have been installed upside down also; thus, reversing the "ON" and "OFF" positions.

It's not installed upside down because when it's in the "On" position, the crossover adjustment knob works. That is, when turned, you can actually hear the change. When in the "Off" position, turning the knob has no effect.

Zing, on the back, does it say where it's "Made in"?

It does not say anything, surprisingly. But my sub is about 4 or 5 years old and, back then, SVS was using BASH amps made in Canada.

 

[soundhound]

Glad you got it working, but it's mysterious to me why setting a crossover at 80Hz had such an effect on the frequencies well below that. Yeah I mean I know the xo point is a shoulder, so there's some slight falloff below it, but still shouldn't it basically be a low pass filter? Why does it seem to be boosting the 30-70Hz range so much?

Not arguing that your RTA result isn't real, just wondering how what you did accomplished that...

The higher frequencies might be interacting with ones below in destructive or reinforcement to those frequencies. Also, the shift from one driver to the other for the higher frequencies could have an effect.

I cured a problem I had at 200Hz by crossing over my subs at 100Hz. Moving the source of frequencies between between 60Hz and 100Hz from my mains to my subs solved the problem.

 

[Flint]

I have a theory as to why this happened.

Crossovers are all gradual, so the even though the crossover on the mains may be set to 80Hz, there can still be some output below that frequency which is interacting with the output from the subwoofer.

It is possible the crossover introduces a phase shift to the signal which better aligns the phase of the subwoofer output to match the output of the main speakers.

Just a theory, but I am not 100% confident in it.


Crossovers, signal interaction, and room effects all play a role in what you experience, and tuning a system really is a very complex game. This is why I am always very cautious about encouraging people to build their own speaker systems and design the crossovers themselves. Even with the best tools available, there is a ton of "art" required to get it right.

 

[Flint]

By the way, you still have a room mode at 45Hz and 65Hz (resulting in nulls and nodes at higher octaves).

The main improvement is in the range of 40Hz to 80Hz, which is where the sub and the mains are overlapping. 40Hz is only one octave below the 80Hz crossover point.

 

[nwboater]

The purple trace is the typical response I've been dealing with since moving the sub up front. A few different adjustments here and there have altered it in some fashion but I'm always left with some form of that W-shaped null between 35 and 90Hz.

I had some time this afternoon to play around and the yellow trace is the result. I say that's a significant improvement. What say you?

I'm wondering if the original problem isn't the result of room modes and some phase cancellation between your sub and mains. Have you run a graph with just the mains, and then just the sub? Might be informative?

Does your sub amp have a phase adjust? Or at least a 'Reverse switch'?

If no to both of those, it might be informative to reverse the phase between the sub and mains. With the built in sub amp you can't easily do it there, but you could reverse the leads to your mains.

Does the BFD have a variable phasing ability?

Good luck.

Rod

 

[soundhound]

Does the BFD have a variable phasing ability?

No it does not. Its really made for one purpose, that being feedback suppression in PA systems, and a phase adjustment would not be called for in that application. The fact that it has 20 channels of fully parametric equalization per channel makes it also very useful for flattening subwoofer response.

I would definitely not use it above around 100Hz though for even mildly critical home reproduction, as its converters are, in spite of being "24/96", pretty crappy. A good analogy would be a cheap point and shoot digital camera which has "14 megapixels"; the trouble is those pixels are small, noisy, and the lens can't discern the resolution anyway.

 

[DIYer]

SH, do you only trim with your FBD? I hear some say boosting with EQ is a bad idea so I'm wondering.

 

[soundhound]

SH, do you only trim with your FBD? I hear some say boosting with EQ is a bad idea so I'm wondering.

I use 14 of the 20 available bands, and all of them except for two are trimming. Those two have boosts of I think 2dB or so. While it is preferable to trim-only, boosting in some bands won't cause any harm as long as they are relatively modest. The problem with overly boosting a band is the possibility of clipping either the electronics or the subwoofer. My system has gobs of headroom so there's little possibility of that.

I could have lowered every band by 2dB except for the two I boosted, but I decided that the trouble was not worth the effort. In addition, doing that would screw up the gain structure between the subs and the mains (the overall levels in the Feedback Destroyer Pro are not adjustable.

 

[Zing]

Does your sub amp have a phase adjust? Or at least a 'Reverse switch'?

My sub has a variable phase adjustment between 0 and 180. In a previous round of RTA testing, I was able to determine that a setting of about 125 degrees gave me the smoothest response.

Truthfully, I don't know the exact phase relationship. The 0 mark is at the 7:00 position, the 180 mark is at 5:00 and its current setting is somewhere between 1:00 and 2:00. So assuming 12:00 is 90 degrees, it's only a guess that it's currently set to 125 degrees.