Double Bass Phase Cancelling?

[Bass_Machine]

Hi all,

I'm using one of the Shadow nanoflex systems on my bass, and have recently been warned of a phase cancelling effect occurring between the two elements. Is this a recognised issue with this pickups system? Can I get around this by flipping one of the elements upside down to get them back in phase with each other? Would that be preferable to using just one of the elements?

Thanks,

BM

 

[Francois Blais]

I think the sole answer is to experiment by yourself.
Your instrument may or may not suffer from phase cancellation, and if that happens you'll need to try a few things to see what works best.
Sorry for not being more positive!

 

[Gary E. Bartig]

Keep in mind that whenever you have more than one pickup element on an acoustic instrument some distance apart, the phase relationship between the two elements will be changing as you play up and down the neck. The further apart the elements are located, the more wacky things can get.

The reason is vibrational nodes (nodes and anti-nodes) move physical location with every different note that is played. That means one pickup could be moving up, while the other is moving down at some point in the frequency range.
The two pickups (some distance apart)on certain notes could be 2 degrees apart, on other notes they might be in phase, on other notes they could be 40 degrees apart and so on.

My advice is to find the best location using one element and EQ to taste.

And when using EQ, do your best to only cut, not boost. With the vast majority of EQs, when you boost a frequency band, you cause some degree of electrical phase shift.

This stuff is maddening because of all the variables. You have the frequency response of the instrument, that of the pickup and pickup location, that of the amp, and speakers, and that of the room you are playing in.

Good luck!

 

[drurb]

^^^^^Generally good advice. The boost vs. cut suggestion arises largely out of misunderstanding. Both boosts and cuts in these active circuits produce frequency-dependent phase shifts. Applying too much boost can overload the next device in the chain and that's largely the reason that people recommend cutting rather than boosting. In truth, the best approach is to apply the shaping that results in changes across the fewest number of bands and then adjust the overall gain to prevent overload.

The reason for applying changes across the fewest number of bands is to prevent "ripples" in the spectrum. Graphic EQ devices are made up of a set of discrete non-constant-Q filters. Even if you're not familiar with such terms, the results are pretty easy to understand. Suppose you have a 10-band equalizer and you apply a 6-dB cut to all of the bands. You might think that you now have a flat spectrum reduced by 6 dB. Not even close. What you will have is a rippled spectrum with "troughs" in the response occurring at the center frequency of each of the filters.

So, let's say that you need to tailor your response to compensate for a dip around 500 Hz. You should boost that band rather than cut all the ones around it. Doing the former will lead to the smoother overall response; doing the latter will produce a rippled spectrum.

Here's a view. Notice the response that results from boosting or cutting all of the bands by various amounts:

 

[Gary E. Bartig]

Drurb, thanks for the EQ clarification!

 

[Michael Eisenman]

Yes, very clearly stated for us non-engineering types. Thx.

 

[Bass_Machine]

Wow, thanks for the detailed information!

Will definitely bear that EQing info in mind. I tend to cut the bass and treble response for the room I play in most often (both my regular groups use the same rehearsal space), and I think that's a good place to start as a guideline for how my bass sounds through my rig, which I can adjust for different live venues. She's going in for some soundpost adjustments and a restring soon though - could be opening pandora's box....

I think, being a jazz guy rather than an acoustic engineer, I'll just fall back on the classic "using my ears" approach! (Never ever hurts to have an academic basis for what I'm doing though, knowledge is pretty powerful...). I'll try playing with one element to see if there's something I never knew I was missing, but honestly I've never been overly troubled with the sound of both elements together.

 

[JtheJazzMan]

An obvious problem with dual element double bass pickups is they are often on opposite sides of the bridge. Im surprised people can avoid phasing issues at all with this setup. In fact if you could split the 2 elements and analyse both signals Im sure youll always find phasing. You may be lucky that the phasing is in frequencies which dont detract from your idea of a good amplified tone.