Pickup placement and phase cancellation: Calcs?

[Gruuve]

With the thought of possibly putting together a semi-custom bass from Warmoth or other parts, I'm fascinated by the possibilities that the phase cancellation introduced by pickup placement introduces (assuming it's two pups...or hey, maybe more, but let's start with two).

 

Here's an article that talks about phase cancellation for two microphones recording the same source: http://www.recordingeq.com/articles/321eq.html

 

I think it's safe to say that you can model pickups the same way (they are essentially "microphones" that only pickup vibrations from metal strings rather than vibrations in the air, right?). There are of course other factors (resonance of the pickups, resonances of the woods used, etc.) but if you take all those factors out, it seems to me that you could at least get a decent estimate of what frequencies to expect would be reinforced/cancelled based primarily on the distance between pickups (and likely their distance from the bridge as well). Agreed anyone?

 

I'm searching for any article that actually shows how to calculate this...there's got to be a mathematical way to model the phase cancellation I'm sure, just gotta find it. If anyone knows of any articles on the web that might have equations or sample calcs (whether it's for pickups, microphones, whatever), please post 'em up please!

 

Dave

 

[Gruuve]

Good article...no calcs:

 

http://www.school-for-champions.com/science/noise_cancellation.htm

 

[Gruuve]

Interesting article on beat frequencies (ie. fluctuations in amplitude) due to phase cancellation/reinforcement: http://www.school-for-champions.com/science/sound_beat.htm

 

Hmmm...I wonder if beat frequencies contribute to what we perceive as "growl" in some basses? I don't know if you get beat frequencies if you mix (for instance) a fundamental tone and an even-ordered harmonic of the fundamental tone (what you'd presumably get from a musical instrument, right?), but I'd suspect you do.

 

Ah...just read a little lower in the article. The beat frequency is simply the difference between the two summed frequencies. So, it seems to say that for instance if you combined a 60 Hz wave and say a 120Hz wave (the 1st even ordered harmonic), you'd get a 60Hz beat frequency. Interesting...this suggests that an even-ordered 1st harmonic would always create a

beat frequency the same as the fundamental (assuming they are equal amplitude, which is a simplifying assumption).

 

Dave

 

[Steve Force]

Interesting article on beat frequencies (ie. fluctuations in amplitude) due to phase cancellation/reinforcement: http://www.school-for-champions.com/science/sound_beat.htm

 

Hmmm...I wonder if beat frequencies contribute to what we perceive as "growl" in some basses? I don't know if you get beat frequencies if you mix (for instance) a fundamental tone and an even-ordered harmonic of the fundamental tone (what you'd presumably get from a musical instrument, right?), but I'd suspect you do.

 

Ah...just read a little lower in the article. The beat frequency is simply the difference between the two summed frequencies. So, it seems to say that for instance if you combined a 60 Hz wave and say a 120Hz wave (the 1st even ordered harmonic), you'd get a 60Hz beat frequency. Interesting...this suggests that an even-ordered 1st harmonic would always create a

beat frequency the same as the fundamental (assuming they are equal amplitude, which is a simplifying assumption).

 

Dave

 

Interesting article--thanks for the link!

[5 string Mike]

Looking through the material, it is interesting. The articles really explain why we get 'dead frets' sometimes on our instruments, as one example.

 

Thinking out loud for a minute-- As far as building it into a bass at the construction level, I wonder if that would work with the low frequencies of a bass. I know that David Gilmour's black strat has a switch that wires the neck and bridge pups together (not sure if Series or Parallel) and it gives an out of phase sound. But, frequency range is different. That is one thing that I would have to find out first is that being on a bass, you might be able to show it on a sine wave graph, but you might not be able to actually hear it.

 

I know there are phasers for bass, but that is taking the sine wave at the wire and adding in the out of phase signal. If you are trying to do it with the natural sine waves that occur while playing, it sounds like quite a challenge.

[jeremy c]

Many people have rewired Jazz basses with a switch between series and parallel for the pickups. That's what the Fender S-1 switch is. Other brands also incorporate this kind of switch.

 

You can hear the difference but I don't think it has anything to do with phase. Nor does a dead spot. A dead spot is there whether the bass is plugged in to an amp or not.

 

Dave, I think you are overthinking this whole thing.

 

You could build a dummy body, put the pickups on rails and slide them around until you found the places that sounded the best.

 

Lane Poor had one of those in his shop.

 

You could just buy a Gibson Grabber and slide the pickup around.

[Big Daddy from Motown]

Phase cancellation between pickups is a very real consideration.

 

It's the phenomenon that produces the Eric Clapton sound on Stratocasters. I have read that it is because of one of the pickups being at the null point of the vibrating string while the other pickup is not. I don't have any ideal how to calculate it.

 

Just pick a 2 pickup bass with the same scale length as the one you want to build and put your pickups the same distance from the nut.

[1111000]

My Jazz has the S1. I can't say there's a dramatic difference between series and parallel. If anything I like in series better because the notes are more clear; however, it seems to lose a little mwah. Still, the differences are very miniscule.

[Gruuve]

You could build a dummy body, put the pickups on rails and slide them around until you found the places that sounded the best.

 

Actually, that's a thought I already had. The only issue is that it might end up sounding a little different with different woods.

 

BUT, something I could do to take all guesswork out of it is order the body with the whole pickup area between the bridge and neck routed, then I could experiment with pickup placement until my heart's content. Once I find the perfect spots, I could simply cover the open routed spaces with "ramps", which is what I actually intend to do anyway. I'm pretty sure I want the bridge pup 1/2" from the bridge, but I could also experiment with that. Not sure where I would want the neck pickup, but wherever it ends up, I can just hide the holes with ramp sections. The more I think about that idea, the more I like it. That takes a little bit of weight out of the bass body as well.

 

Dave

 

[5 string Mike]

You can hear the difference but I don't think it has anything to do with phase. Nor does a dead spot. A dead spot is there whether the bass is plugged in to an amp or not.

 

Right, but isn't the prevailing theory that with the dead spot(s), there is a resonant frequency in the wood that is 180 degrees out of phase with the string frequency, causing them to cancel each other out, resulting in a dead note?

[lug]

Plus, a perfect spot for an open string isn't gonna be the same as fretted on the fourth fret. The "sweet spot" is really a myth, it's just changing the relationship of fundamental and overtones depending location (closer to the neck = larger fundamental).

[Paul K]

Plus, a perfect spot for an open string isn't gonna be the same as fretted on the fourth fret. The "sweet spot" is really a myth, it's just changing the relationship of fundamental and overtones depending location (closer to the neck = larger fundamental).

 

Which also means any phase cancellation that occurs will also change as you go up or down the neck. Hope Dave finds the Goldilocks spot......

[Gruuve]

Hmmm...interesting thoughts here. Yes, the phase cancellation/reinforcement would indeed change as you go up the neck. However, I think the range is fairly wide for lower frequencies, so I make a wild guess that the cancellation you see from say open to 5th fret is going to be similar...5th to 12th fret probably a different flavor. It'll change but I'd expect it to change gradually rather than abruptly.

 

So, I was experimenting a bit on my bass today. One thing I'd love to have for slap is really strong low-end attack WITHOUT any need to boost the lows. (I often like to switch between slap and fingerstyle within a song, and obviously don't like to try to turn knobs or slip switches or even step on pedals mid-song). It sure would be nice to have a bass that gets a ridiculously strong LF attack on slap without having to do anything but...well...slap. So, I tried slapping over the neck (actually more like "sweet spot") and bridge pups on my Tobias. Slapping right over the pickup (regardless of which one) produces a ridiculously strong LF attack. Hmmm...

 

So, I'm thinking of trying one pickup within less than an inch of the bridge, and one pickup right up against the neck (so that when I slap, I'm slapping right over the pickup...I usually slap just short of the neck, hard to double-thump right over the neck unless the action is awefully high). With the pickups spread this far, I would tend to think that might drastically increase the bass's sensitivity to whether you are playing near the bridge or near the neck (hopefully to the point of not really even truly needing a pan knob maybe? That's a nice thought, eh? )

 

I think it's time to tear into an experimental bass. I'll probably get a reasonably priced neck and body from Warmoth, or find a decent use "donor" bass that's 5-string and has 19mm string spacing or near it.

 

Someone recently mentioned that Squire Jazz basses were excellent for the price. For a 5-string, does anyone happen to know the string spacing at the bridge or the neck width at the last fret?

 

Dave

 

[alexclaber]

There's a Java applet somewhere that models this - you plug in string tuning, scale length and pickup positions, and it shows you how the pickup positions affect the frequency response of the open string. It's pretty amazing but I'm damned if I know where to find it!

 

Alex

[Gruuve]

Aha! Bingo! Thanks Alex...

 

http://www.till.com/articles/PickupResponseDemo/index.html

 

Very cool...add a 2nd pickup, and try moving the level sliders...amazing how the FR curve completely changes!

 

Dave