Stiffening Rods and Dead Spots

[Beej]

All necks, regardless of construction, have resonances - not just one, but multiple frequencies within the range of the bass.

When a neck vibrates at one of its resonances, different places move more (antinodes) and others move very little (nodes). When the fret that you are playing is at an anti node of the same frequency, the neck (which moves easily at that point at that frequency) takes energy from the string readily, the energy leaves the string quickly, and you have a dead spot.

A lot of talk on here about “bridges coupling energy into the body” and such is complete hogwash. Any time you couple energy from the string into anything else, you are taking energy from the string, which kills sustain. To have good sustain, you need a stuff, massive anchor at both ends of the string, so that energy stays in the string.

Dead spots occur because the neck (which is less stiff and less massive than the body end) is stealing energy from the string at certain point at certain frequencies. I understand the Physics well (I studied this stuff in college). The common wisdom you read on here and other places is a complicated mess of information and misinformation.

Yes, I understand how dead spots work with regard to antiresonance, but I'm personally more interested in my last line:

"It would be interesting to understand some of the engineering approaches that could be utilized to somewhat predictably alter the coupled oscillator relationship and eliminate dead spots."

Any thoughts?

 

[micguy]

Yes, I understand how dead spots work with regard to antiresonance, but I'm personally more interested in my last line:

"It would be interesting to understand some of the engineering approaches that could be utilized to somewhat predictably alter the coupled oscillator relationship and eliminate dead spots."

Any thoughts?

1) Semantics - as you're asking an Engineer, I'll say that we use the word resonator to describe what we have here - the strings and the neck resonate, which means they move in repetitive motions, but they die out over time. An oscillator is a device with includes some means of supplying energy on an ongoing basis, so the thing will run continuously, (without dying out). A bass with good sustain is a resonator, a bass in feedback though the PA is an oscillator.

OK, that nit pick out of the way,

There isn't a practical way of making a neck that has no resonances. Thus, you're not really going to get rid of dead spots, you're going to manage them. Wood is a very special material - if you look at the periodic table of elements, there aren't any solid (at room temperature) elemental things that are in the same density range. So, you're going to be working with something hollow or a composite structure with a lot of spaces in it as a material. You want something that, assuming it has non - isotropic properties (hollow or fibrous structures tend to be non - isotropic) has stiffness oriented in the one direction you really need it - along the length of the neck. Wood is actually a very good material for our purposes, and reinforcing it with something to deal with constant stress (a truss rod) is a good strategy. So, you find species that are stiff enough and not too heavy; what you get with those stiffness and mass properties is something that is resonant in line with them. Wood has the advantage of having some damping (a lot of other materials have less) which reduces some of the resonance issues.

Once you've decided on a wood species (maple is a good choice generally), then you have geometry to deal with. A floppy neck is generally a poor neck - not much sustain, and more obvious dead spots - a Fender J neck without reinforcement rods is a good example of this, but there are necks that are even floppier (Ibanez trades off stiffness for a very thin neck - they play "fast" (according to folks with small hands at least), but you gotta play a lot of notes to cover the short sustain.

Last, but not least, you put some tuners on it - tuners are a mass load on the end of the cantilever that is your neck, and they affect the frequency at which it resonates by virtue of how much mass they add.

Once all this is decided, you now have a neck system that you can start your experimentation with. As the math that tells you what resonant frequency a neck is has square roots in it, things don't move too fast - you may need to make big changes to various parameters to change the resonant frequency significantly.

OK, you take that neck mount it to a body, and start vibrating it. With the right instrumentation, you can find the resonant frequencies of the neck - the lowest one (where the whole neck moves together) is usually not a problem, except some headless designs will have an issue with this around the open A string - the worst (by far) dead spot I have ever encountered on a bass was the open A on a headless bass.

The next mode is the one that we're most familiar with - the neck moves in two sections - the headstock is moving one direction, most of the rest of the neck is moving the opposite direction. The frequency is typically somewhere around the C - the 5th fret on the G string. If the spot where it moves the most is also at the same fret that corresponds to that resonant frequency, you have an obvious classic dead spot.


There are really only three strategies that are useful in helping this:

1 & 2) - You either try to tune the neck frequency up, or down - either way, you're trying to separate the resonant frequency of the neck from lining up with the fret where the string has that frequency on it. I have had good luck with moving thing higher w.r.t. resonant frequencies. To do this, you use a bigger neck (more stiffness due to the thicker cross section) and/or a stiffer species (roasted maple is stiffer than regular maple) or a lighter species - roasted maple, is lighter then regular maple, this is unusual, but very good, in that lighter woods typically are less stiff. Less mass loading on the headstock (a smaller headstock, and/or lighter tuners) is also helpful in driving the resonance up. On my basses, I don't one of these things, I do ALL of them, and it helps. If you want to tune things the other way, people have often added weight to a headstock. As I don't like heavy basses or neck dive, I don't favor this approach.

3) You tune the strings to different pitches - this has the same effect as the first two strategies, in that you're getting things to hopefully not line up. When you try a bass out at a store, the FIRST thing you should do is tune it to pitch - it it isn[t to pitch, it can heave dead spots that you will completely miss. At one point, I had a bass that I kept tuned to D standard - the neck was more consistent in that tuning than in E standard, so I chose it as the bass that got that tuning. Keep in mind that re-tuning is not necessarily a panacea - if you lower the tuning enough, the lowest resonant frequency of the neck (which is usually tuned a bit below the open A string) can start to creep in and make your "downtuned A" string (which might be G or lower) into a dead spot. If Leo had experimented enough with dead spots he might have decided that bass guitar should be tuned to a slightly different range.

One other note - changing the length of the neck also retunes it. i have a Squier Bronco I bought as a beater, and darn if the much shorter neck doesn't hide whatever dead spots it might have extremely well - my main basses have "areas of reduced sustain" that almost noone would call a dead spot, but they are noticeable if you really know what to listen for. That Bronco is even more consistent sustain wise than my main (much more expensive) basses (it does, admittedly have a bit less sustain overall, but it's very consistent.

 

[Beej]

1) Semantics - as you're asking an Engineer, I'll say that we use the word resonator to describe what we have here - the strings and the neck resonate, which means they move in repetitive motions, but they die out over time. An oscillator is a device with includes some means of supplying energy on an ongoing basis, so the thing will run continuously, (without dying out). A bass with good sustain is a resonator, a bass in feedback though the PA is an oscillator.

OK, that nit pick out of the way,

There isn't a practical way of making a neck that has no resonances. Thus, you're not really going to get rid of dead spots, you're going to manage them. Wood is a very special material - if you look at the periodic table of elements, there aren't any solid (at room temperature) elemental things that are in the same density range. So, you're going to be working with something hollow or a composite structure with a lot of spaces in it as a material. You want something that, assuming it has non - isotropic properties (hollow or fibrous structures tend to be non - isotropic) has stiffness oriented in the one direction you really need it - along the length of the neck. Wood is actually a very good material for our purposes, and reinforcing it with something to deal with constant stress (a truss rod) is a good strategy. So, you find species that are stiff enough and not too heavy; what you get with those stiffness and mass properties is something that is resonant in line with them. Wood has the advantage of having some damping (a lot of other materials have less) which reduces some of the resonance issues.

Once you've decided on a wood species (maple is a good choice generally), then you have geometry to deal with. A floppy neck is generally a poor neck - not much sustain, and more obvious dead spots - a Fender J neck without reinforcement rods is a good example of this, but there are necks that are even floppier (Ibanez trades off stiffness for a very thin neck - they play "fast" (according to folks with small hands at least), but you gotta play a lot of notes to cover the short sustain.

Last, but not least, you put some tuners on it - tuners are a mass load on the end of the cantilever that is your neck, and they affect the frequency at which it resonates by virtue of how much mass they add.

Once all this is decided, you now have a neck system that you can start your experimentation with. As the math that tells you what resonant frequency a neck is has square roots in it, things don't move too fast - you may need to make big changes to various parameters to change the resonant frequency significantly.

OK, you take that neck mount it to a body, and start vibrating it. With the right instrumentation, you can find the resonant frequencies of the neck - the lowest one (where the whole neck moves together) is usually not a problem, except some headless designs will have an issue with this around the open A string - the worst (by far) dead spot I have ever encountered on a bass was the open A on a headless bass.

The next mode is the one that we're most familiar with - the neck moves in two sections - the headstock is moving one direction, most of the rest of the neck is moving the opposite direction. The frequency is typically somewhere around the C - the 5th fret on the G string. If the spot where it moves the most is also at the same fret that corresponds to that resonant frequency, you have an obvious classic dead spot.


There are really only three strategies that are useful in helping this:

1 & 2) - You either try to tune the neck frequency up, or down - either way, you're trying to separate the resonant frequency of the neck from lining up with the fret where the string has that frequency on it. I have had good luck with moving thing higher w.r.t. resonant frequencies. To do this, you use a bigger neck (more stiffness due to the thicker cross section) and/or a stiffer species (roasted maple is stiffer than regular maple) or a lighter species - roasted maple, is lighter then regular maple, this is unusual, but very good, in that lighter woods typically are less stiff. Less mass loading on the headstock (a smaller headstock, and/or lighter tuners) is also helpful in driving the resonance up. On my basses, I don't one of these things, I do ALL of them, and it helps. If you want to tune things the other way, people have often added weight to a headstock. As I don't like heavy basses or neck dive, I don't favor this approach.

3) You tune the strings to different pitches - this has the same effect as the first two strategies, in that you're getting things to hopefully not line up. When you try a bass out at a store, the FIRST thing you should do is tune it to pitch - it it isn[t to pitch, it can heave dead spots that you will completely miss. At one point, I had a bass that I kept tuned to D standard - the neck was more consistent in that tuning than in E standard, so I chose it as the bass that got that tuning. Keep in mind that re-tuning is not necessarily a panacea - if you lower the tuning enough, the lowest resonant frequency of the neck (which is usually tuned a bit below the open A string) can start to creep in and make your "downtuned A" string (which might be G or lower) into a dead spot. If Leo had experimented enough with dead spots he might have decided that bass guitar should be tuned to a slightly different range.

One other note - changing the length of the neck also retunes it. i have a Squier Bronco I bought as a beater, and darn if the much shorter neck doesn't hide whatever dead spots it might have extremely well - my main basses have "areas of reduced sustain" that almost noone would call a dead spot, but they are noticeable if you really know what to listen for. That Bronco is even more consistent sustain wise than my main (much more expensive) basses (it does, admittedly have a bit less sustain overall, but it's very consistent.

Thanks for taking the time - regarding the 'semantics' point, I was working from this operational definition of those concepts rather than working jargon: Resonance - Wikipedia

I also didn't mention or allude to building anything 'without resonances', as that's an impossible goal and so underscores an unrelated point. What I wanted to know from the hive mind is if there is a 'predictable' methodology to employ in construction that could move the resonant frequency leading to a dead spot up or down the neck such that it is out of the range of the frequencies of the instrument (or minimized).

"Tuning the frequency" up or down out of the range of the neck (Point 1/2) is what will result in the removal of the dead spot and so that's also been previously established - it's the generic 'how' of how to get rid of dead spots, but does not identify a predictive methodology. It may be that there are too many interdependent variables in the system to be able to effectively and reliably 'build' a dead spot out of a neck, and that's alright if that's the case.

Point 3, finding an alternate tuning of the strings is also not fruitful in this line of questioning as there are established tunings that players expect to utilize and I'm not going to manufacture basses for people that have to be used in an alternate tuning in order to avoid their own inherent construction-related dead spots.

Changing the neck length is also an idea that doesn't widely solve the issue as there already exist widely accepted scale lengths that players are accustomed to, so I'd prefer to find a method that moves the resonant frequency out of the range of the instrument such that a regular long scale bass that has the fit and feel of an established instrument also does not possess the historically 'common' dead spots.

The points you make are basically equating to "here is why it happens, use trial and error to find what works best" which I already do and have done. In my mind I have already successfully utilized several methods to solve the issue, as none of my scratch-built instruments have any dead spots that I or others have found. I think @MPU 's assessment above also identifies the things that 'work', but not exactly how they work!

I'd still like to identify a predictable, intuitive methodology for eliminating dead spots like we have for so many other operational factors in these instruments.

 

[Jon Cowens]

With the right instrumentation, you can find the resonant frequencies of the neck

Would you please provide more details of the instrumentation and sensors.
Thanks

 

[micguy]

Thanks for taking the time - regarding the 'semantics' point, I was working from this operational definition of those concepts rather than working jargon: Resonance - Wikipedia

Changing the neck length is also an idea that doesn't widely solve the issue as there already exist widely accepted scale lengths that players are accustomed to, so I'd prefer to find a method that moves the resonant frequency out of the range of the instrument such that a regular long scale bass that has the fit and feel of an established instrument also does not possess the historically 'common' dead spots.

In my prior post, I said you're not going to be able to get rid of deadspots and resonances, you're going to manage them. I'll elaborate a bit. The lowest resonant frequency on a typical bass guitar neck is at about 50 Hz - that's the one where the whole neck moves in unison. The anitinode of that resonance is the end of the neck, which is why it affects some headless basses around the open a (which is 55 Hz). The dead spot that most of us are most familiar with is the next one up, which is in the range of 130 Hz ( a music C on the G string) - and the anitnode ends up at the fret where that C is played.

To move the resonances to where they are not in the range of the instrument is a logical goal, but an impractical one. The bass by my desk is a 5 string 22 fret instrument - it goes up to an F, clocking in at 350 Hz or so for it's fundamental. If we want to move the resonant frequency of the neck to 3450 Hz, and keep if the same length, we need to raise the stiffness to mass ratio by a factor of the frequency ratio squared (there is a square root in the math that bites us here). So, 350/130, and square that, and you get about 7.25. We need the neck to be 7.25 times stiffer, or one 7.25th of the mass, or some combination.

There is no wood that will get us that much more stiffness. Aluminum won't do it - aluminum is much stiffer, but also much heavier than any wood. I had a few aluminum neck Kramers years ago, and that lower 50 Hz resonance moved up about 10%, to where it was tuned close to the open A, so there was a bit of a dead spot there (not progress). On an aluminum Kramer neck the normal dead spot on the G string was moved enough to not be a practical problem - (that design managed its resonances well, it didn't get rid of them), but that resonance was certainly still within the range of the instrument.

Carbon fiber composite will get you a factor of about 5 - not quite there, but it's in the right direction. It is much heavier, which means you'll need to make it hollow, or the thing will neck dive. Diamond will get you there, though there is some question about ow to orient the crystal planes to optimize this. You won't be able to afford that neck, and nobody makes diamonds anywhere near that big, but it is in theory possible to do what you want with that material. Beryllium (I spent some time working with this material in my career) will also get you there, but it's toxic, and rather expensive, and being a strategic material, if you want to buy enough to make a neck, you're going to have to deal with the government asking you a LOT of questions.

The most economical way to make a neck with the stiffness you'll need is via geometry. If you make a neck that is twice as thick, the stiffness goes up by a factor of 8. As long as you're OK with a 2 inch thick neck, that'll work, and it won't cost much more. You probably won't need a truss rod, so it might actually cost less. It'll be a bit uncomfortable to play. Uprights have rather short necks, and they are also quite thick - by breaking out of the design constraints of a solid body bass format , they likely have necks which are close to doing what you want. My short scale has a neck that is much shorter then my long scales, and it manages it's resonances much better.

My point is, within the usual design limitations (one of which you were adamant about - the neck length), you are not going to get the resonances of a neck outside the range of the instrument. Lots of people have looked at this problem before you (it's a very well know problem), and if there was a practical solution, one of them would have found that solution. Hence my statement that you're going to manage the resonances (and my sharing with you how I do that).

Even with the 2 inch thick neck, the harmonics of the notes you're playing will be in the range of the neck's resonances, so it'll still affect the sound - if you get a bass with a pronounced dead spot on the G string, and you play the same note an octave lower on the A string, you can hear the second harmonic (the octave) die out quicker than the other harmonics. Even the theoretically "stiff enough/no issues" neck will have resonances that you have to manage.

My point is, yes, in some dream scenarios, you can make your ideal neck. Within the design limits placed by people who want to play a bass, though, it isn't practical.

 

[micguy]

Would you please provide more details of the instrumentation and sensors.
Thanks

There are various ways of doing this. A scanning laser vibrometer is one of the best ways. I doubt you can afford that, though I have worked with those. A number of well placed accelerometers on the neck can get you there, though you have to be careful about how many you put on there, as they add some mass. If you have a fretless neck with a flat fingerboard, you can place the thing horizontal, drive it with a coil and a magnet, and sprinkle salt on the thing. At resonance, the salt will move to the nodes, and the place between those are the antinodes. Sugar will also work, but you'll attract ants.
Good finite element analysis software can help you predict resonances - there is a lot of work getting correlation between the modeled behavior and the real world, but once you get control of that, you can evaluate ideas without having to build al the iterations.

 

[Gary_M]

The lowest resonant frequency on a typical bass guitar neck is at about 50 Hz

According to what research or measurement?

Edited to add: Cite your sources.

 

[micguy]

According to what research or measurement?

Edited to add: Cite your sources.

That is one determined from experiential data - you tune your A string down, and you lose sustain. You often get a bit of that on the E string st that same freq, but it’s less dead because you’re further from the anti node. The worst dead spot I ever encountered was a headless (less mass loading, as it has no tuners), which was a godawful dead spot right at tge open A - 55 Hz in that case.

That experiential data, along with a lot of lab experience and classes on the subject, getting a degree in Acoustical Physics, means I really know this stuff.

 

[Gary_M]

That is one determined from experiential data - you tune your A string down, and you lose sustain. You often get a bit of that on the E string st that same freq, but it’s less dead because you’re further from the anti node. The worst dead spot I ever encountered was a headless (less mass loading, as it has no tuners), which was a godawful dead spot right at tge open A - 55 Hz in that case.

That experiential data, along with a lot of lab experience and classes on the subject, getting a degree in Acoustical Physics, means I really know this stuff.

Pretty sure that’s anecdotal evidence.

 

[micguy]

Pretty sure that’s anecdotal evidence.

I did well instrumented tests in the Physics lab at the University which educated me on this. Once you understand the Physics and have established that your observations, the theory, and all your measurements line up, you’re not guessing anymore.

Oppenheimer, at the first atomic bomb test, dropped some grass when the blast hit, and calculated the yield from thst observation. He was pretty close to the calculated yield that they got after crunching the numbers from the instruments. He was educated on that subjects, and knew enough Physics to make his “anecdotal” evidence meaningful.

I’m not Oppenheimer, but I have done the work (4 years of Univerdity study and over 40 years designing audio gear) to where I know what certain observations mean. The frequency of an observed resonance, if you can nail it to a specific musical note, you know the frequency within 3%.

If you put a Snark tuner on the end of your bass’s neck (without strings on it) the neck rings long enough that the tuner will tell you what note it is, and if it’s a bit sharp or flat from that. One of my roasted necks, when you do this, registers as a bit sharp of G. The G in that octave is 49 Hz. If you want to believe me when I tell you necks resonate (the lowest mode) around 50Hz, fine. If you choose not to believe me, feel free to learn enough to do your own experiments.