- Apr 18, 2008
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Winter Build Off 2020 - Sub-woofer On A Stick
- Thread starter 2groggy
- Start date
Okay, now you've reached the critical part of the whole instrument: the top (or diaphragm) over the round ring. That diaphragm is the main moving part that moves the air. It pumps air off of the front of the instrument, and on the backstroke, it pumps air down into the drum and out through the serpentine duct. That's why the sound in the duct is 180 out of phase with the sound off the front.
Here's the technical challenge: To produce low frequencies with any volume, that diaphragm has to have a lot of travel. Like moving 1/4". Look at the speaker cone in your amp cabinet when you tap on the E string. The cone jumps about 1/4". That's what it takes to move enough air across the space to your eardrum, at those low bass frequencies.
That's why bass drums are so large in diameter; the large diameter means that the center has a lot of travel as the head flexes when you hit it. That's also why the body of an upright bass is so large. It isn't to make a large volume of air inside, although that's a side effect. The real reason is to make the top (the diaphragm) large enough that the wood can flex enough (without breaking) to get more travel in the center. That's how it pushes air at low frequencies, driven by the strings through the bridge. Lots of air.
So, if you want to create low frequencies out of this bass, the diaphragm over the top of your center ring needs to be able to move up and down a lot, like 1/8" to 1/4". A thin wood top isn't going to do that, not even close. What are you going to do???
You could: Use a flexible material stretched over the ring. Like a drum head. Like a Banjo!
Or, make a disk or cone that's fairly rigid, but is mounted in a flexible surround ring. Like a speaker, or the guts of a resonator guitar.
Or......Come on, summon up your inner Mad Engineer.
Once you figure out how to have a diaphragm with vertical travel, then you have to figure out how to drive it with the bridge. Without causing anything to explode.
Here's the technical challenge: To produce low frequencies with any volume, that diaphragm has to have a lot of travel. Like moving 1/4". Look at the speaker cone in your amp cabinet when you tap on the E string. The cone jumps about 1/4". That's what it takes to move enough air across the space to your eardrum, at those low bass frequencies.
That's why bass drums are so large in diameter; the large diameter means that the center has a lot of travel as the head flexes when you hit it. That's also why the body of an upright bass is so large. It isn't to make a large volume of air inside, although that's a side effect. The real reason is to make the top (the diaphragm) large enough that the wood can flex enough (without breaking) to get more travel in the center. That's how it pushes air at low frequencies, driven by the strings through the bridge. Lots of air.
So, if you want to create low frequencies out of this bass, the diaphragm over the top of your center ring needs to be able to move up and down a lot, like 1/8" to 1/4". A thin wood top isn't going to do that, not even close. What are you going to do???
You could: Use a flexible material stretched over the ring. Like a drum head. Like a Banjo!
Or, make a disk or cone that's fairly rigid, but is mounted in a flexible surround ring. Like a speaker, or the guts of a resonator guitar.
Or......Come on, summon up your inner Mad Engineer.
Once you figure out how to have a diaphragm with vertical travel, then you have to figure out how to drive it with the bridge. Without causing anything to explode.
I really like this approach. I also stumbled across this old paper where the interplay between the lowest resonances on a double bass are discussed at length: http://www.speech.kth.se/prod/publications/files/qpsr/1970/1970_11_2-3_050-054.pdf (If you cannot access it pm me)
You may or may not find it helpful. My take-home message from a quick read was that the lowest resonance is the Helmholtz air resonance you are building with the duct (A0 in the paper, around 55-60 Hz), which then interacts with the lowest few top plate resonances. The paper explicitly discusses tonal qualities and gives power spectra for fundamentals and harmonics over a wide range. I think you are in better shape than you fear with your approach.
Well...there could be a little explosion...?
During R & D, there may be a few. Wear safety glasses.
And video is a MUST.....
Thanks @Bruce Johnson and @Avigdor. I certainly have some thinking to do.
My thought process was more about lowering the natural frequency as the means to increasing the amplitude at bass frequencies. I doubt that I can get 1/4 inch of travel without trading off tuning and intonation stability.
I'll keep mulling over how to get the flexibility required and toughness (meaning resistance to repetitive strains).
meanwhile, I'll get a template made for that last panel and keep working on the neck.
My thought process was more about lowering the natural frequency as the means to increasing the amplitude at bass frequencies. I doubt that I can get 1/4 inch of travel without trading off tuning and intonation stability.
I'll keep mulling over how to get the flexibility required and toughness (meaning resistance to repetitive strains).
meanwhile, I'll get a template made for that last panel and keep working on the neck.
Remember that there's no point in building a finely tuned speaker cabinet....if you don't mount a speaker in it and drive it with a signal from an amp. The wooden cabinet by itself is just going to sit there quietly. Something has to drive sound into the cabinet in order for it to work. You pluck the bass string, the amp sends an AC signal to the speaker coil, and the speaker cone moves in and out, pumping air, at the frequency that you played on the bass.
That's the driver. It pushes vibrating air directly at you, which is most of the sound that you hear in your ears. At the same time, the air behind the speaker cone gets driven back into the cabinet. The design and geometry of the cabinet may then modify the sound being pushed through it by boosting or softening parts of the frequency range. Coming out the ports, that adds to the overall sound you hear. Again, the cabinet doesn't make sound by itself; it has to have sound pushed into it by some kind of driver.
An acoustic bass is a similar setup, except that you've taken out the electrical stuff: the pickup, the amp, and the speaker coil. You're mechanically linking the bass bridge right on the center of the speaker cone. When you pluck an E, the bridge is forcing the speaker cone to move in and out, at the frequency of E. The speaker cone is the top of the bass, the diaphragm that's forcing the air to move. The air off the front of it makes you hear an E directly. Meanwhile the air on the back side is pumping an inverse E (out of phase) down into whatever you've done with the body.
As with the speaker cabinet, the back chamber and serpentine duct aren't going to produce sound by themselves. You have to physically drive sound into them from the diaphragm for them to go into any resonances and do anything.
That's why the motion of diaphragm is critical to getting sound out of an acoustic bass. If the diaphragm is a piece of wood that only moves 0.010" when you pluck the string, you aren't going to get any serious low frequency sound. It has to have travel and horsepower driving it to pump low frequency sound.
That's the driver. It pushes vibrating air directly at you, which is most of the sound that you hear in your ears. At the same time, the air behind the speaker cone gets driven back into the cabinet. The design and geometry of the cabinet may then modify the sound being pushed through it by boosting or softening parts of the frequency range. Coming out the ports, that adds to the overall sound you hear. Again, the cabinet doesn't make sound by itself; it has to have sound pushed into it by some kind of driver.
An acoustic bass is a similar setup, except that you've taken out the electrical stuff: the pickup, the amp, and the speaker coil. You're mechanically linking the bass bridge right on the center of the speaker cone. When you pluck an E, the bridge is forcing the speaker cone to move in and out, at the frequency of E. The speaker cone is the top of the bass, the diaphragm that's forcing the air to move. The air off the front of it makes you hear an E directly. Meanwhile the air on the back side is pumping an inverse E (out of phase) down into whatever you've done with the body.
As with the speaker cabinet, the back chamber and serpentine duct aren't going to produce sound by themselves. You have to physically drive sound into them from the diaphragm for them to go into any resonances and do anything.
That's why the motion of diaphragm is critical to getting sound out of an acoustic bass. If the diaphragm is a piece of wood that only moves 0.010" when you pluck the string, you aren't going to get any serious low frequency sound. It has to have travel and horsepower driving it to pump low frequency sound.
I am liking this thread for all sorts of reasons.
Clear and concise comments, Bruce. I'll jump to your summary.
Good points. But a vibrating string can only make a certain amount of energy. We can't create energy without amplification. The best that we can do is rob Peter to pay Paul, making a low frequency range louder at the expense of a higher range. If we can direct all of the vibration into the diaphragm by making the rest of the body stiffer, and then tune the body to respond to the bassier frequencies, we should end up with something louder than a conventional acoustic bass guitar. That's the goal, but I don't expect miracles.
So let me turn the question around. Given a round space for a diaphragm, how might we convert the most energy from string vibration and pump as air at bass frequencies as possible?
Good points. But a vibrating string can only make a certain amount of energy. We can't create energy without amplification. The best that we can do is rob Peter to pay Paul, making a low frequency range louder at the expense of a higher range. If we can direct all of the vibration into the diaphragm by making the rest of the body stiffer, and then tune the body to respond to the bassier frequencies, we should end up with something louder than a conventional acoustic bass guitar. That's the goal, but I don't expect miracles.
So let me turn the question around. Given a round space for a diaphragm, how might we convert the most energy from string vibration and pump as air at bass frequencies as possible?
- make the diaphragm more flexible?
- make it heavier?
- mount it on a spring?
- dampen the rest of the body?
- stiffen the rest of the body?
- any special shape?
- what else? (all of my lists end with this question)
- wait, did I hear somebody "cheat" and add an onboard battery powered pre-amp that drives an actual speaker? That wasn't me. Cool idea, whoever said that BTW, but not for this project.
- spider bridge?
I've never touched a banjo. How much does the drum skin deflect? Do you have to set the action high to avoid fret buzz from the bridge vibration?
On a banjo the head doesn't deflect much at all. On a internally ported acoustic bass, with what would have to be a smaller head? Who knows, you'd have to mock it up and try to see what the heck happens (see cautions about explosions earlier in the thread
). I'd be concerned about getting enough downward pressure to transfer the energy into the resonating mechanism (whatever it ends up being). Maybe a neck angle with a tall bridge? Sorta DB-esque?You could try to pickup a 8" Tom Tom on Ebay and make a test rig to see if it might work.
If you are wondering about my knowledge and experience with diaphragms driven by bass bridges, etc:
Bass Banjo
Quick answer: Yes, to drive bass-frequency sound, you need mechanical horsepower to push that diaphragm and pump that air. On a mechanical (acoustic) bass, you have to pluck the strings harder than you would if the bass were power-assisted by a pickup, amp and speaker. That's another of the hurdles of making an acoustic bass. Power doesn't come from nowhere.
Bass Banjo
Quick answer: Yes, to drive bass-frequency sound, you need mechanical horsepower to push that diaphragm and pump that air. On a mechanical (acoustic) bass, you have to pluck the strings harder than you would if the bass were power-assisted by a pickup, amp and speaker. That's another of the hurdles of making an acoustic bass. Power doesn't come from nowhere.
I think that my head would 'splode before the SWOAS (Sub-woofer On A Stick)
Lots of experience driven good advice from @Bruce Johnson here and after seeing the banjozilla (and the ingenious way he overcame all kinds of challenges there) I am convinced you'll manage to do it. Given the shape and varying cross section of your resonator "tube" I am sure that the frequency response will be quite wide, esp. with that sharp bend in there. So it should do its job nicely, reinforcing those low frequencies.
Now I'm really intrigued to see and hear the final instrument.
(And you're quite right - calculating from first principles will only get you so far in a real build. But it can be really fun to try to see how far it CAN get you, and then it can be a great supplement to gut feeling.
)
Now I'm really intrigued to see and hear the final instrument.
(And you're quite right - calculating from first principles will only get you so far in a real build. But it can be really fun to try to see how far it CAN get you, and then it can be a great supplement to gut feeling.
)
Man, you guys have vision and I am wearing bi-focals!
Great thread/great project!
Great thread/great project!
I'm back from a 2 week hiatus.
Before I left, I prepared a test front with a dog bowl resonator.
I have a few other panels so I can make different test fronts with different resonator configurations. I expect to make a single cover that will work for all of them.
I wasn't happy with the neck so I started again. Here's a fresh POW (pile of wood) shot.
I;m reclaiming another window sill. This one turns out to be poplar too, but with more discolouration. Maybe the zebra wood fingerboard will make the streaks stand out less.
Tomorrow, I'll pick up another truss rod and print out my fret spacing so I can catch up with some of the other builds.
Before I left, I prepared a test front with a dog bowl resonator.
I have a few other panels so I can make different test fronts with different resonator configurations. I expect to make a single cover that will work for all of them.
I wasn't happy with the neck so I started again. Here's a fresh POW (pile of wood) shot.
Tomorrow, I'll pick up another truss rod and print out my fret spacing so I can catch up with some of the other builds.
