Not sure if this belongs in this forum or the instrument one, here goes.
I am building a self contained electric acoustic u-bass, basically I bought a u-bass from:
Kielder Bass ukulele by Buzzards Field Ukuleles
and I want to put the amp and speaker in the body of the bass, powered by a battery, for a mini self contained bass. Amp, driver and battery as follows:
TPA3116D2 Subwoofer Digital Power Amplifier 100W AMP Board Audio Module UK | eBay
Buying a SB Acoustics SB13PFC25-4 woofer? | SoundImports - SoundImports
Power Bank XT-16000Q3 with 24V Output from XTPower
So now I need to design an enclosure that fits inside the body of the bass, and gives me the necessary response. I would like to do a vented enclosure, but it may be too big. How do I go about calculating the volumes, port size, etc.? Will I be able to get good response down to FS of 44Hz in a sealed enclosure?
Any advice is appreciated.
I am building a self contained electric acoustic u-bass, basically I bought a u-bass from:
Kielder Bass ukulele by Buzzards Field Ukuleles
and I want to put the amp and speaker in the body of the bass, powered by a battery, for a mini self contained bass. Amp, driver and battery as follows:
TPA3116D2 Subwoofer Digital Power Amplifier 100W AMP Board Audio Module UK | eBay
Buying a SB Acoustics SB13PFC25-4 woofer? | SoundImports - SoundImports
Power Bank XT-16000Q3 with 24V Output from XTPower
So now I need to design an enclosure that fits inside the body of the bass, and gives me the necessary response. I would like to do a vented enclosure, but it may be too big. How do I go about calculating the volumes, port size, etc.? Will I be able to get good response down to FS of 44Hz in a sealed enclosure?
Any advice is appreciated.
You're going to have serious trouble with feedback. I'd recommend stopping before you spend any money.
Here's why:
- Acoustically, when you pluck the string the energy will conduct into the body of the instrument, exciting resonances and producing sound from the larger radiating area.
- To convert some of that energy to electricity, we fit a pickup. They detect vibrations and turn those into electricity.
Here's the problematic bit: when you use a speaker to amplify those vibrations in close proximity to the source, the speaker will vibrate the body of the instrument. That goes back into the pickup, back through the speaker, and gets louder and louder until something gives up.
You've heard mic feedback, I'm sure, where someone does something silly with the microphone (like pointing it at a speaker) and there's a loud screech.
Something similar will happen with your E-U-bass. You could turn it down low to reduce the speaker vibrations feeding into the pickup, but at that point you might as well not bother.
My recommendation would be to build the components you've chosen into an external amp. I'd probably stick with a sealed box, since U-basses can put out some very low frequency signals, which will be problematic for a ported box unless you have a highpass filter.
I'd also consider trying a 6" PA midbass speaker. The main reason being that if you feed them low-frequency signals, they tend to reach mechanical limits before actual damage occurs. HiFi speakers usually stay linear for as long as possible, which means it can be pretty easy to smash the coil into the back plate.
Chris
Here's why:
- Acoustically, when you pluck the string the energy will conduct into the body of the instrument, exciting resonances and producing sound from the larger radiating area.
- To convert some of that energy to electricity, we fit a pickup. They detect vibrations and turn those into electricity.
Here's the problematic bit: when you use a speaker to amplify those vibrations in close proximity to the source, the speaker will vibrate the body of the instrument. That goes back into the pickup, back through the speaker, and gets louder and louder until something gives up.
You've heard mic feedback, I'm sure, where someone does something silly with the microphone (like pointing it at a speaker) and there's a loud screech.
Something similar will happen with your E-U-bass. You could turn it down low to reduce the speaker vibrations feeding into the pickup, but at that point you might as well not bother.
My recommendation would be to build the components you've chosen into an external amp. I'd probably stick with a sealed box, since U-basses can put out some very low frequency signals, which will be problematic for a ported box unless you have a highpass filter.
I'd also consider trying a 6" PA midbass speaker. The main reason being that if you feed them low-frequency signals, they tend to reach mechanical limits before actual damage occurs. HiFi speakers usually stay linear for as long as possible, which means it can be pretty easy to smash the coil into the back plate.
Chris
Hi Chris, thanks for your reply.
I post on diyaudio. I build speakers. I play guitar, banjo, bass and French Horn. I understand how electric instruments work. Like you, I do a lot of live sound work, so I do know what feedback is as well.
Attached is a picture of a similar project a friend of mine did in the States earlier this year. He also has a lot of experience in sound, and his instrument works well. He used two Dayton Audio ND91-4 fr drivers in separate sealed enclosures made from PVC pipe caps, and acoustically isolated from the body of the instrument with Velcro. He has no feedback issues, so I don't anticipate any. BTW, my bass uke has a piezo pick-up, since bass ukes use thick PU strings that are a lot like round rubber bands.
I chose the driver in my original post because it:
It handles 40W RMS, do you really think it will have trouble with what it will get from the bass when it's driven by a TPA3116?
I'm happy for the advice re sealed rather than ported, sealed will obviously be easier to build into the instrument body. Do you think the SB13PFC25-04 will have the requisite response (I assume down to around 45 Hz for a bass) in a sealed enclosure?
I post on diyaudio. I build speakers. I play guitar, banjo, bass and French Horn. I understand how electric instruments work. Like you, I do a lot of live sound work, so I do know what feedback is as well.
Attached is a picture of a similar project a friend of mine did in the States earlier this year. He also has a lot of experience in sound, and his instrument works well. He used two Dayton Audio ND91-4 fr drivers in separate sealed enclosures made from PVC pipe caps, and acoustically isolated from the body of the instrument with Velcro. He has no feedback issues, so I don't anticipate any. BTW, my bass uke has a piezo pick-up, since bass ukes use thick PU strings that are a lot like round rubber bands.
I chose the driver in my original post because it:
- is very sensitive
- is fairly small and light
- has an Fs of 44 Hz, which seems low enough for a bass
- is not expensive
It handles 40W RMS, do you really think it will have trouble with what it will get from the bass when it's driven by a TPA3116?
I'm happy for the advice re sealed rather than ported, sealed will obviously be easier to build into the instrument body. Do you think the SB13PFC25-04 will have the requisite response (I assume down to around 45 Hz for a bass) in a sealed enclosure?
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> has an Fs of 44 Hz, which seems low enough for a bass
Except Qts is 0.29. On infinite baffle the response is down 10dB at 44Hz relative to midrange. It will tend to fall from 150Hz (44Hz/0.29).
A resonance will bump some of this range and reduce anything below. A huge ported box could be "flat" to ~~70Hz, but have trouble with slapping on the low string unless pre-filtered.
This is not inappropriate. That small body is clearly not supporting 42Hz acoustically (even a stand-up double-bass is too small for good 42Hz). A "true boost" can have considerable bass-slump.
Yes, the thing will feed-back. My suspicion is that you can hardly get 6dB gain of output before feedback. Which suggests that 80-100 Watts is not necessary unless you pluck it with a claw-hammer. However hunred-watt amps are now cheap enough to just use regardless.
Except Qts is 0.29. On infinite baffle the response is down 10dB at 44Hz relative to midrange. It will tend to fall from 150Hz (44Hz/0.29).
A resonance will bump some of this range and reduce anything below. A huge ported box could be "flat" to ~~70Hz, but have trouble with slapping on the low string unless pre-filtered.
This is not inappropriate. That small body is clearly not supporting 42Hz acoustically (even a stand-up double-bass is too small for good 42Hz). A "true boost" can have considerable bass-slump.
Yes, the thing will feed-back. My suspicion is that you can hardly get 6dB gain of output before feedback. Which suggests that 80-100 Watts is not necessary unless you pluck it with a claw-hammer. However hunred-watt amps are now cheap enough to just use regardless.
The good news is you have a successful project to copy ideas from.
The bad news is that putting the speaker in the bass itself - rather than in separate acoustically isolated enclosures - is the very thing that's raising the eyebrows of some of our experienced members.
I know there was a short-lived era of novelty/cheap electric guitars (not basses) with built-in amps and tiny speakers, basically a transistor pocket-radio built into an electric guitar amp. I've read a few articles about some of these, and nobody mentioned acoustic feedback being an issue. But those speakers were operating at much higher frequencies than you're planning (bass response was not an issue with
those guitars - unsurprisingly, pocket-radio electronics and speakers got you pocket-radio sound!)
So the question is whether the lower frequencies you're dealing with, and the increased power levels, will be a game-changer, or not.
You may know that the laws of physics are not kind to the combination of small speaker and low frequencies. Efficiency falls off a cliff when you try to get deep bass out of a small speaker. This is compounded by the fact that our ears are very insensitive at low frequencies, so you need quite a lot of SPL to even be able to hear them.
My guess is that the only way this will work is if you cheat like crazy. Forget about reproducing the fundamental frequency of the strings, and settle for reproducing the 4th harmonic up, which you might actually be able to do with a tiny Uke body and powder-puff speaker.
I think this will sound much better than you might expect, because we don't pick bass guitar strings near their midpoint, and as a result, the fundamental is very weak in the actual string vibration itself; from some spectrograms I've seen, it often actually is the 4th harmonic that's the strongest part of the bass signal. Pluck a 41.2 Hz low E, and most of the pickup signal is at 164.8 Hz...
-Gnobuddy
The local Lafayette Radio Electronics store (like a step down from Radio Shack) had one on display when I was a kid. They would let me play it whenever I was hanging out there. I spent a lot of time there or at the slot car track next door. This was mid 1960's.
It had a transistor amp of no more than 1 watt, probably a lot less, in a solid body guitar with a small speaker, maybe 3 inches. It ran on 4 AA batteries, and sounded best when the batteries were nearly dead. There was no way to get feedback from this thing at full crank since it wasn't very loud and had no low end.
Since that era I have built 3 versions of self amplified guitars (not bass guitars). One even had a tiny tube amp. This is what I have discovered:
There are 3 possible modes of feedback when trying this:
Electrical feedback that occurs when the magnetic field from the OPT or speaker voice coil gets into the magnetic pickup. This was the killer in my tube amp in the guitar experiment. Advancing the gain enough to get serious distortion created a feedback path that caused a screeching sound in the 1 to 3 KHz range with no strings on the guitar. Testing for parts placement before making the guitar body might have prevented this, but it was incurable once the guitar was built.
Acoustic feedback from the speaker into a piezoelectric pickup can also lead to feedback with out any string vibration. I was using a Graphtech Ghost pickup system which incorporates a piezo element into each string saddle. These can pick up sound directly, or more commonly through vibrations in the guitar body.
If you have your target guitar already, connect it to an amp crank up the gain / volume and speak into the sound hole. If you hear your voice in the speaker, you will have feedback problems. You can try putting various different tones through a small speaker aimed into the sound hole to find the problem frequencies before getting too involved.
Acoustic feedback from the speaker which causes sympathetic vibrations in the guitar strings. This is usually picked up and transmitted through the guitar's body and is the most prevalent and hard to control feedback mode.
Isolating the speaker system from the guitar body helps as does playing with the pickup phasing. There will always be some frequency or frequencies where the whole thing wants to go unstable, but it can be made to work at reasonable volume levels with a standard guitar. It is slightly louder than an acoustic guitar before weird effects begin to occur. And there is a range of volume where the effects can be used to produce long sustained notes, and peaky responses.
I was using a Faital Pro 3FE22 speaker mounted on a rubber suspension. Power came from a cheap 10 watt Chinese class D board. Low frequency cut was applied to keep the little speaker from bottoming out and reduce feedback. Cranking the volume up full but muting the strings produces no sound, but one or more strings will start vibrating on it's own when you let go of them. Stuffing a piece of felt between the first fret and the nut allows for some cool tapping effects though.
That guitar was made from Strat parts and was in pieces when I had to pack up and move a few years back. It's still in a box on the shelf awaiting some mods and a rebuild. Major issue was playability due to odd size and shape. It started out as a wood shop experiment that I didn't expect would work.
It would help if you converted the instrument into a solid body equivalent, tying the bottom of the bridge to the neck block. Isolating the speaker from the top would also help. What I would do is leave the bass alone and pick up a neck from China and build a body specifically with the speaker in mind. Built like a speaker box, not meant to resonate. I came across a perfect example of what might work. At a thrift store someone made a guitar shaped object, it did not produce much sound.
There is no binding, what looks like it is 1/4" thick wood, the sides I am guessing would be thicker. With a semi-hollow arrangement with the center section from the bridge to the neck should keep feedback at bay for a while.
There is no binding, what looks like it is 1/4" thick wood, the sides I am guessing would be thicker. With a semi-hollow arrangement with the center section from the bridge to the neck should keep feedback at bay for a while.
I understand where you're coming from. I suspect there might be some challenges with scale-length and hardware, due to the string thickness.
The little U-basses use short but extremely thick plastic strings, which need enormous holes in the tuners, enormous holes in the tailpiece (or bridge), and enormous slots in the nut.
Kala U-basses are so ridiculously expensive I've never even bothered to look at one, but I assume they have custom tuners (tuning gear mechanisms) on them - you probably need pretty fat tuner posts just to be able to drill the large-diameter string holes through them. I'm not sure if such tuners are available to the DIY builder.
On the other hand, if you ditch the plastic strings, you are now into fairly conventional territory - short-scale electric basses have been around a long time. But now you have far more string tension to deal with (from the metal strings), and short-scale basses tend to be "thuddy", with not much sustain and not a particularly attractive sound (which is why they've remained on the fringes of the bass world.) Not to mention, the scale length is much longer than a U-bass.
For most people looking at a plastic-string U-bass, a metal-string bass wouldn't be in the running anyway, because of the supposedly acoustic-bass-like sound of the plastic strings. So I guess the question is how to get or modify hardware for the fat plastic strings.
If I ever need to beat off a charging moose with a guitar, I know which one to pick!
-Gnobuddy
When the guitar becomes a weapon.....or an AXE, you need the Aluminator!
Rogue Aluminator 90's Super Rare Aluminum Body Guitar | Reverb
Some of the early versions had an aluminum neck...the tuning changed with the temperature.
> If I ever need to beat off a charging moose with a guitar, I know which one to pick!
We've had moose in the yard.
Odds are you will find an Aluminator in the trash before you have to fight a charging moose. While Maine has "Beware of moose" signs on highways into the state, and moose-crashes are serious, they are also very rare because moose are super-shy. (That's a 2-foot stick. No deer!)
We've had moose in the yard.
Odds are you will find an Aluminator in the trash before you have to fight a charging moose. While Maine has "Beware of moose" signs on highways into the state, and moose-crashes are serious, they are also very rare because moose are super-shy. (That's a 2-foot stick. No deer!)
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The tuners on the bass look like regular bass tuners. The 'strings' are surgical tubing and looks like they get stretched and placed in the slots. It looks like the bridge gust has a hole for the tubing to go through and is more than likely just knotted inside. The nut just takes a little file work.
I think all of my guitars go sharp when the temperature gets colder. It's been more noticeable since I left Los Angeles and moved to BC, where we actually have four seasons.
I think the metal truss-rod shrinks when temperatures fall, and the wooden neck doesn't, so the neck gets pulled slightly backwards, and the string tension goes up as a result.
Invar ( Invar - Wikipedia ) truss-rods, anyone?
-Gnobuddy
I once watched a documentary film about a large group of moose which had taken over a small city golf course.
After the moose chased and scared a number of people (in fairness to the moose, these people had usually behaved like idiots and invaded the moose's personal space before it chased them), a fairly effective solution was found: police constables riding Segway transporters! Apparently the moose didn't like the Segways at all.
Another diyAudio user told me about Alaska's most famous moose, Buzzwinkle, who liked to get drunk eating fermented apples, after which he would paint the town red: Recalling Alaska's most notorious drunken moose, the street-smart Buzzwinkle - Anchorage Daily News
-Gnobuddy
On a U-bass? I would have thought four standard bass-guitar tuners on a Ukulele neck would have about the same effect on balance as a lead brick on a kitten's tail.
The nut is definitely the easiest part.
Some years ago, I put a set of (metal wrapped) plastic strings on a Chinese-made short scale violin bass for a friend who wanted to try playing bass. Those strings fit the tuner holes okay, and I filed out the nut with a set of ordinary needle files. But I had considerable headaches at the other end (bridge and tailpiece.)
That particular violin bass was quite heavy, and came with standard bass-guitar tuners on the headstock.
The strings worked fine with the magnetic pickup, but I didn't like their sound - thuddy and with an unattractive timbre, and they always sounded out of tune no matter what I did (yes, I set and reset the intonation, several times.)
-Gnobuddy
Wow, thanks for all the feedback and suggestions.
I've been in touch more about some of the issues raised here with my friend who has built a version of this project already, and he has told me he does have some feedback issues, but he is able to get sufficient gain before feedback to allow the ubass to compete with acoustic instruments, which is all that's needed. He has braced from the bridge to the back in his instrument, like someone suggested, so I'll be doing that.
Otherwise, I'll let everyone know how I get along.
Wish someone would answer my original question about how best to calculate the volume of a sealed enclosure for the SB13PFC25-04 driver...
I've been in touch more about some of the issues raised here with my friend who has built a version of this project already, and he has told me he does have some feedback issues, but he is able to get sufficient gain before feedback to allow the ubass to compete with acoustic instruments, which is all that's needed. He has braced from the bridge to the back in his instrument, like someone suggested, so I'll be doing that.
Otherwise, I'll let everyone know how I get along.
Wish someone would answer my original question about how best to calculate the volume of a sealed enclosure for the SB13PFC25-04 driver...
WinISD or Hornresp will let you run simulations.
I'd consider ported, too. The simulations will show you why.
Chris
