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Vented box for bass guitar design - HELP!
Vented box for bass guitar design - HELP!
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Old 21st October 2017, 03:24 AM   #1
ChurritoTierno is offline ChurritoTierno  Paraguay
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Exclamation Vented box for bass guitar design - HELP!

Hi guys!
I was reading the Dickanson's book to try to design a vented box for a bass guitar amp.
I have many doubs so,.. i wish that you could help me
Attached is a pdf of the story of this adventure.
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File Type: pdf vented box.pdf (615.7 KB, 73 views)
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Old 21st October 2017, 04:30 AM   #2
leadbelly is offline leadbelly  Canada
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Is this to be a serious cabinet used for actual practices and performance? If so, there are so many issues.

Full range bass cabs are not tuned for hi-fi type bass extension. You've got a 10" driver with 3mm Xmax. It's not even close to being up to the job. Muti-way bass cabs that are tuned for flat bass use PA subwoofer drivers: 12" and 15" drivers with much larger Xmax.

If you have already bought that driver I think your only option is to go with a sealed cabinet and try to protect it from overexcursion.
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Old 21st October 2017, 07:49 AM   #3
chris661 is offline chris661  United Kingdom
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Nothing wrong with a 10" driver and a 3mm Xmax. One of the most commonly used 10" drivers is the Eminence Delta-10 which has 3.5mm.

Trick is managing expectations. A 1x10" cab is never going to give the bottom-end feel of a 2x15". That said, a single 10" driver can put out useful SPLs above 80Hz or so, which is the octave above the low-E string. Given that the harmonics are usually louder than the fundamentals anyway, I'd aim for a smallish box tuned to 80Hz and then add a highpass filter if the bass driver farts when unloaded. Some will overload gracefully when driven past Xmax, and some will just make bad noises.

Chris
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Old 21st October 2017, 08:05 PM   #4
freddi is offline freddi  United States
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Vented box for bass guitar design - HELP!
if just one 10 in a cabinet - then I'd suggest a Karlson enclosure. xrk971's "XKi" tread has dealt with dual stacked drivers with smallish front cavity and K-aperture. Rear chamber volume and system tuning would be set for "punch" - probably no more than 35L per driver. ChurritoTierno might ask xrk971 to run an XKi alignment for that (Eminence?) 10. I have a 0.62 scale "K15" - cutoff with even Beta10cx is quite high - Vb rear ~25 liter). A change in port area will change "voicing" but nothing much on a graph (why ?) The little K has 3-5dB more output in the 100-200Hz octave than Beta10cx in a 5 cubic foot Karlsonator tuned to 37Hz. Under sine conditions, when I played with obstructing part of the inner port with a narrow slot in a board, a lot of harmonics were introduced (modulated noise?)

this is probably ~ 1Pi - the green trace is for the cabinet whose total bulk is a bit less than 2 cubic feet.

Click the image to open in full size.
Click the image to open in full size.

Dual driver XKi - for little fullrange

for the 10, I'd expect system tuning to be in the mid-60s depending upon rear chamber volume
a Karlson 12 would be an upper limit imo so around 36 liter or so per driver, xrk971
can do an akabak sim with your numbers. To save time on his part - put your driver's T-S
in this form

Def_Driver 'Fane Sovereign Pro10" | Fane 300W rated speaker with 4mm VC overhang, Qts = 0.32, 2.5" VC, 56oz ferrite magnet
Sd=378 cm2
Fs=58 Hz
Mms=37g
Qms=6.08
Qes=0.34
Re=5.7 ohm
BL=15.3 T/m
Le=1.68mH
Vas=41L


Vented box for bass guitar design - HELP!

Last edited by freddi; 21st October 2017 at 08:35 PM.
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Old 22nd October 2017, 08:20 PM   #5
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by ChurritoTierno View Post
Attached is a pdf of the story of this adventure.
Hello, and welcome!

In your pdf is the sentence "Also the cone excursion will be half of that a single speaker Xmax =1.05mm." I do not think this is correct; the point is that, with two speakers operating, there is twice as much speaker piston area moving air. So, for the same sound pressure level, each speaker cone needs to move only half as far.

But this is not the same as Xmax, which is the maximum distance one speaker cone can travel. That is a mechanical property of the driver, and stays the same, whether you use one driver, two, or four.

So, for a given SPL, each speaker only needs half the "x" compared to one speaker alone. But Xmax for each speaker is still the same 3 mm as before. That means, when both speakers are driven all the way to their Xmax limit, you are now moving twice as much air as before, making +3 dB SPL more than one speaker alone in the process.

Actually it gets better than that - having twice the speaker cone area also increases speaker efficiency by coupling to the air more efficiently. So you should get about another +3 dB more from that effect.

Combining both effects, if you drive two speakers in phase with the same low-frequency signal all the way to their Xmax limit, you should get nearly 6 dB more SPL than you could get from one single speaker. (Of course it takes twice as much power to drive the speakers, that's +3 dB more power. But the extra +3dB on top of that is a free gift, because of the improved coupling to the air.)

The other question you asked - "Could I take some intermediate value for the box volume?" is an interesting one. Firstly, published Thiele-Small parameters for drivers are never exact, and there are also manufacturing tolerances to deal with between one driver (speaker) and another. Then there are correction factors like the volume of the loudspeaker itself (volume of magnet, plus conical volume of speaker cone) which subtract from the volume inside the box.

So there is really no way to get exactly the right volume the first time - usually, if you want it exact, you have to build, measure, modify, measure, and so on, until the speaker behaves the way you want. This is the normal way prototype speakers are developed in a factory, before the design is finally frozen and ready for production.

But how exact does it have to be? If the box volume isn't exact, the low-frequency alignment won't be exactly what you want. This means the frequency response won't be exactly the same shape you intended. But if the difference is slight, you probably won't be able to hear it; particularly because simply moving a perfectly designed bass cabinet into a different room will usually completely mess up the frequency response due to room interactions!

And let's not forget, you have tone controls, and maybe a graphic EQ, on your guitar and bass amp, which will also help you make small changes to frequency response, to correct for whatever minor imperfections your design has.

So if you already have a box that is nearly the right size, or perhaps you wind up using one less sheet of plywood by slightly changing the volume of the box, I would go ahead and just do it. It will probably make too little difference to matter.

This is bass guitar we're talking about, not precision audio monitor speakers, where you really want everything flat to less than +/- 1 dB if you can manage it.

And I agree about the bottom octave not being important: I usually use a 60 Hz high-pass filter with my 5-string bass, which has a low B tuned to 30 Hz. It sounds wrong if you come from the Hi-Fi world, but cutting off the bottom octave of the bass actually makes the bass sound better, less boomy, and it sits better in the mix with the drums.

-Gnobuddy
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Old 22nd October 2017, 08:59 PM   #6
freddi is offline freddi  United States
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Vented box for bass guitar design - HELP!
I think with tube bass guitar amps, that limited primary inductance may also improve things vs a "hi-fi" output transformer with enough inductance to work down to 20Hz - saves weight too for a given power. I've custom 1K p-p M6 transformers sitting for 2-decades which are rated to 40Hz 240W.
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Old 23rd October 2017, 12:33 AM   #7
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by freddi View Post
I think with tube bass guitar amps <snip>
As far as I know, all bets are off with a tube amp - the output impedance is so high that the loudspeaker Thiele-Small parameters are worthless, Q of the driver goes up to Qms rather than Qts, and because Qms is huge (far greater than 0.707) there is no hope of designing a controlled bass frequency response.

However, I don't think anyone mentioned using a tube bass amp so far in this thread.

-Gnobuddy
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Old 23rd October 2017, 07:00 AM   #8
xrk971 is online now xrk971  United States
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Vented box for bass guitar design - HELP!
Default Dual 10in drivers in XKi works well

Freddi has a good suggestion for an XKi for this purpose. I agree that tuning this to maximize 80Hz and limit excursion is the way to go. I used the Vas spec to size this at 46 liters for both drivers and this will tune it a bit higher for more output and less cone movement. With a 12in wide cabinet and qnty 2 x 12in wide x 1in deep x 9.0in long vents, make the front chamber a "V" with a vertex in the middle so that 20% of the box volume is in the front chamber and 80% is in the rear chamber. Wire the drivers in parallel and here is the predicted SPL vs frequency for 2.83v input. It is 100dB sensitive with 03dB at 72Hz. This will play the 80Hz notes very clearly and loud. Ignore the falloff above 300Hz as this model is for the bass response only (I do not have the direct radiation from the cone passing through the aperture captured here):

Vented box for bass guitar design - HELP!-xki-guitarcab-2-83v-spl-png

Here is the cone excursion at 23vrms input, use a 51Hz high pass filter:

Vented box for bass guitar design - HELP!-xki-guitarcab-max-displ-png

Here is the max SPL at 23vrms - about 118dB at 1m, not too shabby:

Vented box for bass guitar design - HELP!-xki-guitarcab-max-spl-png

XKi - X's ab initio Karlson 6th Order Bandpass

Make the box like this (but out of wood of course):
Click the image to open in full size.

Click the image to open in full size.

I would open the aperture up a bit to let more highs out...
Click the image to open in full size.

It is actually a pretty compact cabinet. If you want deeper extension, say to 40Hz, that is a challenge as the fs is 54Hz. It can be done but overall SPL will be very limited, and cabinet will be double the volume.
Attached Images
File Type: png XKi-Guitarcab-2.83v-SPL.png (7.6 KB, 195 views)
File Type: png XKi-Guitarcab-max-displ.png (8.8 KB, 194 views)
File Type: png XKi-Guitarcab-max-SPL.png (15.1 KB, 194 views)
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Last edited by xrk971; 23rd October 2017 at 07:04 AM.
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Old 23rd October 2017, 07:14 AM   #9
chris661 is offline chris661  United Kingdom
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Valve (tube) amps add a couple of ohms output impedance. This raises Qes (and thus Qts), but not to infinity (which would leave Qms as the only damping).

Hornresp has the option to add series resistance to see what long cable runs or amplifier output impedance would do.

Chris
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Old 23rd October 2017, 08:13 AM   #10
David McBean is offline David McBean  Australia
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Quote:
Originally Posted by chris661 View Post
Valve (tube) amps add a couple of ohms output impedance. This raises Qes (and thus Qts)
Just to clarify - Qes and Qts are Thiele-Small driver parameters, they are independent of the source.

Qes is the driver resonant electrical Q at the driver resonance frequency, due solely to inherent electromagnetic damping.

Qts is the driver total Q.

Changing the value of the amplifier output resistance has no effect on Qes or Qts. This can be readily confirmed using Hornresp by entering a large value for amplifier output resistance Rg and checking the driver Thiele-Small parameter values by double-clicking on the Sd input box. Qes and Qts will remain unchanged.
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