Hello All,
When making a Sonotube subwoofer of a given volume, lets say 200 liters you could for example:
What factors would you consider to choose one of them if SQ was your only goal (not aesthetics, not practicality of placing in your room)?
When making a Sonotube subwoofer of a given volume, lets say 200 liters you could for example:
- use a 36" diameter sonotube which would result in ~12 inch tall
- use a 24" diameter sonotube which would result in ~26 inches tall
- use a 16" diameter sonotube which would result in ~60 inches tall
What factors would you consider to choose one of them if SQ was your only goal (not aesthetics, not practicality of placing in your room)?
Relevant factors I can think of with my limited knowledge
- How do I model/calculate the harmonics that might occur along the length, or radially? Maybe one of the aspect ratios avoids those harmonics better.
- The tube is stiff radially because circles distribute pressure very well. However the end caps are just MDF like any other subwoofer box. Since they are the least stiff component maybe one should try to make them as narrow as possible to limit deflection? What would the effect of lack of stiffness be? Some sort of damping/loss? Is it negligible?
Calculating for a simple pipe is easy. Both ends are nodes, so basically, what frequencies can have an integer number of wavelengths fitting exactly from end to end?
For a subwoofer application, you want as short as possible so that the resonant frequencies are well outside (>2-3 octaves if possible) the top end of the passband.
As for SQ, well, as long as the length doesn't contribute to any resonance in the passband, I wouldn't think any specific aspect ratio would affect SQ very much. If you are very concerned, just wrap the sonotube in a damping material and/or stiffening material.
For a subwoofer application, you want as short as possible so that the resonant frequencies are well outside (>2-3 octaves if possible) the top end of the passband.
As for SQ, well, as long as the length doesn't contribute to any resonance in the passband, I wouldn't think any specific aspect ratio would affect SQ very much. If you are very concerned, just wrap the sonotube in a damping material and/or stiffening material.
In engineering terms, only stiffness is relevant for subwoofers. Damping is not important if the cabinet walls only have modes above the operating frequency band. Thick cardboard already is stiff when loaded in-plane, which happens in a sonotube subwoofer.I would consider only its soundproofness.
Guessing that it is made from cardboard, nothing good it's gonna happen unless you pack it externally with: lead sheet, then gum(neoprene) sheet, then another lead sheet
If I understand you correctly f = (speed of sound / length):
- For the 36" diameter x 12" option, 372hz across the diameter, 1116hz along the axis
- For the 24" diameter x 26" option, 558hz across the diameter, 515hz along the axis
- For the 16" diameter x 60" option, 837hz across the diameter, 223hz along the axis
Assume you want to set your LPF at 120hz. So the 36" only has 1.63 octaves between 120hz and the first harmonic. 24" has 2.1 octaves, 16" has 0.89 octaves.
So the more square-like aspect ratio seems the best from the harmonics point of view? Did I understand you correctly 454Casuli
Actually I just looked it up and the formula for the first harmonic standing wave in a pipe is
f = speed of sound / 2 L
For the 36" diameter x 12" option, 186hz across the diameter, 558hz along the axis
For the 24" diameter x 26" option, 279hz across the diameter, 258hz along the axis
For the 16" diameter x 60" option, 419hz across the diameter, 111hz along the axis
Assume you want to set your LPF at 120hz. So the 36" only has 0.63 octaves between 120hz and the first harmonic. 24" has 1.1 octaves, 16" has -0.11 octaves.
f = speed of sound / 2 L
For the 36" diameter x 12" option, 186hz across the diameter, 558hz along the axis
For the 24" diameter x 26" option, 279hz across the diameter, 258hz along the axis
For the 16" diameter x 60" option, 419hz across the diameter, 111hz along the axis
Assume you want to set your LPF at 120hz. So the 36" only has 0.63 octaves between 120hz and the first harmonic. 24" has 1.1 octaves, 16" has -0.11 octaves.
Check that the formula is for a pipe with both ends closed (and not one end closed, one end open).
Yes, a sub with lower overall aspect ratio would perform better in this regard, except in the event that you are trying to do a transmission line or variant thereof.
I am curious though as to why you would want an LPF so high if you are OK with a sub that looks like a sonotube... are your main speakers bookshelf-type?
Yes, a sub with lower overall aspect ratio would perform better in this regard, except in the event that you are trying to do a transmission line or variant thereof.
I am curious though as to why you would want an LPF so high if you are OK with a sub that looks like a sonotube... are your main speakers bookshelf-type?
Bla...bla...bla...engineering, modes above the operating range...I was talking about sound ( uh Sorry, air pressure variation) passing thru the 'box' walls and a way to limit It.
Greets!
There's a 'mismash' of info too numerous to respond to individually, so first need to know what type of Sonotube alignment you're wanting, i.e. vented, sealed or simple TL: Resonances of open air columns
vented, TL = 1/4 WL closed cylinder
sealed = 1/2 WL open cylinder
Re SQ; pipe length/type sets tuning/axial harmonic modes, pipe area sets modes' amplitude, so the higher the aspect ratio, the lower the tuning and the greater its modes' damping on the driver for a given net volume [Vb], ergo the greater its pass band 'ripple' amplitude, which in turn requires more damping [stuffing].
In short, you're 'robbing Peter to pay Paul' in that there's an optimum aspect ratio for each type TL based on driver specs that in general will require a [much] larger [Vb] to achieve a near identical response in a smaller sealed or vented alignment, which in turn can be damped to bring the SQ gap even closer, though still technically not quite as low in overall distortion.
Re Sonotube stiffness; as previously noted, a cylinder is extremely rigid along its axial length, though when using high power it's wise to run threaded rod through the top, bottom, driver to tension/preload it in entirety to insure sufficient rigidity to shift its structural [Fs] at least an octave above its XO BW, not to mention to keep it from damaging a top's airtight seal [been there, done that].
Ditto WRT to woofer box material, bracing construction also.
There's a 'mismash' of info too numerous to respond to individually, so first need to know what type of Sonotube alignment you're wanting, i.e. vented, sealed or simple TL: Resonances of open air columns
vented, TL = 1/4 WL closed cylinder
sealed = 1/2 WL open cylinder
Re SQ; pipe length/type sets tuning/axial harmonic modes, pipe area sets modes' amplitude, so the higher the aspect ratio, the lower the tuning and the greater its modes' damping on the driver for a given net volume [Vb], ergo the greater its pass band 'ripple' amplitude, which in turn requires more damping [stuffing].
In short, you're 'robbing Peter to pay Paul' in that there's an optimum aspect ratio for each type TL based on driver specs that in general will require a [much] larger [Vb] to achieve a near identical response in a smaller sealed or vented alignment, which in turn can be damped to bring the SQ gap even closer, though still technically not quite as low in overall distortion.
Re Sonotube stiffness; as previously noted, a cylinder is extremely rigid along its axial length, though when using high power it's wise to run threaded rod through the top, bottom, driver to tension/preload it in entirety to insure sufficient rigidity to shift its structural [Fs] at least an octave above its XO BW, not to mention to keep it from damaging a top's airtight seal [been there, done that
]. Ditto WRT to woofer box material, bracing construction also.
OK, that's between you and the physics community that came up with the descriptions.
It's what I understood also, but no one explained when which resonant pipe was required for typical 'Sonotube'/cylinder subwoofer alignments, so nowhere did I imply/include just a vent, but to be complete, if vented, then the vent pipe is an 'open cylinder'.
It's what I understood also, but no one explained when which resonant pipe was required for typical 'Sonotube'/cylinder subwoofer alignments, so nowhere did I imply/include just a vent, but to be complete, if vented, then the vent pipe is an 'open cylinder'.
Thank you for the response GM! Personally I plan to go with vented for my next build. However I was interested in learning how to properly model any type of alignment for future reference.
Do you have any advice or links regarding how to add the threaded rod without obstructing the vented pole piece?
You're welcome!
You'll need Hornresp or similar to sim TL/horn alignments: Hornresp
??? Well, there use to be numerous pictures showing detailed construction, but google no longer provides us with the kind of service it once did, so didn't find any beyond a few that didn't use any tensioning rods, but as I posted, everything is tied together with the rods going through the [4] driver mounting holes and with large drivers were often extended down to be its legs also, so has nothing to do with the motor.
If bottom vented, then the woofer is just attached to a baffle and the threaded rod just ties the top/bottom together and maybe used for its legs, preloading [pinching] the tube a small amount with washers and tube spacers to ensure the proper torque.
Regardless, that's the way I did/recommend it, though many just relied on the tube's rigidity or placed a circular panel stiffener or two along its length with the cheaper, thinner concrete column tube formers like I used.
Basic calculator/sectional view: Sonosub - Free Speaker Design Software
edit: Found a 'flexi' design, though not quite like I described and missing some pics:
Tempest Sonosub
You'll need Hornresp or similar to sim TL/horn alignments: Hornresp
??? Well, there use to be numerous pictures showing detailed construction, but google no longer provides us with the kind of service it once did, so didn't find any beyond a few that didn't use any tensioning rods, but as I posted, everything is tied together with the rods going through the [4] driver mounting holes and with large drivers were often extended down to be its legs also, so has nothing to do with the motor.
If bottom vented, then the woofer is just attached to a baffle and the threaded rod just ties the top/bottom together and maybe used for its legs, preloading [pinching] the tube a small amount with washers and tube spacers to ensure the proper torque.
Regardless, that's the way I did/recommend it, though many just relied on the tube's rigidity or placed a circular panel stiffener or two along its length with the cheaper, thinner concrete column tube formers like I used.
Basic calculator/sectional view: Sonosub - Free Speaker Design Software
edit: Found a 'flexi' design, though not quite like I described and missing some pics:
Tempest Sonosub
Huh, maybe I was misinterpreting OP wrong. I figured he was asking what happens when you make the longest dimension of the box too long.OK, that's between you and the physics community that came up with the descriptions.
It's what I understood also, but no one explained when which resonant pipe was required for typical 'Sonotube'/cylinder subwoofer alignments, so nowhere did I imply/include just a vent, but to be complete, if vented, then the vent pipe is an 'open cylinder'.
(because those online calculators don't help with that)
No, just that there was no way to answer it without knowing his 'game plan' and the ensuing posts just further 'muddied the waters', so to speak as he tried to make sense of a very confusing subject and until we know the specs of the driver, desired bandwidth [BW], I still can't answer it to any degree of certitude, but we're now at least moving in the right direction [hopefully] as a 'team'.
This is my plan so far (will probably buy the materials in the next couple days unless yall point out something to reconsider)
- Driver: UM12-22 ($219)
- Sonotube: 24” in diameter likely results in most cube like aspect ratio, but 30” would also work
- Chamber: 177 Liters
- Port: 21.9 Liters, air velocity <17m/s, very gradually flared 3D printed ports, 4 ports total, all on front face of subwoofer along with the driver. Intended operation is with 1 of 4 ports blocked (175cm^2 port area, 16.3Hz Helmholtz freq) for subsonic bass extension, or zero ports blocked (233cm^2, 18.5hz Helmholts freq) for flat response.
- Orientation: sonotube axis is parallel to ground to allow speaker to operate without sag reducing xmax
- Passband: Subsonic Mode:+/-5db from 16hz to 135Hz, Flat Mode: +/- 2db from 20 to 135hz
- Digtial EQ: High Pass Filter @13Hz to prevent subsonic frequencies from driving speaker outside of Xmax. Low Pass Filter at 60hz (my towers play down to 45hz), parametric eq to flatten response in subsonic mode, 111Db max SPL after flattening with EQ. No eq is used in flat response mode, it should play 115db from 20-130hz.
- (In subsonic mode) There is an impedance spike of 8.5 ohms at 17.2 hz, the driver needs 260W@17.2Hz of electrical input power but my amplifier (Crown 1002) could only supply about 210W. I would consider this a reason to buy a better amplifier rather than something to fix (unless there is an easy fix)
- Aesthetics: Not sure how to make the sonotube look best, perhaps bondo + sanding to make it smooth, then paint it? Or perhaps stretch cloth over it?
- Group delay is above 30ms below 21.9Hz, peaking at 120ms in the sub-audible frequencies, not sure how much this matters
- What is the maximum velocity of port compression if you have very very well flared ports? I have heard 10m/s, 17.5 m/s and 30 m/s given as the upper limit. The higher velocity I go the more narrow and short the ports can be, which increases the upper frequency the subwoofer can play before it breaks up. So I would like to make full use of whatever velocity will avoid port compression
- I don't understand if I should put polyfill in the subwoofer, and if I do how that might affect the size of the box or the frequency response
- I see that driver compression due to heat can really change the low-frequency behavior. I suppose I should design with the assumption of some power compression but not sure how many degrees C is reasonable for home theatre use.
- What is the best geometry for port's flare? I am 3D printing it so I can do anything. I imagine a shape that increases in cross-sectional area at a constant rate would be best? So that’s a logarithmic curve? But then it should also have some nice smooth boundary condition where the port becomes tangent to the flat front face...
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