First - the best woofer is the one POSITIONED BEST in your room! That's the major benefit of a subwoofer, you can chose a better room position as with your main speakers.
Read some stuff about positioning subwoofers, SBIR or lambda/4 chancellations, multi subs, bass arrays etc. This is the way to good bass reproduction.
Flush mountig doesn't help at all when the position is bad btw. It helps against SBIR with ONE surface.
Enclosure - a closed volume keeps a 2nd order resonant system, it just pushes the frequency higher and makes Q higher. All other systems which gain level with some sort of resonant behaviour gives you a higher order RESONANT system. When your goal is best sound - stay with a lower order system. Just stick with a closed woofer.
Force chancellation works to have less vibration at the cabinet. You still need a STIFF cabinet - lot's of bracing between walls give you that.
MDF is not BAD! I did plenty of material testing and measuring - it's less stiff as plywood, but not THAT much. Aluminium or Iron is stiff. Acrylic glass has great dampening. But wood stays in it's area. Good plywood is still a preferable material but great stiffening with bracing is WAY more important.
Stay small enough with the dimensions of your woofer enclosure that no standing waves happen with at least an octave over your cross over frequency and shape of the volume is not very important. I prefer to add dampening in the enclosure to be sure there is nothing happening at higer frequencies but depending on your usecase it's not needed.
The art of subwoofer building in a few sentences ...
Heavy, not very stiff. MDF is not a great speaker building material/ Its prime asset is CHEAP.
In a woofer you want stiff to push potential resonances up above the bandwidth of the box so they are not excited. We built a dual 10” push-push sub, and except for the top 15mm was sufficient.
It takes something like 18mm MDF to maybe be as stiff 9as the 15mm).
Invariably a biggish box, the difference for your back between 18mm MDF and 15mm ply is considerable. As an example you could lift the push-push sub with a couple finger sin the driver cut-out (not so easy when the woofers are installed).
dave
In a woofer you want stiff to push potential resonances up above the bandwidth of the box so they are not excited. We built a dual 10” push-push sub, and except for the top 15mm was sufficient.
It takes something like 18mm MDF to maybe be as stiff 9as the 15mm).
Invariably a biggish box, the difference for your back between 18mm MDF and 15mm ply is considerable. As an example you could lift the push-push sub with a couple finger sin the driver cut-out (not so easy when the woofers are installed).
dave
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Density of MDF is between 600 (thick) and 750 (thin, black) kg/m³. Birch plywood is 690kg/m³. E-module is about 2500 N/mm² for MDF and 5000 N/mm² for plywood. Coincidence frequency for both is out of the range for a subwoofer!
In real live just getting a few mm thicker MDF does the trick. As we don't build speakers for carrying around for HiFi a higher weight is potentially benefitial and 6mm more dimension ... is not a big deal.
Plywood has other benefitial behaviours (moisture, surface, ...) and mdf is still just glued dust and not the best material under many aspects - but ACOUSTICALLY the difference is not big. Good bracing is the key to get a stiff enclosure.
In real live just getting a few mm thicker MDF does the trick. As we don't build speakers for carrying around for HiFi a higher weight is potentially benefitial and 6mm more dimension ... is not a big deal.
Plywood has other benefitial behaviours (moisture, surface, ...) and mdf is still just glued dust and not the best material under many aspects - but ACOUSTICALLY the difference is not big. Good bracing is the key to get a stiff enclosure.
When building a sub, don't forget that the very worst shape you can build is a cuboid - they require extensive and heavy bracing to overcome their obvious and inherent flaw of lower frequency panel resonances compared with more considered and considerably more inherently stiff shapes. My 18" subs have their first major resonance at over 500Hz with minimal 6mm thick MDF bracing.
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I won't claim that I know what the theoretical best sub enclosure is, only that I like this one the best:
http://jazzman-esl-page.blogspot.com/2011/01/ripole-subs-are-underway.html
http://jazzman-esl-page.blogspot.com/2011/01/ripole-subs-are-underway.html
I've looked at that design several times. Would be keen to hear them against a typical dipole.
Pretty much, plus airtight! Standing waves are not usually a problem for subs as the wavelengths rarely fit in the enclosure.
I just finished two of them to assist a set of Quad ESL 57's. A provisional short review is: "Wow!"
Incidentally when I go to the loo I notice that in another room it's not half as loud as I expected even if in my listening spot it was quite loud. Certainly no booming or rumbling! Since I live in an apartment that's a big plus!
A German company, Modal Akustik, markets a Ripol subwoofer (photo above) licensed by the patent holder, Axel Ridtahler, and it happens that both this sub and my DIY version use the same Peerless SLS 12 woofers.
Six Moons Audio reviewed the Modal Akustik sub and said this about it:
"For music-first listeners who prioritize speed, articulation and enunciated clarity, it's the long awaited messiah."
Srajan Ebaen, 6moons.com
I share that sentiment.
The Ripole design principal is not well known or understood-- I'm still struggling with it myself. Modal Akustik explains it as follows:
"The RiPol-principle explained:
The dispersion pattern of regular bass principles is, below a certain frequency, omni-directional. A dipol is physically clearly defined and shows a typical 8-shaped dispersion. The RiPol is a special kind of bass-dipol.
Its special enclosure gives a RiPol not that exact symmetrical dispersion. Because the intensity radiated from the front in relation to the back is different, a RiPol has an asymmetrical dispersion pattern, which leads to a beneficial in-room frequency response. Because the dispersion of a RiPol does not follow the classic 8-shape, a RiPol is not a typical dipol but one of its own kind. Dispersion diagrams show clearly the difference compared to a classical dipol, which justifies its own name, RiPol (as a combination of "dipol" and the last name of its inventor, Axel Ridtahler).
The RiPol-enclosure has a strong effect on its dispersion pattern. It prevents, that the air produced by the chassis movement gets out of the way too quickly, so that the radiation resistance increases. The natural resonance of the drivers is lowered when mounted in a RiPol-enclosure, which improves the reproduction quality of the lower bass region the smaller the enclosure gets (!!). There is however a limit, that is why the dimensions of a RiPol-enclosure have to be iteratively optimized, calculated and simulated. The arrangement of the two drivers opposite of each other in addition realizes a benefitial impulse compensation. At the same time the enclosure has a very small footprint.
A RiPol disperses low frequencies in certain directions, to the front and ,with different intensity and 180° revered phase, to the back. This attribute reduces harmful standing waves in the room. These are energy-retention effects within a room, also called room modes. In a listening room standing waves can cause the impression of inertial bass energy that does not fade away. Bass-heavy speakers float the room with bass energy and leave the impression of a slow or sluggish bass. A RiPol, through its rear chambers, produces "antimatter" at low frequencies, that prevents this effect to emerge. A bass impulse is created and, through its counter-wave that comes from the back, the continuance of the standing wave is prevented. As a result the signal is not blurred by its own overlay. These attributes make the RiPol one of the most musical bass-principles on the market.
If you are asking about the woofers in the Ripol subs, those are 8 Ohm 12" Peerless SLS wired in parallel (4 Ohm).
They don't require huge power to play at [what I consider] a normal listening level. In fact; in a Ripol, as with any open baffle, they won't take a lot of power before bottoming out at X-max (+/-12mm).
I drive my pair (4 woofers in two subs) with a Carver TFM-25 (225W/Ch) and pushing the meter needles more than about half-way will dive the woofers to X-max.
My Ripols are the cleanest, most non-boomy subs I've ever experienced but they aren't as efficient as boxed subs so I always recommend a pair, not just one. Also, if you will be playing really loud home theater sound tracks, apply a limiter in the crossover (I use a digital crossover) or go easy on the throttle or fuse them for protection.
They don't require huge power to play at [what I consider] a normal listening level. In fact; in a Ripol, as with any open baffle, they won't take a lot of power before bottoming out at X-max (+/-12mm).
I drive my pair (4 woofers in two subs) with a Carver TFM-25 (225W/Ch) and pushing the meter needles more than about half-way will dive the woofers to X-max.
My Ripols are the cleanest, most non-boomy subs I've ever experienced but they aren't as efficient as boxed subs so I always recommend a pair, not just one. Also, if you will be playing really loud home theater sound tracks, apply a limiter in the crossover (I use a digital crossover) or go easy on the throttle or fuse them for protection.
I've heard a single SLS 12 on an OB and a pair in a RiPol, and I'll share my impressions.
Aside from the quantitative difference of 2 woofers versus 1 on the OB, there's a qualitative difference as well.
Both exhibit the characteristic dipole roll off in the far field so I will give my impressions regarding the near field.
The RiPol is more controlled/tighter/cleaner overall, and especially on low notes near 30Hz. The RiPol has a big advantage down low because it constrains the woofer's fundamental resonance by driving it 5-7Hz downward, whereas the OB can't do that, so that resonance is audible near the woofer's FS (31Hz for the SLS 12).
So; if you use a woofer with a low FS to start with, the RiPol can push even further downward where it's unlikely to get excited because most music contains little or no energy down that low, and even if it did, a resonance near the threshold of a baby's hearing might be felt but you wouldn't actually hear it as a tone.
Above 150 Hz the OB has a clear advantage in that it doesn't have an enclosure so there are no cavity resonances. Whereas, the Ripol has a front chamber and two rear chambers, and each one resonates when excited by a sympathetic note.
In my SLS 12 RiPol; the front chamber resonance is loud peak at 299Hz, and each rear chamber exhibits a less-loud peak at 294 Hz. However, because I use a steep-sloped crossover at 70 Hz, these cavity resonances are not excited and do not occur.
Neither a Ripol nor an OB sub are going to hit you in the chest like a sledge hammer, so if that's your desire, you're better off with a sealed or ported box sub. If you prioritize super clean non-boomy bass over thump and efficiency, then a Ripol is wonderful.
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The late Brian Cheney of VMPS mixed speakers of various sizes in one chamber in his designs - lots of fiberglass fill - a downwards into slot firing passive radiator (eventually sags) Nestorovic mixed speaker types in a Speakerlab product where the larger speaker
also acted as a passive radiator.
also acted as a passive radiator.
I do like the clean non-boomy bass of open baffle and have been running that for about 13 years now, but I would like a little more. Unfortunately (and fortunately because they are very expensive) I can't get the 18" drivers I have because the company went out of business a long time ago. So I am stuck with a single driver each side, but I have very little in the way of baffle (on one driver none at all) I am currently looking at a modified version of a H baffle.