So, tell me please, how a sub can produce 11hz @ 105 db spl with two 6.5" drivers.
@amirm please test!
Very easy! Using the calculator, you need two 6.5 inch drivers with 7.55 inches of Xmax--basically, a 15 inch stroke and done!
Alas, you have been a victim of smoke and mirrors--KEF never claimed it puts out 105dB at 11 Hz. No! They claimed a peak output of 105dB on one line and a frequency response on another line--they never, in any shape or form claimed the two specs are related to each other.
I find this hysterically funny--a sealed subwoofer is the easiest sub to calculate output--it is the most basic air pump you can have as there is not horns, no bandpass, no ports or anything to calculate. It is purely bore X stroke that produces liters of air (or less than a liter) Liters of air movement directly result in specific SPL at specific frequencies.
Look at the drivers, they are conventional with spiders and the frames have an angle which limits the diameter of the spider. The smaller the spider, the less Xmax is available so you want to stuff in the largest diameter spider in you can, use a deep surround to allow more cone movement and so on. No LOOK at that driver! The spider is not the largest you can stuff inside AND the surround is not deep. I looked at Sundown, they make very long travel drivers for car audio, they have a 6.5 with vertical sides to cram in the largest spider they can, very large magnet and a larger surround than the KEF. The specs at 70% BL is 10mm of Xmax and an Xmech of 20mm. Xmech is when the voice coils slams into the magnet and you break things.
TangBand has a 6.5 inch that has 12mm of Xmax and a Neo motor and there is a 6.5 incher that uses an XBL2 motor to punch 12.5mm. That is about your limit because of the natural limit of the diameter of the spider. You might have noticed this because a Dayton UM8 8 incher has an Xmax of 16mm, some 10 inch subs have an Xmax of 19mm and a CSS SDX12 has an Xmax of 28mm. As subwoofer frames get larger, you can have longer Xmax or stroke because the spiders become larger to allow this.
A good example of the "magical DSP" is a design from around 9 or 10 years ago called the "VBSS sub" Basically, it is a 18" PA woofer in a 6.75 cubic foot box two 4" flared ports tuned to 20Hz (or 19 Hz, 18 Hz or one port plugged for 14.5 Hz) The woofer runs $90 to $98 in price, and is limited to about 9mm of Xmax. The "magic" in it is simple, the parametric EQ has a boost of +12dB at 20Hz so you get even response up to around 105dB. To protect the driver from such massive boost, it uses dynamic limiting which limits the power allowed at specific frequencies to protect the woofer. Basically, the woofer is only allowed to move 9mm of Xmax before the power is limited. A certain amount of power is allowed at 20Hz, more power is allowed at 25Hz, even more power at 31.5Hz and no limits at 40Hz. For example, at 40 or 50Hz that sub can punch out higher than 120dB because the limiter now becomes the amp limiter.
The magical amp is called a Behringer iNuke 3000DSP which costs less than $300 (now called NX3000D) Into the 8 ohm load of the woofer it will punch out 300 watts into 8 ohms. Strap two of those VBSS subs to each channel and it jumps to 600 watts X 2 into 4 ohms. Total cost to build four of those VBSS subs is $90 X 4 or $360 for the drivers, $60 or so for the eight flared ports and four sheets of plywood--maybe $279 for the amp. $600 in parts and four sheets of 3/4" plywood--throw in some Duratex and we are talking $800 for four 18" subwoofers, 1,200 watts of total power, many bands of parametric EQ, delays, adjustable limiters and so on. If you have four of them in a typical room (generaly basement theaters) you will get room gain down around 20Hz and the subs will start to couple with each other at low frequencies (depending on distance) so very easy to get 115dB at 20Hz or "reference levels" Output at 30 or 40Hz is well into the 120+dB range and the dynamic limiters will protect the driver down low but allow full power up higher to go completely rockstar with drums and bass guitars.
Anyone can build/design such a thing--hell, I've done it! No, I don't have the VBSS but helped a friend build one and it works very well. There is nothing physics defying about it, no magic, no messiah has come down from the clouds to alter Hoffman's Iron Law--just parametric EQ boost and dynamic frequency limiters available in an amplifier for under $300. If you like, get a Eminence 21" stadium sub, slap it in a 16 cubic foot box tuned for 18Hz, throw 2,000 watts at it and apply dynamic limiting and EQ boost to get 120 to 130dB depending on frequency. It is not hard, no EE degree required, just a working knowledge how speakers work, how EQ and dynamic frequency limiters operate--a bit of software to model how to do proper setup and done. Yep, one guy and a weekend and you can do it start to finish. If a fool like me can do it--as long as you have opposable thumbs, can read and won't cut your hands off with a power saw all is well.
There is a guy in Canada, the single guy engineering type that has his basement underground, fully treated with walls over 2 inches thick. He took 16 of those Dayton 18" PA woofers, built two columns with 8 woofers each (4 on each side opposed) in 24 cubic foot sealed enclosures.
Strapped them to a very stout PA amp that pushes 8,000 watts and obtained 115dB at 2 watts of power. Ran it full throttle in his room at 8 KW and exceeded 150dB (why the basment is underground and his nearest neighbor is over a mile away) He posted his distortion at 115dB and it was very low, down to a few tenths of a percent. Typically, he never goes over 130dB peaks in the bass region so content. Total cost for the 16 drivers at $85 each, a clone stadium amp at $1,000, he has audio processors for everything already and the wood/bracing to build two 24 cubic foot sealed enclosures was under $3K. Oh yeah, he has another 15 subwoofers to do the low stuff--those TC Sounds 18's with more stadium amps but such is required to get a frequency response flat down below 2 to 4 Hz. Those 16 PA woofers have no high pass filters for protection and are not EQ'd below 40Hz where the TC Sounds add in.
So yeah, I find what KEF is doing is laughable--but they need to make a profit so give incomplete, misleading specs--like audio gear on Alibaba. They specifically create magic but they know their customers better than me so they can profit from it. Yes, it makes KEF look like those flea market car amps that promise 4,000 watts and contain a single 20 amp fuse but it sells. They are selling to specific groups, the people that want a tiny, sexy little sub to match the room decor, people that are the KEF fanboys and faithful and others that use specs to determine how good they are, how large their manhood is etc. Granted, anyone that knows/understands acoustics won't buy such a thing but KEF does not make high performance subwoofers anyway so no big loss.
So when somebody actually measures ones and it is obviously weak will this hurt KEF? No, they aspire to the lifestyle market which is much larger than the performance at all costs HT/music market. The Phantom sells well, it obviously has bogus specs and KEF wants that money--honey! It has all the buzzwords for the clueless, a thousands watts! 11 Hz! 105dB! and it is expensive enough so the audiophile claim of "You are just jealous because you can't afford it" applies. Sure, they damage their rep but it's not like KEF was the engineering powerhouse of the late 70's and 80's anyway. They went coax to simplify in the 90's and now are angling to go full lifestyle. It worked for Bose! No worries, just business so for the KEF owners that will get butt hurt because they refuse to produce world class performance with lifestyle audio...just the way it is. You will adjust, plenty of KEF fans from the 70's/80's with 104.2 towers etc. saw those little coax bookshelves from the 90's and they left the church...and KEF survived so welcome to the 20's.
KEF lost its direction after making its last batch of BBC-licensed monitors, and eventually went into
receivership in 1992.
[6] It was acquired by GP Acoustics, a member of the Hong Kong-based
Gold Peak Group.
[1]
Makes sense, they went bust in 1992 and was purchased by a China based company... sooo... yeah.