Does anyone here own a KEF KF92 sub yet? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 7 Old 02-13-2020, 12:52 PM - Thread Starter
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Does anyone here own a KEF KF92 sub yet?

The newest sub from KEF was released at the end of last year, but I can't find any reviews written anywhere or videos on YouTube about it. Been interested in it ever since it was announced because it is a small form-factor dual 9" driver sub which they claim goes down to 11Hz which is perfect for a living room environment where placement limitations and aesthetics are more of a consideration over a dedicated space. Is the $1999 US price tag keeping most interest away from this model?

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post #2 of 7 Old 02-13-2020, 02:02 PM
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2K gets a person a lot more bang for the buck with any of the ID brands. 2x or more performance for the same price I'd venture to say.

Notice the spec's do not state the F3 point.

They claim it plays down to 11htz, but how far down? Not -3db or they would proudly state that. -6? -10? -12?

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post #3 of 7 Old 02-13-2020, 03:49 PM
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Quote:
Originally Posted by derekmoore View Post
2K gets a person a lot more bang for the buck with any of the ID brands. 2x or more performance for the same price I'd venture to say.

Notice the spec's do not state the F3 point.

They claim it plays down to 11htz, but how far down? Not -3db or they would proudly state that. -6? -10? -12?
The specs actually do say +/- 3db but I have a hard time believing that.

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post #4 of 7 Old 02-14-2020, 06:46 AM
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^ you are correct. The KEF site does state 11hz as the F3 point. I previously checked on BB which did not.

Impressive, but still a lot more bang for buck elsewhere.

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post #5 of 7 Old 02-14-2020, 07:29 AM
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11Hz from dual 9" drivers and 500W? I'm unimpressed. The much-cuter B&W PV1D - dual 8" drivers and 400W - had an FR rating of 7.5-450Hz +/-3dB.
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post #6 of 7 Old 02-14-2020, 08:34 AM
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Hmmm,

Now to break it down--very easy to do considering it is a typical direct radiator type subwoofer. Two 9" drivers with 500 watts each and down at 11 Hz. OK, I would hazard to guess it uses two long stroke drivers with EQ in a sealed box running opposed--very conventional. They do claim 110dB peak output but (of course) won't tell you at what frequency and won't show a SPL maximum chart but what do you want for two grand? 1110dB at 1000 watts with two drivers in a sealed box is very, very inefficient--doing the math...carry the two. 1000 watts = 30dB of gain for power so the sub has an efficiency of 80dB 1w/1m which is horrific but does follow Hoffman's Iron Law so very predictible.

This is not magic, not an engineering feat or anything--two small, long Xmax drivers driven with a lot of power and huge boost down low. They do use opposed drivers so it won't vibrate (or walk in the worst possible case) As is always the case with direct radiator subs, it comes down to bore X stroke and two 9" drivers have about the same surface area (bore) as a single 12. Knowing how subs are made, the issue with small drivers is it has a natural limit how large the spider can be (the coil on the base of the cone that keeps the voice coil lined up in the gap) You can't get 38mm of Xmax in a 9" driver because the coil will need to have a huge diameter to allow that which won't fit in such a small frame. Long stroke drivers like a Dayton UM8 has 16mm of Xmax for an 8 incher and CSS uses a 10 inch driver that has 19mm of Xmax because of that limit. Although you can mimic the surface area of larger drivers, you have a very real limit of spider size which limits stroke--why you should use the large diameter drivers at the start.

The other issue is the longer the stroke, the more power it takes to push it that far. 500 watts might seem impressive to a driver but to do that takes large diameter voice coils with a lot of copper to handle the heat load. Alas, this also makes the moving assembly very heavy which demands a lot of power to move the thing. The farther a cone moves, the more it distorts so the design has to attempt to control that movement. Sealed helps, the trapped air acts like a spring or why they call it "acoustic suspension".

This is not new information, very basic info and millions of car audio subwoofer builders know this and attempt to squeeze bigger drivers in if they can. Sealed boxes with EQ boost is normal no matter if you call it DSP or "music engines" or whatever. You require EQ to force a sealed box to put out reasonably accurate frequency response so why not market it as a "feature".

You can go online and find a subwoofer SPL calculator--plug in 9 inch drivers, throw in what frequency and SPL you demand and it will spit out how far the cone has to move to reach that spec. No magic there, just bore X stroke X number of drivers. This only applies to direct radiators (sealed, ported) but not apply to bandpass or horns but the KEF is a direct radiator. Will the sub put out 110db at 11 Hz? No, it won't because it can't as the spider required for that kind of stroke won't fit in such a small frame. How much SPL will you get out of that sub depends on the Xmax of the driver which is not listed. You can, if you so choose select 19mm of Xmax which is what the CSS XBL2 motored 10" driver does--give it a shot. This will show you the maximum output it can do (in theory) the KEF might produce. Spoiler alert--you won't like the results!

Other factors to consider, human hearing gets very poor as the frequency drops so demands to be much, much louder just so you can detect it. I have not memorized the chart but I think it is around 90dB at 16Hz so it would be reasonable to expect around 95 to 100dB at 11 Hz to hear it... basically, IF you have the sub really close to you a person might detect 11Hz but it would have to be very close and a small room helps. Sure, I can grab a Sundown 6.5 inch subwofer, stuff it in a box and EQ it flat down to 8 Hz if I like--but I won't hear it.

A good reason you won't see people on a forum that fully understand how subwoofers and hearing works going nuts about a dual 9" driver sealed sub rated at 11 Hz makes sense. It is at the edge of marketing VS reality and is not aimed at performance users so would be ignored. Once you know how it works, very simple to reverse engineer subwoofers that are direct radiators--math is very useful in that case. I'm not saying KEF is bad, I'm sure they pushed it's performance as far as can be expected from something so small. It is aimed at KEF coaxial speaker people, probably be a good choice considering those speakers have a limited SPL potential so might be a good fit with their smaller bookshelf speakers. However, they are not aimed at "power users" people that use other more efficient speaker systems or full HT systems.

Realistically, I would not go below 10 inch drivers if you want decent performance below 25Hz--the CSS XBL2 motored 10 inch subwoofers with a pair of them opposed would be OK if you use at least two of them in a small room (room gain) with enough power applied. Since you need two subwoofers to get reasonable frequency response to avoid peaks/nulls of one sub--a few tens might do something or get close. Sure, I have built plenty of 8 inch subwoofers but they were either used in cars or I built one as a computer speaker sub for basic tasks. The wee little subs are great and I will be building a 28Hz tuned TangBand 6.5 inch Neo sub to be driven by a chip amp this year (boombox project) The KEF might be fairly decent if you get four of them, one sub in each corner to give you the most efficiency boost while evening out the response. I could think of far better options with $8,000 though but if you like the looks or style of the things--go for it! Just don't be under any illusion that one of them will get you anywhere near "reference levels" or will reproduce your pipe organ collection with clarity. With deep bass, it is how much air can be moved to produce a certain SPL at a certain frequency--forget science, it is basic math so understand the equation.

Hope that helps...
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post #7 of 7 Old 02-14-2020, 08:41 AM
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Quote:
Originally Posted by 18Hurts View Post
Hmmm,

Now to break it down--very easy to do considering it is a typical direct radiator type subwoofer. Two 9" drivers with 500 watts each and down at 11 Hz. OK, I would hazard to guess it uses two long stroke drivers with EQ in a sealed box running opposed--very conventional. They do claim 110dB peak output but (of course) won't tell you at what frequency and won't show a SPL maximum chart but what do you want for two grand? 1110dB at 1000 watts with two drivers in a sealed box is very, very inefficient--doing the math...carry the two. 1000 watts = 30dB of gain for power so the sub has an efficiency of 80dB 1w/1m which is horrific but does follow Hoffman's Iron Law so very predictible.

This is not magic, not an engineering feat or anything--two small, long Xmax drivers driven with a lot of power and huge boost down low. They do use opposed drivers so it won't vibrate (or walk in the worst possible case) As is always the case with direct radiator subs, it comes down to bore X stroke and two 9" drivers have about the same surface area (bore) as a single 12. Knowing how subs are made, the issue with small drivers is it has a natural limit how large the spider can be (the coil on the base of the cone that keeps the voice coil lined up in the gap) You can't get 38mm of Xmax in a 9" driver because the coil will need to have a huge diameter to allow that which won't fit in such a small frame. Long stroke drivers like a Dayton UM8 has 16mm of Xmax for an 8 incher and CSS uses a 10 inch driver that has 19mm of Xmax because of that limit. Although you can mimic the surface area of larger drivers, you have a very real limit of spider size which limits stroke--why you should use the large diameter drivers at the start.

The other issue is the longer the stroke, the more power it takes to push it that far. 500 watts might seem impressive to a driver but to do that takes large diameter voice coils with a lot of copper to handle the heat load. Alas, this also makes the moving assembly very heavy which demands a lot of power to move the thing. The farther a cone moves, the more it distorts so the design has to attempt to control that movement. Sealed helps, the trapped air acts like a spring or why they call it "acoustic suspension".

This is not new information, very basic info and millions of car audio subwoofer builders know this and attempt to squeeze bigger drivers in if they can. Sealed boxes with EQ boost is normal no matter if you call it DSP or "music engines" or whatever. You require EQ to force a sealed box to put out reasonably accurate frequency response so why not market it as a "feature".

You can go online and find a subwoofer SPL calculator--plug in 9 inch drivers, throw in what frequency and SPL you demand and it will spit out how far the cone has to move to reach that spec. No magic there, just bore X stroke X number of drivers. This only applies to direct radiators (sealed, ported) but not apply to bandpass or horns but the KEF is a direct radiator. Will the sub put out 110db at 11 Hz? No, it won't because it can't as the spider required for that kind of stroke won't fit in such a small frame. How much SPL will you get out of that sub depends on the Xmax of the driver which is not listed. You can, if you so choose select 19mm of Xmax which is what the CSS XBL2 motored 10" driver does--give it a shot. This will show you the maximum output it can do (in theory) the KEF might produce. Spoiler alert--you won't like the results!

Other factors to consider, human hearing gets very poor as the frequency drops so demands to be much, much louder just so you can detect it. I have not memorized the chart but I think it is around 90dB at 16Hz so it would be reasonable to expect around 95 to 100dB at 11 Hz to hear it... basically, IF you have the sub really close to you a person might detect 11Hz but it would have to be very close and a small room helps. Sure, I can grab a Sundown 6.5 inch subwofer, stuff it in a box and EQ it flat down to 8 Hz if I like--but I won't hear it.

A good reason you won't see people on a forum that fully understand how subwoofers and hearing works going nuts about a dual 9" driver sealed sub rated at 11 Hz makes sense. It is at the edge of marketing VS reality and is not aimed at performance users so would be ignored. Once you know how it works, very simple to reverse engineer subwoofers that are direct radiators--math is very useful in that case. I'm not saying KEF is bad, I'm sure they pushed it's performance as far as can be expected from something so small. It is aimed at KEF coaxial speaker people, probably be a good choice considering those speakers have a limited SPL potential so might be a good fit with their smaller bookshelf speakers. However, they are not aimed at "power users" people that use other more efficient speaker systems or full HT systems.

Realistically, I would not go below 10 inch drivers if you want decent performance below 25Hz--the CSS XBL2 motored 10 inch subwoofers with a pair of them opposed would be OK if you use at least two of them in a small room (room gain) with enough power applied. Since you need two subwoofers to get reasonable frequency response to avoid peaks/nulls of one sub--a few tens might do something or get close. Sure, I have built plenty of 8 inch subwoofers but they were either used in cars or I built one as a computer speaker sub for basic tasks. The wee little subs are great and I will be building a 28Hz tuned TangBand 6.5 inch Neo sub to be driven by a chip amp this year (boombox project) The KEF might be fairly decent if you get four of them, one sub in each corner to give you the most efficiency boost while evening out the response. I could think of far better options with $8,000 though but if you like the looks or style of the things--go for it! Just don't be under any illusion that one of them will get you anywhere near "reference levels" or will reproduce your pipe organ collection with clarity. With deep bass, it is how much air can be moved to produce a certain SPL at a certain frequency--forget science, it is basic math so understand the equation.

Hope that helps...
@18Hurts
Great explanation! This is why i come to this forum.
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