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Kef b300 in smaller enclosures
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qguy
Intermediate Contributor 75+


Joined: 08 May 2003
Posts: 85

PostPosted: Tue Oct 17, 2006 12:11 am    Post subject: Kef b300 in smaller enclosures Reply with quote

What would happen if I reduce the current internal volume of my KEF B300 (70 liters) enclosures by half ?

Do I loose efficiency ?
Do I loose frequency extension ?
Do I loose booth ? if I loose efficiency and LF extension, can this be "corrected" by using an equalizer ?
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Tue Oct 17, 2006 12:10 pm    Post subject: Re: Kef b300 in smaller enclosures Reply with quote

[quote="qguy"]What would happen if I reduce the current internal volume of my KEF B300 (70 liters) enclosures by half ?

The resonance frequency and the system Q will both go up by a factor of 1.4 (sq rt of 2). The piston band sensitivity will remain the same because you have not changed the moving mass or the magnet strength.

You can eq back to the original response with a single second order biquad filter. You could use the one described by Linkwitz here http://www.linkwitzlab.com/filters.htm or you could use the Friend Biquad as used in the KEF Kubes and described in "Principles of Active Network Synthesis and Design" by Daryanani.

I think the Linkwitz filter is difficult to design using regular 1% or 5% standard values. It also uses 4 capacitors when only 2 are used in the Friend circuit.

I have written a Windows program which will calculate the Friend circuit. You are welcome to have a copy if you want. You choose starting values for capacitors, enter the system and required fs and Qs and the program will calculate all the resistors. What you will be doing is boosting the drive to the speaker below the new box resonance at 12dB/Oct as far down as the original box resonance.

The downside to this is that the speaker system will require more voltage than the large box design for the same excursion. SO you will need a bigger power amp and you can only take this approach so far before you run the risk of thermal damage to the woofer.

SG
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qguy
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PostPosted: Thu Apr 05, 2007 7:27 pm    Post subject: Reply with quote

Currently the b300 is driven by a subwoofer amp and operate around 30 - 40 hz

I am happy with th eoutput, but the size of the boxes are too large

If I reduce the box size to around 50 liters, do I just need to crank the gain a little bit higher ? to get same output ? if not , Can I go around this problem by adding a port ?
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speakerguru
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PostPosted: Fri Apr 06, 2007 8:52 am    Post subject: Reply with quote

qguy wrote:
Currently the b300 is driven by a subwoofer amp and operate around 30 - 40 hz

Sounds like you're not so sure what you've ended up with and you have no means of measuring the response.

qguy wrote:
I am happy with the output, but the size of the boxes are too large
Make up a smaller box and see if you're still happy Wink .

qguy wrote:
If I reduce the box size to around 50 liters, do I just need to crank the gain a little bit higher ? to get same output ? if not , Can I go around this problem by adding a port ?

The output from a smaller box will change the response. It will be the same above resonance, go up at resonance (because of the increase in Q) and go down below resonance because of the increase in box air stiffness. You need to measure what you have now, that you are happy with, and then eq the new small box system to get back to it. You will lose out on maximum output unless the amplifier output voltage (not watts) is increased by the inverse ratio of the box decrease.
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Lee in Montreal
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Joined: 22 Aug 2009
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PostPosted: Sun Oct 11, 2009 4:03 pm    Post subject: Re: Kef b300 in smaller enclosures Reply with quote

speakerguru wrote:
The resonance frequency and the system Q will both go up by a factor of 1.4 (sq rt of 2).


Very interesting and probably the insight I was looking for. The initial question was about decreasing the volume. Mine (posted a few days ago) was about increasing the volume from 70 litres to 108 when using a B300 in 105/2 configuration...

If I increase the volume by 50% (150%) my understanding is that the sq.root being 1.22, I will therefore reduce the system resonance frequency by this factor. The 105/2 resonance being 38Hz, the new resonance shall therefore be 31Hz, with no change of sound level above the initial resonance frequency, but an increase below.
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SaSi
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PostPosted: Sun Oct 11, 2009 7:20 pm    Post subject: Reply with quote

Reducing or increasing the enclosure volume (with all other parameters remaining identical), changes the transient behavior of the system. Decreasing Q factor improves transient response and increasing the Q factor worsens transients.

All things are ofcourse relative and the BBC monitor has a Q factor of about 1.2, much higher than the "surgical precision" 0.7 that most British loudspeakers had at the time.

A high Q factor makes the system more "boomy" and "rich in bass", although less precise.

Trouble is, I bet the 105 already has a low Q alignment, so increasing the volume might not serve you well - acoustically.

Why not enter the B300 params to WinISD and see how it performs with various enclosure sizes?
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SaSi
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PostPosted: Sun Oct 11, 2009 7:34 pm    Post subject: Reply with quote

OK, I did it myself - had the parameters entered some time ago.

For an alignment of 0.707, WinISD gives an enclosure size of 176 litres. The 70 litre enclosure comes up with a Q=0.933 and your suggestion of increasing the enclosure to 108 litres would result in a Q=0.811

So, I was rather wrong that the 105 has a low Q alignment after all. Just below 1, which results in the maximum extension to the low frequency whith a 1dB bump above the cutoff frequency.

There might be a problem though. If we trust WinISD, then the 70lt enclosure gives a max excursion at 150W just below 12mm. With a 108lt enclosure, excursion would exceed 12mm below 42Hz and reach 14~16mm below 30Hz. Increasing the enclosure might require a high pass filter tuned at around 30Hz to counter the excursion.
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Lee in Montreal
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PostPosted: Sun Oct 11, 2009 7:53 pm    Post subject: Reply with quote

Interesting calculations. It reminds me why the Isobaric system working in band pass (like a 107) cabinets are so great. They have smallish sealed enclosures, meaning minimal diaphragm excursion. But working in pairs, it allows phenomenal pressures.

So, maybe I need to find an extra set of B300 and make "bigger" 107s.

Lee
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SaSi
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PostPosted: Sun Oct 11, 2009 9:18 pm    Post subject: Reply with quote

Are you sure the 107s are isobarics? I was under the impression it's a 4th order band pass enclosure.
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Lee in Montreal
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PostPosted: Sun Oct 11, 2009 9:38 pm    Post subject: Reply with quote

SaSi wrote:
Are you sure the 107s are isobarics? I was under the impression it's a 4th order band pass enclosure.


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SaSi
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PostPosted: Sun Oct 11, 2009 10:23 pm    Post subject: Reply with quote

I know what you mean with the picture. But an isobaric configuration has everything to gain from a small between-drivers chamber and lots to go out of control with a large space including stuffing material.

Could it be that bottom of the speaker is open?

It certainly is a departure from the 104.2
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Lee in Montreal
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PostPosted: Sun Oct 11, 2009 11:00 pm    Post subject: Reply with quote

SaSi wrote:
Could it be that bottom of the speaker is open?


The opening is on top, right below the heads. A very different design from the 104/2.
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speakerguru
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PostPosted: Mon Oct 12, 2009 8:56 am    Post subject: Reply with quote

SaSi wrote:
It certainly is a departure from the 104.2

Look at it more closely. It is exactly the same as a 104|2 except that the port turns through 90deg to come out at the top of the box instead of in the middle, at the front.

Both 104|2 and 107 are bandpass designs with acoustic second order low and low pass roll-off characteristics. The high pass ends up being third order because of the series capacitors.

A bandpass design is the same as a reflex box but using just the port acoustic output. The driver's direct acoustic output (from the cone) is covered by the second box. The added complication is that most KEF designs use two drivers in two boxes feeding a common third port box. That way the drivers mechanical vibration can be force-cancelled via the interconnecting rod. Think of it like a humbucker. The drivers are out of phase mechanically AND out of phase electrically. They end up inphase acoustically but the vibration gets cancelled. Clever or what Laughing ?

KEF would never do an "isobaric" because that is a misnomer. It is just equivalent to a single driver of twice the mass and twice the motor strength. If KEF had wanted that, they would have just built a single driver with those parameters.
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SimplySound
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Joined: 08 May 2012
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Location: Albuquerque, NM

PostPosted: Tue Mar 26, 2013 2:53 pm    Post subject: Reply with quote

speakerguru wrote:
KEF would never do an "isobaric" because that is a misnomer. It is just equivalent to a single driver of twice the mass and twice the motor strength. If KEF had wanted that, they would have just built a single driver with those parameters.


Guru... can you comment about the design used in the Maidstone 109? Isobaric? From this article it states;

"Taking a close look at the low frequency end of the spectrum revealed that the ‘Interport’
Coupled Cavity system was fulfilling its role of providing accurate, well defined
bass in a compact enclosure. However, when the possibility of designing a new High-
End Uni-Q based system came along the KEF engineers thought long and hard about
the best way of reproducing the frequency range below 400Hz. Without any major
constraints on system cost and the physical size and shape of the cabinet, a number of
serious possibilities arise. Concepts used in older KEF speakers like the Reference 105
and KM1, for example, were re-evaluated to test their suitability. Finally a decision
was made, the R109 would use direct radiators for the low frequency and lower
midrange. New drive units would be specially designed to best exploit the direct
radiator principle.


I know the R109 Bass Driver was a VERY special unit... I'm sure practically worth it's weight in gold! Wink

Thanks as always for your excellent insight!
_________________
Kef 107's-"One of the few full-range loudspeakers that I have experienced to touch the soul—not just of the music, but of the listener."—John Atkinson
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clubsport911
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Joined: 26 Aug 2012
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Location: Cheltenham, UK

PostPosted: Tue Mar 26, 2013 9:01 pm    Post subject: Reply with quote

@ speakerguru

As a student of matters "coupled cavity bass" I keep returning to the same question. If truth is universal, and the twin coupled cavity bass is so "clever" why do more people not use it now ? Why was it not used much or copied then? Why did KEF stop using it ?

Remember, i am a KEF owner and proud.
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