Kensho Two – M3

Updated 20170416!

Concept:

Kensho Two – M3 is a small sized 2-way stand-mount bass-reflex loudspeaker. It’s ideal for small to medium sized listening room. The tweeter level can be fine-tuned according to personal preferences and taste.

“Kensho” is Japanese for “seeing nature.” It means seeing one’s true nature; perceiving the self, but can also in Zen Buddhism mean “a brief experience of enlightenment”.

Loudspeaker driver units:

 

Kensho Two – M3 uses a 5″ mid-woofer and a 1″ soft dome tweeter from SB Acoustics.

The SB15NRXC30-4 is a low distorting 4Ω mid-woofer with a coated Papyrus fiber cone.

The SB Acoustics SB26STAC-C000-4 is a 4Ω, 1″ coated textile dome tweeter.

For further details see:

SB Acoustics SB15NRXC30-4
SB Acoustics SB26STAC-C000-4

Cabinet:

 

The Kensho Two – M3 uses a 11 liter “Dayton Audio” enclosure. This is a very nicely built curved cabinet with a 25mm (1″) MDF front-baffle. The enclosure walls are made of 18mm MDF as well as a 15mm brace from the bottom to the top. The enclosure net volume is approximately 10 liters (enclosure volume – drivers, filter and port).

Cabinet drawing: Dayton 11 liters Curved Cabinet #302-711

 Baffle drawing and driver unit layout:

Box simulation:

 

Bass-reflex 11 liter Fp=43Hz

-6dB = 37Hz
-12dB = 32Hz

Bass-reflex port: 45x155mm

Impedance measurements:

 

(click on picture to zoom)

Blue: Left loudspeaker

Green: Right loudspeaker

Minimum impedance of 3,28 Ohm @ 170Hz.

Cross-over design:

The Kensho Two – M3 have an acoustical fourth-order Linkwitz-Riley (LR4) filter topology.

The mid-woofer cross-over filter section is an electrical third-order and consists of two coils and a cap (L3+L4+C4) that shapes the cross-over slope to an acoustical LR4 roll-off with a targeted 2.1kHz cross-over point. The mid-woofer cross-over filter section also have a frequency response shaping circuit (C5+L5+R4). The L3 and L4 coil should have a summed total series resistance of 0.4 Ohm +/- 0.1 Ohm.

The summed resistance of the L5 coil and R4 resistor should have a summed total resistance of 11.38 Ohm +/- 1 Ohm.

The tweeter cross-over filter section consists of two resistor (R1) and (R2), together with a third-order electrical filter (C1 + L1+C2) that shapes the cross-over slope to an acoustical LR4 roll-off with a targeted 2.1kHz cross-over point. (C3+L2+R3) flattens out the tweeters impedance peak at the resonance frequency and it’s necessary in order to shape the filter to a LR4 filter slope.

Mid-woofer frequency response shaping circuit:

Tweeter frequency response shaping circuit:

Cross-over simulation:

 



(click on picture to zoom)

Top: 15deg off-axis frequency response.

Left: 15deg off-axis frequency response, reverse polarity.

Right: Individual driver unit phase tracking.

The reverse polarity simulation shows a deep reverse null indicating an excellent phase tracking between the two driver units. The tweeter has a rising response above 12kHz.

Frequency measurements:

 

The following frequency measurements of the finished loudspeaker are made at a 2m distance.

All frequency response charts are presented in a 50db scale with 1/24 octave smoothing and the measurements are valid down to 375Hz.


(click on picture to zoom)

Upper Left: On-axis frequency response @ 2m.

Upper Right: On-axis frequency response, tweeter reverse polarity @ 2m.

Lower Left: 15deg off-axis frequency response @ 2m.

Lower Right: 15deg off-axis frequency response, tweeter reverse polarity @ 2m.

The dip around 3.5kHz is caused by baffle diffraction and smooths out in the off-axis frequency responses.

(click on picture to zoom)

Left: On-axis, 15 and 22.5deg off-axis frequency response @ 2m

Right: 30, 45 and 60deg off-axis frequency response @ 2m

 

Baffle Step Compensation (BSC):

The following near-field frequency measurements are made at a 0.25m distance at woofer-axis. The measurements are valid down to 140Hz.

(click on picture to zoom)

Blue:  Mid-woofer frequency response without cross-over.

Red:  Mid-woofer frequency response with cross-over.

This near-field frequency response plot shows the mid-woofer cross-over effect on the loudspeaker baffle step.

Distortion measurements:

 

Measurement setup:

  • Tweeter-axis near-field measurement at 20cm
  • Frequency Range: 200-10000Hz
  • Baffle size WxH: 20,3×35,6cm

The distortion measurements are done in near-field and the amplifier output level was adjusted so that the fundamental is 90dB and 95dB at 1m. This setting simulates medium-high and high listening levels.


(click on picture to zoom)

Upper left: Left loudspeaker, 90dB @ 1m
Upper right: Right loudspeaker, 90dB @ 1m
Lower left: Left loudspeaker, 95dB @ 1m
Lower right: Right loudspeaker, 95dB @ 1m

This is a very low distorting loudspeaker!

Component list:

 

This loudspeaker costs about 300 US $ to build (drivers + filter components), excluding cabinets.

Summary:

 

Kensho Two – M3 is a low distortion low cost loudspeaker with premium performance. As an entry level DIY loudspeaker it can compete with far more expensive loudspeakers on the market.

The SB Acoustics 5” mid-woofer is a low distorting and bass capable mid-woofer with a nice mid-range presentation. In this price range, it’s as good as it gets and the SB Acoustics SB15NRX coated paper cone mid-woofer is among the very best 5” woofer available, especially considering its moderate price tag.

At this price range, it’s not much complain about this loudspeaker. The bass is fast and deep and is perceived as larger than it actually is. The tweeter has an extended frequency response with high resolution.

The Kensho Two – M3 has a generous sound stage with a nice width, height and depth.

Recommended loudspeaker placement:

  •         No or little toe-in.
  •         >0.5 meters from back wall.
  •         3 meters apart.
  •         2.5-3 meters listening distance

The loudspeaker is optimized for a listening distance of >2.5m, but everything between 2-3.5m works fine. As always it pays off to test different loudspeaker placements and see what fits the room and personal preferences the best.

Technical specification:

 

Kensho Two – M3
Type: Two-way small sized stand-mount loudspeaker
Enclosure type: Bass-reflex
Woofer driver unit: 5” coated Papyrus fiber cone. SB Acostics SB15NRXC30-4
Tweeter driver unit: 1″ coated textile dome tweeter. SB Acostics SB26STAC-C000-4
Cross-over frequency: 2.1kHz.
Cross-over function: LR fourth-order (acoustically)
Frequency response: 37-25000Hz
Sensitivity: 86db 1m/2.83v, on-axis @ 1kHz
Nominal impedance: 4 Ohms
Minimum impedance: 3.28 Ohms
Enclosure volume: 11 liter
Weight: 10,0kg
Enclosure dimensions: (height x width x depth): 35,6 x 20,3 x 28,8cm

 

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Thank you!

/Göran

Author of the “AudioExcite Loudspeaker Design” website