S15 - Econowave DSP

The Original S15 Econowave DSP 
Eminence Beta 12a + Selenium D220Ti & Pyle WG 
(C1-W1-CD1)

Econowave is a cost effective controlled-directivity loudspeaker design. DIY folks at AudioKarma had investigated numerous driver + waveguide combination which would achieve the optimum polar response using off-the-shelf products.

Note: You could get lost in the thousands of posts in AudioKarma. But here's my refence links which summarise the variations of the designs: AudioKarma and  TechTalk

For this DSP build, follow my post in AudioKarma and DIYAudio forums.

Specifications
Topology: Active 2-way with pro-woofers and compression drivers
Radiation pattern: Monopole - Controlled Horizontal Directivity
Directivity: Omni-directional at lower  frequencies, smooth transition to 90 deg beam at 1.2khz
Frequency response: 20hz - 17khz. Critically damped LF Q of 0.5
Sensititivity: ~92dB
Dimension (H, W, D): 57 x 37 x 33cm
Vertical lobe (Listening position) : Tweeter axis, toe'd in 5-10deg
Amplifiers: 4-Channels required (2x stereo amps)
Crossover: DSP (MiniDSP + 2way Advanced Plugin)
 

Cabinet construction

The cabinet is basically made from 33cm, 18mm MDF cutouts. The offcuts are glued into the inside panels to provide additional strength and rigidity. The finished speakers are very heavy.

Cabinet C1

 Cabinet C2
(future design ...)

A handsaw was used and could not provide precise cuts. Hence the  construction can be made square and flush effectively by ofsetting 0.5mm and sanding.

Stuffing

The primary reason for stuffing is to dampen modes inside the box structure. With so much variations on stuffing material (dacron, acousta-stuff, natural wool fibers, etc.) how much stuffing requires some experimentations. I use dacron (those pillow-stuffing) and starts about this much, very loosely filled. After some listening I removed some. The goal is to have enough to reduce the modes, but not too much that it overdamps and reduce the box volume. It is a competing factor.

DSP Crossover Configurations

The DSP configuration is available below.

## WARNING #
I configured the speakers using an older version of MiniDSP plugin which had a bug in it
https://www.minidsp.com/forum/software-support/10857-minidsp-4x10-hd-slp-q-biquad-errors
The effect is that if you use the newer plugin (which is the fixed one) the bass will sound heavy. So please be aware of it. I will revisit these in the future with the updated plugin.

Important: do not forget to implement  L-PAD for the tweeters.

Woofer DSP crossover  

Woofer W1: Eminence Beta 12

Cabinet C1 

• XO:  1250hz, LR2
  Crossover point is chosen by careful consideration of distortion, group delay, power handling, and polar response. See my investigation here on dispersion pattern of various xo points.
• EQ1:  Notch Filter
  PEAK 618hz / -2.8db / 2.6Q
• EQ2:  Linkwitz Transform Biquad (LT)
  LT: Select "Advanced", Copy and paste below, and click "Process".
  

  biquad1,
  b0=1.00329685429188,
  b1=-1.99470503770825,
  b2=0.991546613279961,
  a1=1.99477083461872,
  a2=-0.99477767066137

  The woofer is equalised to F3=20hz with critically-damped Q of 0.5. This means -3db@40hz and -6db@20hz. Please note that these values are driver and cabinet volume-dependent. If you change the cabinet volume drastically or change the Woofer, then you need to recalculate and remeasure the transform. No you can't just punch in factory-provided TS parameters.
  
  Read more about LT Biquad here and here, and also a calculator.
  
  Raw woofer measurement (nearfield)
  LT Biquad (simulation)
  Woofer with LT, Notch, and XO (measured nearfield)
  Green=raw, Black=Equalised woofer, Red=Target frequency response
• EQ3:  Baffle step compensation
  HIGH-SHELF 206hz / -6db / 1Q
  Baffle step consist of shelving highpass with -6db/oct gain. It compensate the transition from 4pi (omni-directional) to 2pi (forward radiating) of box speakers. Read about baffle step here.
  Woofer response with Baffle Step (measured, nearfield)
  Woofer response from 1m
  Black=with baffle step
• Ignore values of EQ4.. etc. 
• Attenuation, Phase, and Delay
  With CD1: Attenuation 0, Polarity NORMAL, Delay: 0ms
  With CD2: Attenuation 0, Polarity NORMAL, Delay: 0.06ms

Tweeter DSP crossover

CD1 (Selenium D220Ti)

Compression driver: Selenium D220Ti screw-on

Waveguide:  JBL 1" thread-on Progressive Transition, Part# 338800-001, or Pyle Pro PH612, or Dayton clone. In Australia: DJCity YD-L033
Directivity pattern measured from 0-90deg, 10deg increments:

• XO:  1250hz, LR2
• EQ1:  Waveguide loading notch
  PEAK 1956hz / -4.8db / 2.6Q
  Raw and equalised Compression Driver + Waveguide (from 80cm)
  Green=raw, black=equalised.
• Passive L-PAD
  Passive L-PAD is used at the compression driver to provide main attenuation for woofer level matching. The benefit is that noise from Amplifier will also be reduced. The remaining few dB is adjusted from MiniDSP.
  L-PAD:  -11db using 5.6 Ω  and 3.3 Ω 5w resistors
  
• Attenuation, Phase, and Delay
  With W1: Attenuation -5db, Polarity INVERT, Delay: 0.04ms
  Measurement from 1m at Tweeter level
  Ignore lower frequencies.

I have a quick iphone capture of these loudspeakers in youtube:
http://www.youtube.com/watch?v=SwSK8dmeqgU

CD2 (B&C DE250)

Compression driver: B&C DE250 + DJCity adapter (photo)
The adapter needs to be enlarged to 1 inch, so better ones may be found from PartsExpress.
Waveguide:  JBL 1" thread-on Progressive Transition, Part# 338800-001, or Pyle Pro PH612, or Dayton clone. In Australia: DJCity YD-L033
Directivity pattern measured from 0-90deg, 10deg increments:

• XO:  1250hz, LR2
• EQ1:  Waveguide loading notch1
  PEAK 3981hz / -4.1db / 1.2Q 
  Raw and eq notch 1 (measured  1m)
•  EQ2:  Waveguide loading notch2
  PEAK 1269hz / -3db / 1.5Q 
  Fully eq'd Tweeter (measured 1m)
• Passive L-PAD
  Follow the passive -11db LPAD configuration as CD1 above.
• Attenuation, Phase, and Delay
  With W1: Attenuation -5db, Polarity INVERT, Delay: 0ms (measured 1m)
   

CD3 (Celestion CDX1-1745) 

Compression driver: Celestion CDX1-1745 (photo) + PE Adapter

Waveguide: As per CD1 and CD2 (Pyle) 

I configured and listened briefly this compression driver at recent GTG and was pleasantly surprised as how similiar the frequency characteristic and sonics is compared to B&C DE250.

A longer-term experience and listening may be needed to confirm, but there were three of us who felt the same during that brief setup. We even set it up playing both drivers together left and right.
(update: this has been confirmed by another guy that CDX1-1745 is a poor man's DE250!)

Please follow the exact configuration of CD2 (DE250) for this compression driver.

Optional variations

35hz Linkwitz Transform
If the woofers (W1) are equalised to 35hz, Q=0.5 (instead of 20hz above), then further +5db can be obtained and that raises the loudspeaker's sensitivity to ~97dB. This also relieves the strain of the woofers when playing loud. In order to have this:

• W1 EQ2 biquad needs to be changed to 35hz, Critically damped Q=0.5
• W1 EQ3 Baffle Step SHP needs to be changed to Shelving Lowpass
• W1 EQ4 biquad of -1db is added.

Crossover
Second-order Linkwitz-Riley crossover topology (LR2) was chosen for group delay, however you might want to play the loudspeakers at very high SPL continously. In this case a 4th order crossover providing 24db/oct slopes may be a better approach. This can be done by selecting Woofer and Tweeter crossover to LR 24db/oct and changing the Tweeter's Polarity to non-inverting.

Using Subwoofer

Subwoofers can greatly relieve the stress to the pro-drivers which has highish Fs. I would recommend at least a pair to minimise excitation of room modes. The optimum scenario is to cross with LR2 slope (Q=0.5) and this can easily be done by modifying EQ2 (LT) biquad values. For example, if you would like to cross with the Subs at 80hz the biquad values for the LT would be like this.

"Geddes" Foam Plug

Earl Geddess popularised and patented the 30ppi foam plug approach on his GedLee line speakers. Many users reported improvements in the sound quality. I also  tried it and posted my experience which was different. The foam will attenuate the high frequency. So you need to increase the tweeter level by 2dB. 

In Australia you can buy them from FlexFoam. Ask for "30ppi Mericell foam". They are also referred to as filter or reticulated foam.

Loudspeaker Toe-In

There is a view that these speakers are best listened at extreme toed-in angle, as much as 45degrees. It does seem to provide uniform coverage across listeners but with soundstage depth compromise. I find about 5-10deg off the normal triangle is satisfying.

Room response analysis

Elias Perkonen kindly performed wavelet analysis of the in-room response of these speakers.

Native frequency response, left and right:








Longer time windows, 150ms and 300ms:









Update 11/2/2011
Zilch (Evan) who is the champion of this design passed away. He will be sorely missed.
http://audiokarma.org/forums/showthread.php?t=350053
http://www.youtube.com/watch?v=WvNH23XfeVI

Update 3/2/12
Production version of original S15 documented and  posted.

Update 4/2/12
B&C DE250 variant posted
Celestion  CDX1-1745 variant posted

Update 14/9/12
Added information on stuffing

Update 16/9/15
Check out these youtube videos :)
https://www.youtube.com/watch?v=SwSK8dmeqgU
https://www.youtube.com/watch?v=dvTsjRXJgZQ

Update 19/6/16
Biquad values will be different for newer 96khz boards such as 4x10HD or 2x4HD. Use these two biquads instead. The first biquad is to shape the original response to 35hz with Q=0.7, and the second one is 35hz Q=0.7. Together they produce acoustic shape of 35hz LR4 highpass.

Try to remove the baffle step compensation first.  I did not need it in my room.

biquad1,
b0=1.00156217438683,
b1=-1.99720737914148,
b2=0.995679854523304,
a1=1.99722277903866,
a2=-0.997226629012953

biquad1,
b0=0.998612353349429,
b1=-1.99722470669886,
b2=0.998612353349429,
a1=1.99722278171542,
a2=-0.997226631682297