S20-Z Compact Open Baffle using Zaph Kit

This design was announced as winner of Zaph Design Contest. The design had gone through some changes but you can read the original submission.

Follow the build discussions at HTGuide Forum.

Compact and economical Open Baffle design, similar dimensions to a typical 5" hifi floorstander!

While open baffle loudspeakers provide the outmost transparency and openness due to absence of box, they are normally very large and expensive to build. Zaph had done the hard work of providing an excellent 2-way loudspeaker kit, and we simply take this kit as a basis for this design.

The loudspeakers need additional woofers to reach 20hz, and any 10” with decent xmax will do. Extremely large subwoofers, in fact, are not advisable due to size constraints. I use Jaycar 10” woofers bought for about $15 each.

Design : 3-way Open Baffle using Hybrid crossovers
Frequency Response : 20Hz – 20kHz
Polar response : Dipole 20hz to 1.8khz, then forward directional
Crossovers : 1.8khz M-T, 220hz W-M (Asymmetric). With dipole correction
Placement : 80cm minimum from front walls, 50cm from sides
Size (H x W x D): 97cm x  23cm x 27cm
Tweeters: Vifa DQ25SC16-04
Midbass: Zaph|Audio ZA14W08
Woofers: 4x 10"

The loudspeakers require 4 channels of amplification and use hybrid design to achive this:

Midrange-Tweeter section uses ZA5.2TM kit’s crossover, passing the signal for dipole eq first. The Dipole EQ is quite simple: a notch filter and lowpass-shelving filter

Bill of Material
Zaph ZA5.2TM Kit (Madisound), pair $200
10” woofers, 4x $80 - $100
XO / dipole eq options:
  - MiniDSP $100, or  
  - DIY op-amp based $30-$40  

MiniDSP Crossover Settings

Woofer Section:

Image - 220Hz LR4 Crossover
Image - 330Hz/-16dB/2.8Q quarterwave resonance notch filter (measured)
Image - 20Hz/0.7Q Linwitz Transform biquad correction (measured)
Image - 6db/oct Dipole rolloff EQ

The above Linkwitz Transform is for Jaycar CW2139. It will be different for other woofers, depending on the original Fs and Qts of the driver.

A very good value Woofer is Dayton Audio SD270A-88. Low Fs, good xmax, good price and reliably sourced from PartsExpress. If that is the chosen woofer, the Linkwitz Transform Biquad will be as follows:

Using Dayton SD270A-88 Sub
(Credit to SVEN from DIYAudio for providing the measurements):
Image - Linkwitz Transform for Dayton SD270A-88.
Image - Dayton SD270A-88 subwoofer FR modelling

Note: If you have other woofers in mind, please provide me with .frd nearfield measurement on the cabinet mouth and I will gladly publish the LT biquad parameters.

Midbass & Tweeter Section

Image - 120Hz LR4 Asymmetrical crossover
Image - 650Hz/-4.2dB/1.4Q Notch for dipole peak EQ (measured)
Image - 6db/oct Shelving-Lowpass dipole rolloff EQ
Image - Overall response of the panel. Note the dip at 3.8khz which needs to be investigated

Most critical of these settings is the 650Hz dipole notch. It is directly related to the front-rear separation and hence the baffle width. The final loudspeaker build had a baffle width of exactly 21.5cm. If your baffle is wider, the notch will be moved to lower frequency. The notch frequency can only be reliably measured outdoor/anaechoic conditions.

The crossover is 120hz asymmetrical to compensate floor reflections. This puts large demand on the 5" midbass. I play loud enough to fill in a 4.5m x 8m living room with no issues. But for extreme SPL, you may want to change it to 220Hz.

A null is observed at 3.8khz which deserve further investigation.

Other Settings

Image - 2π to 4π radiation space transition
Image - delay and attenuation

As with any other loudspeaker design, the top half of S20Z fires to full space (4π), while the bottom half fires to half-space (2π). This needs to be compensated accordingly using shelving highpass filter.

The combination of CW2139 and Zaph kit does not need further attenuation. With SD270A-88 above it might need minor tuning.

10/7/12 - Drivers shipped and on their way to down under !
17/7/12 - Drivers arrived.
29/7/12 - First dipole sub built and measured.
12/8/12 - Woodworking completed. The final design is actually smaller than planned.

15/8/12 - Preliminary crossover system and listening impressions observed.
14/9/12 - Outdoor measurements taken.
19/10/12 - Crossover configuration and measurements posted
9/7/13 - Other narrow dipole sub possibilities: Z Frame, Z Frame2, W-Frame, M-Frame
10/7/13 - wmcarpenter's adaption using Zaph MTM kit and 4x Bipole woofers
12/7/13 - danvprod's build progress using Markaudio fullrange driver