f JdM12 Audio Project 2018

The Magical Number Seven, Plus Two

This DIY loudspeaker system explores the use of multiple small wide-range drivers in a multiway configuration. The goal is to combine controlled directivity with coherence and power handling. A further objective is to improve timing accuracy through phase control using FIR filters. This version is an evolution of my 2018 system, featuring updated filtering and the addition of lateral "cheeks" to the speaker columns.

   

wideband

The main speakers are based on seven E.J.Jordan JXR6 HD reused from a previous linear array system.
Arranged as closely as possible, they form three groups sharing a common acoustic center:

The system can be viewed as a virtual coaxial driver.

   

The 20-liter sealed enclosures are made from 30 cm PVC pipes filled with polyurethane foam and polyester fleece. A tapered internal partition helps reduce standing waves, starting around 600 Hz. Construction is deliberately simple: a PVC KGU sleeve (with rubber seals) and two KGM end plugs, assembled without glue. The baffle is covered with a 5 mm felt layer to reduce diffraction and ensure flush driver mounting.

Measurements were performed using a logarithmic sweep from 16 Hz to 24 kHz (duration: 21.8 s).
The magnitude response remains within ±3 dB from 200 Hz to 20 kHz.
The increase in group delay below 250 Hz reflects the low-frequency roll-off of the system.


Average measure of the six groups of small drivers at 50 cm


Group delay (blue), minimum group delay (gray), and excess group delay (black)

Subwoofer

Low-frequency extension is provided by two 20 cm SB Acoustics SW26DAC76-4 per channel. Thanks to their dual aluminum cone structure and moderate moving mass, these lightweight neodymium drivers can be used in relatively compact sealed enclosures.
The 21-liter enclosures follow the same construction principle as the main speakers. Internal standing waves start around 600 Hz, well away from the intended operating range. Design parameters: Qtc = 0.86, F3 = 35 Hz.
The subwoofers are positioned above and below the main array to align acoustic centers. Given here their separation of 70 cm, they become directional from 165 Hz.
The response is smooth from 40 to 500 Hz.

 
Average measure of the four subwoofers at 5 cm

 
Group delay (blue), minimum group delay (gray), and excess group delay (black)

Operating

Digital to analog conversion tasks an 8-channel miniDSP U-DAC8 at its best with iFi Audio iPower and iGalvanic3.0.
A Behringer Ultralink Pro MX882 V2 splits the stereo signal into the dual subwoofers.
Power is supplied by two Rotel RMB-1565 5-channel Class D amplifiers, one for each stereo channel.

Instead of using dedicated DSP hardware, all signal processing is performed on a computer. This approach allows the use of long FIR filters for precise phase control, which would be difficult to achieve with conventional DSP units.
This tutorial may help configuring the PC: Windows PC as a FIR Audio Processor.

The system is implemented using Equalizer APO with FIR filters generated by rePhase.
Operating at 48 kHz — miniDSP U-DAC8's native resolution — prevents unnecessary sample rate conversion.
With the chosen settings, frequency resolution is 1.46 Hz up to 24 kHz, latency is 341 ms.

Equalization

Since the drivers operate in sealed enclosures, their 12 dB/octave low-frequency roll-off and phase shift must be compensated. The wideband drivers are equalized using biquads in Equalizer APO, with phase linearization built into the FIR filters. Subwoofer equalization is directly integrated into its FIR low-pass filter.
You may copy the settings to clipboard and load them in rePhase.


Equalization for wideband with (red) and without (blue) phase linearization


Equalization for subwoofers with (red) and without (blue) high-pass filter

Crossover

The loudspeaker already provides a highly satisfying presentation with conventional crossover alignments, namely Butterworth and Linkwitz–Riley filters in the following sequence: But24–But18 at 160 Hz, LR24–LR24 at 640 Hz, and LR24–LR24 at 2560 Hz.
However, despite the additional design complexity and tuning effort involved, the Horbach–Keele implementation delivered the most coherent, stable, and convincing overall presentation among all crossover topologies evaluated, even though the physical layout does not fully comply with the theoretical assumptions of the method.
Horbach–Keele filters are based on the 2007 paper:
Application of Digital Crossover Filters to Pair-Wise Symmetric Multi-Way Loudspeakers.

A key characteristic of this approach is that, at any given frequency, only one or two ways contribute to the output.
With the critical frequencies aligned to the musical notes la, A1 - A3 - A5, the spacing ratio is 4:1 between adjacent bands, resulting in crossover frequencies of 158 Hz, 632 Hz, and 2654 Hz.
Careful window selection is essential. With the Albrecht formulation, increasing the number of terms from 2 to 4 progressively crushes out-of-band energy, mitigating driver intermodulation and off-axis interference.
Scaling tap counts alongside window complexity preserves temporal window length, keeping transition steepness uniform across frequency bands.

To ensure consistent spatial radiation and optimal acoustic summation, the exact same window function and tap counts are applied to both the low-pass and high-pass filters flanking each crossover point:

To implement symmetrical windowing at each crossover point, the signal processing requires two distinct filtering stages — high-pass and low-pass — for the low-frequency and midrange channels. These can either be cascaded directly within Equalizer APO or pre-combined into a single impulse response using a Python script. Because varying filter lengths introduce different group delays, precise time-alignment offsets must be applied across channels to preserve phase coherence. See the Equalizer APO configuration file config.txt.


rePhase settings for crossovers.


Average measures at 50 cm, 1/12 octave smoothing.

Results

For the measures, I used John Mulcahy's REW analysis software.
After equalization, phase responses are well aligned across all ways, and the usable bandwidth extends from approximately 25 Hz to 16 kHz.


Average measures of the left and right loudspeakers at 50 cm.

Conclusion

The lightweight aluminum drivers provide a clear and detailed presentation. Their use in this closely integrated configuration results in a highly coherent full-range response. Balanced directivity ensures consistent tonal character over a wide listening area, contributing to a natural and engaging listening experience. After several years of use, this latest evolution of the system confirms the benefits of the 7+2 driver design.
As a possible limitation, the cylindrical enclosures may not be acoustically optimal; more refined baffle geometries could further improve performance. Nevertheless, the advantages of the concept remain clear, and the approach may be of interest to experimental loudspeaker designers.

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