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.

Wide range

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 central driver (effective diameter 5.6 cm) operates as a tweeter.
The upper and lower drivers, wired in series (equivalent diameter 7.9 cm), reproduce the midrange.
The four side drivers, arranged as two series pairs in parallel (equivalent diameter 11.2 cm), handle the low frequencies.

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)

Subwoofers

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. They are positioned above and below the main array to align acoustic centers.
Design parameters: Qtc = 0.86, F₃ = 35 Hz. Internal standing waves start around 600 Hz, well away from the intended operating range.
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. The E-APO configuration file looks like this:

Device: USBStreamer - USBStreamer Multi-channels # Headroom Preamp: -3 dB # Channels (7.1) # 1 2 3 4 5 6 7 8 # L R C LFE RL RR SL SR # 3.1 source # Copy: 1=L+0.5*LFE 2=R+0.5*LFE 3=L+0.5*C 4=R+0.5*C 5=L+0.5*C 6=R+0.5*C 7=L+0.5*C 8=R+0.5*C # Stereo source Copy: 1=L 2=R 3=L 4=R 5=L 6=R 7=L 8=R # Convolution Channel: 1 2 Convolution: IR_HP-sub.wav Convolution: IR_LP-sub.wav Channel: 3 4 Convolution: IR_HP-low.wav Convolution: IR_LP-low.wav Channel: 5 6 Convolution: IR_HP-mid.wav Convolution: IR_LP-mid.wav Channel: 7 8 Convolution: IR_HP-high.wav Convolution: IR_LP-high.wav # Levels Eval: mainLevel=0 Channel: 1 2 Preamp: `mainLevel` dB Channel: 3 4 Preamp: `mainLevel-9` dB Channel: 5 6 Preamp: `mainLevel-3` dB Channel: 7 8 Preamp: `mainLevel-3` dB

With the chosen settings, frequency resolution is 0.73 Hz up to 48 kHz, latency is 682 ms.

Equalization

Since the drivers operate in sealed enclosures, their inherent 12 dB/octave low-frequency roll-off and associated phase rotation are corrected using inverted second-order high-pass filters (rePhase compensate mode).
You may copy the settings to clipboard and load them in rePhase.

rePhase settings for main frequencies, with (red) and without (blue) high-pass filters

rePhase settings for subwoofers, with (red) and without (blue) high-pass filters

Crossover

Horbach–Keele filters are used following their 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.
Although the physical layout does not fully comply with the theoretical assumptions of the method, Horbach–Keele filters consistently provided the most stable and coherent results in listening tests.

Given here their separation of 70 cm, the subwoofers become directional from 165 Hz. With critical frequencies set to the musical notes la A1-A3-A5, spacing ratios are 4 and crossover frequencies are 158, 632 and 2654 Hz. The wavelength of the critical frequency of the midrange (1760 Hz) provides a vertical beamwidth of about 50°.

With Horbach–Keele filters, careful window selection is essential. Each window is chosen based on time-domain behavior and perceptual impact within its frequency range. The same window is applied to both sides of each crossover to ensure consistent spatial radiation and proper inter-way summation:

Hann is used between the midrange and the high-frequency sections, reducing FIR-related pre-ringing and edge discontinuities while trading off transient sharpness for a smoother, more natural sound.
Between the low-frequency and midrange sections, Lanczos is selected for its inherently smooth time-domain tapering and natural transition behavior.
A 5-term Albrecht formulation shapes the crossover between the sub-bass and low-frequency sections, effectively controlling sidelobes and limiting spectral leakage.

To implement symmetrical windowing at each crossover, processing is split into two successive convolutions:

a first stage handling equalization and high-pass filters
a second stage dedicated to low-pass filters

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.

2018 - 2026