3-way front stage active with rear passive and 15 inch subwoofer in a Volkswagen Polo

boot design.jpg

I was approached to help in building a setup that could be used for both SQ as well as a a decent SPL in a Volkswagon Polo.
The owner had quite a bit of equipment of which the most challenging was the 15 inch Digital Design subwoofer.
The car being a hatchback, it was tricky to design one to satisfy both the needs of listening which oscillated from pure sound quality to a bit of head banger as well.

Amplifiers - 3: Audison LRX (4-channel amp), Hertz HDP4 (4-channel amp) and a Hertz HDP1 (mono).
Speakers: Frontstage (Fully Active) - Audible Physics RG50 tweeters, Audible Physics NZ (3.5 inch) wideband and Audible Physics Brahma (6.5 inch) midbass
Rear Fills (Passive) - Hertx Mille components
Subwoofer: Digital Designs 15 inch (4 ohm single or as 2 ohm dual voice coil)
Head Unit: Pioneer 5000 series
Digital Source: Allo Digione
Processor: Helix DSP mkii pro
Cables: Belden coaxial, Connectix interconnects and Belden Speaker Cables

SUBWOOFER
The biggest challenge in this setup was to design the subwoofer due to limitation of space.
I did my Maths and decided on a 3.2 to 4 cubic feet 2nd order bandpass enclosure.
As it was the persons daily ride, and mostly within city, he was open to give away the wheel well for the fiberglass/wood fabrication.

I will let the pictures speak here.

sub well.jpeg

sub 1.jpeg

sub 2.jpeg

sub finish.jpeg

Boot Cover made with thick MDF and two vents to let the bass into the cabin

boot cover.JPG

This way of designing the subwoofer had many advantageous with respect to this car.
1. i could get good depth in the enclosure and get both the bandpass accurate.
2. The 1st bandpass located at the bottom, trapped the back waves and directed it out via the 2nd bandpass which was facing up and at the farthest end from the sub.
3. As the base of the sub was completely locked into the wheel well (which was completed damped on all sides), there was no rolling or vibration even at the highest SLP levels.
4. It left enough space to mount all the three amplifiers in the boot along with the DSP and the digione. All I had to do was run one RCA cable from the Pio head unit to the DSP. I could keep rest of all the signal cables short, except ofcourse the speaker cables.

Front Tweeters and Wideband:
This was another big challenge due to the shape of the dashboard, limited depth, airbags, puny A Panel and external air vents close to the A Panel in the dashboard.
For the wideband to play its capable bandwidth from as low as 250hz, I needed minimum 1 to 1.3 liter enclosure.
I managed to fabricate this by cutting out a portion of the A Panel cover and utilizing the entire cavity of the A Panel. Then the wideband and tweeters were properly angled to keep them as away from the windscreen and side windows as possible and still minimize it from getting off axis to the driver's seat. The wideband compensated for stage height and the tweeters for width in the design.
The A Panel at he bottom had a little vent for routing cables etc, this I left deliberately open to reduce encosure resonance. The metal face of the A Panel was covered with a 8mm layer of rock wool pad to prevent reflection from the metal surface.

The pics:

A Pane; 1.jpeg

left A Pane;.jpeg

right A Panel (1).jpeg

Rear Tweeter fabrication:
As the rears were only fills to be used when there were rear passengers, I fabricated the tweeters as close to the midbass on the doors.

rear tweeter.JPG

The Helix DSP pro is a marvel, a super SQ beast with tons of active tuning features

I finally settled for
Wideband: High passed at 289 hz (at 12db). (No low pass
Tweeters: High passed at 12600 hz
Midbass: Bandpassed between 80 hz (18db) and 289 hz (12db)
Subwoofer: Low passed at 80 (18db).
Db levels set individually for complete blending of the all the drivers.
Time aligned for proper imaging and staging.

Totally phase aligned, time aligned and gain matched with ears using a collection of tuning tracks and later fined tuned with familiar songs.

The Director was installed under the HU.
Three preset options:
One pure SQ setting with only frontsatge, one with the rear also playing and one full SPL setup.

Impressions:
The depth is almost a foot outside the windscreen, superb height and width as well. No digital grain or fatigue. Expecting further refinement in another 100 to 150 hours of break-in.
The output from digione has better fidelity than the output taken from Pioneer.