System Design with Zen POWER9
Entire books can be written - and have been - on sound-system design. We won’t try to cover everything, but here are some basic rules of car stereo system design:
Speakers
Front speakers affect sound quality more than rear speakers. Use your budget accordingly.
The lower the note, the more air has to move to play it. For this reason, subwoofers are an essential part of high-fidelity systems - not just loud systems!
OEM systems are limited in the design stage - they can't have a big heat sink, they can't have a big power wire, and so they can't use much current or dissipate much waste heat. This means they never, ever, have enough power. Once a system designer knows there's not much power, there's no reason to use a subwoofer with a large voice coil, high power handling, and high excursion capability - so they don't. This is why, when installing a Zen POWER9 on OEM stock speakers, the subwoofer always comes up short. It's not designed for 500W RMS, and it's often on the small side - which means it has to move even farther to try to move enough air. Always upgrade the subwoofer - even if you don't upgrade the mids and highs. (This is not applicable to BMWs with underseat woofers, which don't operate on the 500W channel).
A “three-way” front speaker architecture - tweeters, midranges, and woofers - performs better than a “two-way” - tweeters and mid woofers.
The Ford F-150 uses a two-way front speaker set in the B&O 8-speaker system, and a three-way front speaker set in the Unleashed system (the mids are in the dash). A three-way gives you much better upper midrange performance - that's why Ford does it. For improved performance in the 8-speaker B&O vehicles, use an upgraded 2-way kit (more on this below). Some vehicle owners upgrade their 8-speaker trucks by adding midranges to the dash or to the A-pillars, making the front speaker system a 3-way, and running new wires from the Zen POWER9.
BMWs use a three-way, in that the front door midrange and tweeter are bolstered by the underseat woofer (many call it a subwoofer, but using it as a woofer up to 200 Hz provides much better results).
In the Ford Bronco, the 6.5" woofer and the 4" dash coaxial make a 3-way kit (although most coaxials don't allow for active tweeter arrangements with dedicated tweeter channels).
When using a two-way front speaker system, best results come from systems with tweeters that are mechanically capable of lower-frequency extension. The most common two-way speaker arrangement is a 6.5” mid woofer and a 1”-class tweeter. This often suffers from thin, cold-sounding upper midrange, frequencies which the door-mounted 6.5 cannot deliver effectively and which many tweeters struggle with as well. This problem is even worse with larger midwoofers, such as 6x8" and 6x9"-class drivers. Many Ford truck owners upgrade to a 6.5"-tweeter combo where the tweeter plays much lower. Two-way sets with a 6x9-class or 8"-class woofer and a 1” tweeter are not generally recommended due to limited upper-midrange performance.
This means the tweeter can help cover the upper midrange - which would be handled by a small midrange speaker in a three-way system. This is best performed in an active arrangement, where the tweeter gets its own dedicated channel and the crossover filtering is performed inside the Zen DSP (rather than using passive crossover filters that come from the speaker manufacturer). Active systems require dedicated tweeter channels and tweeter wiring.
This problem can also be solved in some vehicles with a 2”-class wideband speaker and a larger woofer(say, a 6x9”-class). This allows the widebander to handle the upper midrange. In these cases, OEM widebanders often don't play as high as aftermarket models.
The most common three-way set is a 6.5 woofer, a 3”-class or 4”-class midrange, and a 1”-class tweeter. In this instance, an 8” woofer can be used in place of the 6.5 as long as it fits in the desired location!
Tweeters don’t need nearly as much power as midranges or woofers. This is why tweeters can be powered off the ACM channels in Fords.
One-Seat and Two-Seat Systems
The off-center listening positions in a car are far more acute than when sitting on the sofa at home - and this causes problems.
A "one-seat system" is one where the car acoustics problems caused by our off-center seating positions are overcome - but only for the driver's seat. The other seats often sound worse than if nothing had been done at all. This doesn't matter when those seats are unoccupied.
Most sound-quality competition vehicles have one-seat systems.
Most premium OEM systems are two-seat systems - they use "upmixing" and some variation of a 7.1 multichannel presentation. The center speaker allows the system to be symmetrically tuned for multiple listeners. Often a dedicated set of speakers in the rear, sides, or ceiling handle ambiance sounds, to make the cabin feel like a larger performance space without muddling the important front stereo sound.
The biggest improvement from adding a Zen POWER9 is the improved power output compared to the OEM amplifier - but often, our DSP tuning can be better as well. Zen POWER9 has advanced center-channel and ambiance-channel algorithms, which can be used to deliver similar imaging to any seat.
One-Seat Stereo System Pointers
In a one-seat stereo system, your front left and right speakers are the most important for sound quality. In some cases, front speakers are upgraded to higher-performance aftermarket speakers, while rear speakers are left OEM.
When possible, run your front left and right tweeters in an active arrangement. This allows more precision in the crossover filtering used, and more tunability. However, this often requires new wires to be run.
Rear speakers contribute to overall output, but their biggest effect can be to damage the imaging in the listening position. Spending a lot on rear speakers rarely makes a large audible impact.
With many one-seat systems, great results can be achieved with distance-driven delays (that is, using delays calculated by the Zen DSP after you enter in the distances to each speaker) and no phase manipulation (using all-pass phase-inverting filters).
Upmixed Two-Seat System Pointers
In a upmixed two-seat system, the center speaker is the most important speaker for sound quality. Don’t upgrade the left/right speakers and leave the center stock!
When upgrading to aftermarket speakers, use a center speaker at least as good as your left and right speakers, if not even better. Many vehicles have small center speakers. If the center speaker size can be upgraded, that often helps the overall system’s performance.
Whenever possible, run your center tweeter in an active arrangement. It’s usually not very difficult to route a new wire to the center to do this. Running the left and right front tweeters actively is not nearly as important in an active arrangement.
We don’t use delay in upmixed systems, or at least we do not use distance-driven, left/right asymmetrical delay. While OEM systems often deliver symmetrical sound, they often fail to deliver symmetrical bass-up-front, so the Zen tuning tools help achieve better sound than OEMs deliver.The Zen DSP section contains DSP functions to deliver midbass and bass-up-front to all listeners, as well. To get good midbass and good imaging, we often need to use all-pass filters.
Rear surround speakers are even less important to upgrade than rear stereo speakers. They can be left OEM as long as the OEM speakers are in good working order (some rear-deck OEM speakers deteriorate quickly due to UV exposure).
Subwoofer Pointers
Humans cannot determine the directional source of pure sub-bass tones (below 80 Hz). That means that when the front mid-bass speakers perform well, and are aligned in phase with the subwoofer speaker, we get the illusion of “bass up front”.
However, in vehicles, subwoofers rarely produce only pure sub-bass tones.
Sounds that are not pure sub-bass tones:
Rattles (vehicle trim, or enclosure)
Driver distortion (originating in the speaker motor)
Port whistling (caused by too small a port cross-section)
Mid-bass output peaks (caused by too small an enclosure for the driver)
So for best results, eliminating trim rattles is important, as is using a subwoofer with a quality motor design which does not add distortion to the sound.
Subwoofer Location
Speaker locations in vehicles are always subject to cancellations from reflections. We cannot eliminate them, so it is best to work around them. One way to work around them is to ensure the cancellation is outside the speaker’s range of output.
Sometimes a subwoofer location that is convenient also suffers from a bad cancellation. Another way to address this is to use a subwoofer location which is less convenient, but does not share the same drawback. This is why front subwoofers are sometimes part of modern car audio systems.
Sometimes both front and rear subwoofer locations are used. Often, these two subwoofers have cancellations at different frequencies, and using them together delivers better overall performance (once the user has phase-aligned all the speakers, of course).
In some cases, better performance can be achieved by using a rear subwoofer with low distortion, eliminating trim rattles, and phase-aligning the sub with the front speakers.