The Three Problems of Car Stereo, and How to Address Them

In teaching automotive sound tuning to aftermarket technicians, I use a concept called The Three Problems of Car Stereo. All three of these problems are much more common in car stereo than in home audio, and are caused by our predictably off-center seating positions in the car.

PROBLEM ONE: One side is louder.

Of course, that’s usually the near side, due to its proximity. This amplitude mismatch pulls the center image to the near side. Many of us have used the balance control in a rental car, in an attempt to improve the imaging. If level was the only problem, this strategy would have worked - unfortunately, we have two more problems.

PROBLEM TWO: One side sounds different.

Due to the off-center seating position, the left and right speakers may have very different angles relative to the driver’s listening position. As we know, a speaker driver can have different frequency-response characteristics at different angles of dispersion. These frequency-response differences between left and right cause the center image to seem to float left and right, as the frequencies of the content go up and down in register - and so the image floats toward the louder speaker at a given frequency.

The severity of this problem varies with the speaker system used.

For years, the most common front speaker system in vehicles was the 6.5" (165mm) woofer/small tweeter 2-way combination, but this system suffers from extremely compromised off-axis performance at the top end of the woofer’s range when the woofer is installed in the typical location (the bottom of the front door). The dispersion patters are also different when seated off-center, since the angle to the near speaker is far more severe than the angle to the far speaker.

A driver complement which works just fine in mini-monitor speaker cabinets aimed at the listener loses its luster when installed in the bottom of a car door, nearly 90 degrees off-axis on the near side! This upper-midrange dispersion weakness is exacerbated in many OEM systems, where the size of the tweeter is limited due to cost - thus limiting the low-frequency extension of the tweeter - and the resulting crossover point must be very high as a result. This unfortunately ensures that the 6.5” driver can’t possibly meet the tweeter at the crossover point, especially on the near side.

The use of 3-way front-stage speaker systems in some premium sound packages handle this off-axis performance problem more directly - the 6.5 can be restricted to its omnidirectional range, and the superior off-axis performance of the small midrange takes care of these upper midrange notes. This does extract a cost penalty, since two more speakers are required, as well as two more amplifier channels to power them.

Another way to address the Second Problem is the now-widespread use by OEMs of a wideband midrange/treble driver paired with a 6x9 woofer, with both wideband drivers in an off-axis orientation in the top of the instrument panel (as described by Shively, et al, many years ago, and now found in many Toyota, Hyundai, GM, and Stellantis vehicles). Many systems in this category use off-axis wideband speakers facing up in the dash (for similar left/right levels and response), and door woofers operating in their omnidirectional range (for similar left-right response in the bass).

PROBLEM THREE: One side arrives first.

The problem of multiple arrivals is explained in detail in another of my LinkedIn articles, here: The comb filter created by multiple arrivals of common-center information damages the frequency response for any content the recording engineer placed in the center of the stage. Since it is intended for the center of the stage, the content is contained in both the left and right channels, in phase, and at similar levels.

There are a few ways to address these problems, but which path we select depends on how many listening seats we care about - one, or more than one?

In how they address the Three Problems, OEM automotive audio systems fall into a few categories.

1. Stereo (unmodified). This was all we saw back in the 1980’s when I started, but it is increasingly rare today, since it doesn’t solve any of the Three Problems. Stereo is of course symmetrical by definition, even if the Three Problems are significant in either seat.
2. Stereo with a mono center speaker. Common in premium vehicles with entry- or mid-level audio package, mostly just to fill the dash grille present for premium systems in Type 5 below. Not a good way to achieve great sound - the comb filter is still present and the stereo image is problematic - but it is symmetrical.
3. Stereo, optimized for the driver’s listening position. Levels and arrival times are tweaked to overcome problems 1 and 3, but only for one seat (the performance in the other seat is worsened). Some OEM systems also use asymmetric left/right EQ to address problem #2. This is the most common high-performance system type sold by the aftermarket, and in those situations asymmetric EQ, level, and delay are all used regularly. There are very few vehicles sold with Driver-Optimized Stereo from the factory. Most OEM implementations which include driver-seat optimization make it a user-selectable setting in the tone-control menu (for example, Ford and Subaru have both recently offered this setting in specific vehicles).
4. Stereo with phase manipulation to partially address problem #3 to some degree, but for both front seats. A few years ago phase manipulation was solely used in premium branded systems, but over the past few years phase manipulation has appeared in base audio systems from Toyota, Hyundai, Kia, and General Motors (possibly due to an intellectual-property lockout expiration?). It should be noted that problems #1 and 2 (near-side amplitude and near-side off-axis response) cannot be solved for both seats unless a speaker system is used which can minimize the near-side asymmetry. For this reason, many premium OEM systems have been moving to the next category of presentation:
5. Upmixed systems with a center speaker, delivering a symmetrical presentation from either side of the vehicle. These are very common now with premium vehicles, with Dolby Pro Logic II, DTS Neural, Harman Logic 7 and QLS, Bose Centerpoint and Panaray, Fraunhofer Symphoria, and more all finding their way into vehicles from Cadillac and BMW, to Honda and Kia. While multichannel systems are often being labeled “immersive”, many implementations seem to include an upmixing algorithm for use with stereo sources (such as Bluetooth streamed from smartphones), and any height speakers are used in some application-specific manner (rather than playing object-oriented content).

Type 1 Stereo solves few of the acoustical problems in a vehicle cabin, and is an endangered species in the OEM market. The presentation is symmetrical, even if it is not optimized.

Type 2 Mono Center is more common than it used to be, and is also symmetrical without being optimized. The center is largely in the center, but so are sounds which should not be, and comb filtering is significant. Some OEMs are shipping pseudo-upmixed systems, with frequency-band-limiting filters for the center speaker (rather than using an actual upmixing algorithm). Some premium OEM systems in the past few years have been found playing 200 Hz and below in the doors, 2000 Hz and above in the left and right dash locations, and the 200-2000 Hz band is only played by the center midrange channel (which of course makes it mono - and terribly disappointing!). These often also delay the rears symmetrically.

Type 3 Delayed, Optimized for the Driver - is rarely found in the OEM market, presumably due to a preference to symmetry on the part of vehicle manufacturers (who are the customers, even if they are not the end audience for the sound system). It should be noted that the pattern of comb-filter cancellations in either seat, caused by the left and right speaker path-length differences, can be completely eliminated in one seat with the proper use of delay, and this explains its appeal in the aftermarket. The aftermarket is currently focused on asymmetric performance in most cases. In general, aftermarket clients are seeking a higher level of performance than the OEM automaker is capable of delivering within the constraints of weight, current, and cost. While it is increasingly difficult to change an OEM in-dash receiver out for aftermarket, many aftermarket receivers now have level, delay, and even asymmetric EQ for driver-seat optimization (although one suspects these tools are rarely used effectively). These often also delay the rears symmetrically.

Type 4 Stereo with Phase Manipulation - was initially only found in premium systems, but we have seen Toyota, Hyundai/Kia, and GM all offering base audio systems with all-pass filters in the output signal. It should be noted that in these base-audio applications, the comb filter pattern of cancellations in either front seat is only partially managed, but not completely eliminated. These often also delay the rears symmetrically.

Type 5 Multichannel Upmixed - These systems - whether or not they are also capable of playing multichannel recordings using technology such as Atmos - can deliver high-performance stereo performance to multiple seats. The center can be in the center (although it should be noted that it rarely is due to tuning decisions, apparently driven by preferences of the OEM automakers as customers).? The algorithms used are usually the strongest part of the system - again, constraints on weight, current, and cost limit the audio performance of upmixed systems just as relentlessly as they do any other type of OEM audio system. One commonly-found shortcoming is center speakers with poor performance and no high-frequency driver element (some center channels even eliminate the treble completely from the signal sent to the center-channel driver!) The aftermarket is learning to improve the performance of OEM systems while taking advantage of the OEM’s licensed upmixing technology, and we hope to see high-performance aftermarket upmixed solutions available soon (at present only low-performance algorithms, unprotected by IP, are available in the majority of aftermarket products offering upmixing).

While Dolby Atmos is slowly finding its way into the automotive space, every Atmos implementation includes an upmixing algorithm to ensure 2-channel content (from a USB drive, from the radio, or delivered over Bluetooth) can be delivered over the multichannel system. For this reason, Atmos can be categorized as a Type 5 system.

In conclusion, before any general tuning technique can be adopted for the automotive environment, the question of “one seat, or two?” must be answered. Then an appropriate stereo-presentation strategy can be chosen, and only then can tuning tools (equalization, level, delay, and phase manipulation) be brought to bear.