Lesson Two: Equalization of Pathlength Differences
Quite possibly the most important functional consideration that a do-it-yourself enthusiast or professional installer should give to the Legatia SE speaker placement is to optimize, as best as possible, pathlength differences (PLD’s) in the vehicle. PLD’s are defined mathematically as follows (this example assumes a right-hand drive vehicle—PLD’s are always a positive number):
X – Y = Z
Where:
X = distance of the center of the left speaker from your left ear.
Y = distance of the center of the right speaker from your right ear.
Z = pathlength difference.
Applying this formula, assume that the distance of the left speaker from your left ear is 140cm, and the distance of the right speaker from your right ear is 100cm, the pathlength difference is 40cm.
Good stereo imaging is completely dependent on arrival times of the fundamental vocal frequencies. Differences as little as 10 microseconds can be detected by the brain. A PLD of 30 centimeters equates to the sound from the nearest channel arriving about 0.9 milliseconds earlier than the furthest channel. It is Hybrid Audio’s opinion that the end-user should try to keep PLDs to less than 30 centimeters in a vehicle that is intended to have good imaging and staging character from both seated positions.
The best way to go about evaluating certain locations in your vehicle is, in general, to look for the potential locations as far forward and away from you as possible, but still with a general “line of sight” to the speakers (particularly the speaker on the far side of the vehicle). An easy way to test various potential locations is
to hold a tape measure or other measurement device from the potential speaker mounting locations, and measure those locations with respect to your ears.
Reference the figure, below. In this scenario, three potential locations for the mounting of the Legatia SE midbass driver are shown:
In scenario “a”, we show the installation of the Legatia SE midbass in the dashboard, high in a door panel, or in the dashboard. As you can see from the diagram, the PLD’s between the left and right speakers are large, due to the proximity of the listener to the near-side speaker. While the mounting of primary drivers in the dashboard or a-pillars has become increasingly popular, this configuration will undoubtedly require both time and intensity domain equalization in most vehicles to ensure a good, focused center image, properly located in the center of the vehicle for one seated position.
There are, however, some rare exceptions, and you may actually find that the dashboard locations provide the best equalized PLD of the available mounting locations; this is very rare though – in our experience, less than one percent of vehicles on the market today have optimized dashboard speaker locations for the midbass drivers.
In scenario “b”, a typical door installation location is shown, and in many vehicles represents a good improvement in PLD’s from the dashboard, high in the door panel, and a-pillar location identified in scenario “a.” The door speaker installation scenario is the one detailed in the basic installation section at the beginning of this manual, and in most vehicles represents a satisfactory location to mount speakers; not ideal but satisfactory. The door speaker installation scenario will likely also require some amount of time and intensity equalization to ensure a centered image in most vehicles; this can be as simple as adjusting the balance control on your source unit, to more advanced ways of digital time and intensity manipulation.
The third and final potential mounting location as shown in this diagram (scenario “c”) represents a kick panel installation, where the midbass are placed far forward in the A-frame cavity of the
kick panels, present in most vehicles. The kick panels are the small panel next to the throttle and brake pedals, down by your feet. While it is not immediately obvious looking at a two-dimensional drawing, in many cases the kick panel location affords the best equalization of pathlength differences for most vehicles. And the reason why this is a good choice for most vehicles is defined in the second full paragraph of Lesson Three, below.
The lesson to be learned here is that by taking a few moments to evaluate the potential mounting locations in your vehicle, in a very short period of time, you will be able to find the best location for your Legatia SE midbass by determining the location with the smallest PLD.
Hybrid Audio Technologies Specifications and Parameters Spreadsheet (Google Sheets) — Advanced System Installation — Lesson One: Off-Axis Response — Lesson Two: Equalization of Pathlength Differences — Lesson Three: The Effect of HRTF, ITD, and IID — Lesson Four: Point-Sourcing — Lesson Five: Reference — Mounting Baffle Considerations — Crossovers — Time Correction — Acoustic Treatment — Acoustic Treatment — Conclusion