It is our humble opinion that time correction should only be used in three different circumstances:
- When the installer recognizes that a vehicle cannot image properly from both seated positions and it seems more plausible to make the vehicle stage and image well from only one seated position.
- To counter the effects of group delay.
- Time alignment between pairs of drivers.
The first scenario is rather elementary. If a vehicle is too small to achieve equalized PLD’s, it doesn’t lend itself well to equalized PLD’s, or the vehicle’s owner doesn’t wish to embark on physical reconstruction of the car to achieve optimized PLD’s, it is a good use of time correction to make the vehicle image well from the single-seated position. It should be noted that it is Hybrid Audio’s opinion that it is always better to improve the car mechanically and attempt to fix mechanical problems with mechanical solutions than it is use to electronics to fix mechanical problems. However, we realize that there is the occasion when there is little desire to try to mechanically optimize one’s listening space, and electronic manipulation is desired. It is important to recognize that, given the fundamentals of ITD and IID discussed previously, time alignment is rendered virtually useless for frequencies above approximately 2,000 Hz.
In the second scenario, the use of time correction is much more cognitive and shows great promise for countering the effects of group delay. Group delay is impulse response over time. Group delay increases significantly at low frequencies and is considerable in larger midbass and subwoofers. An excellent use of time correction would be to delay the smaller Legatia midranges and tweeters with respect to the larger midbass and subwoofers so that the low-frequency delay of these drivers is synchronized in the time domain with the output of the midrange and treble frequencies.
The final scenario is time alignment between pairs of drivers. When employing multiple drivers in order to achieve a unified listening experience within a vehicle, it’s usually a requirement to install the drivers at physically separated locations; for example, your midbass may be located in the doors, and the midrange and tweeter may be located in the dashboard or a-pillar. In order to compensate for this, you might choose to selectively delay certain speakers in the installation so that all of the tones reach your ears at the same time (note that as previously alluded to, time alignment of your tweeters would be rendered virtually useless).
An excellent use of today’s dual-mono equalizers and advanced digital signal processing is the ability to equalize amplitude anomalies between speakers and sets of speakers installed in a vehicle. In a vehicular installation, the frequency response of drivers can sometimes be manipulated for the betterment of the system using independent left and right amplitude adjustment. Virtually any good car audio system can be made better with judicious use of a minor amount of equalization. And while equalization will not cure phasing anomalies in a car, usually the product of vehicular mechanics, they can certainly be helpful in fine-tuning the system to your own personal taste or in the quest for playback accuracy to the original musical composition.
One important consideration of amplitude equalization (also known in some circles as “amplitude alignment”) is the ability to tune those frequencies above about 500 Hz that are not completely affected by time correction. The very best vehicles have some sort of amplitude equalization between the left and right speakers to account for IID and HRTF. After your Legatia system is installed, you may wish to attempt some minor amplitude equalization between the left and right channels to achieve a more stable image that is not frequency dependent, or perhaps to improve image placement or stage coherency.
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