I figured I'd start a thread about the pros & cons of kickpanel speaker locations, because there seems to be a lot of misinformation on the topic floating around this forum Please allow me to first say that I'm not a huge fan of kickpanel installations ... mostly because I find them to be ugly and intrusive ... and for the record I will absolutely admit that I have not heard the finest incarnation of the art, realized by some of the best installers/tuners in the biz. I hope to correct that sometime soon.
A brief review of dominant hearing mechanisms is in order. For the purposes of this discussion, it's VERY important to decompose hearing localization into two dimensions : lateral and vertical. I think the most important points are these :
1. Localization cues in the lateral plane are dictated by the differences in the acoustic transfer function to our two ears. These include inter-aural time differences, mostly significant for lower frequencies in the midbass and lower midrange, and inter-aural level differences, mostly significant for higher frequencies in the upper midrange and treble.
2. Localization in the vertical plane is dictated primarily by the shape of our outer ears and torso. Please consider that, if our outer ears and torsos were absent, we would be left with the classic "sphere with holes in the sides" for our heads ... and the symmetry of this geometry would absolutely dictate that vertical localization would be impossible. It's my understanding that the outer ear has the largest impact on the acoustic transfer function for vertical localization ... and given the dimensions of the outer ear, this implies that vertical localization cues are only significant above ~1kHz or so.
To summarize :
1. Ears are on the sides of our heads. Through ITD, IID we use both ears for left/right cues.
2. Vertical cues are given to us by the shape of our outer ears ... not vertically symmetrical, thank you, but common to both ears ... and the corresponding impact on the acoustic transfer function to our eardrums.
Conclusions : Let's say as an audio enthusiast, you happen to care about a realistic soundstage in your vehicle ... laterally as well as vertically. Furthermore, let's say you happen to care about a realistic soundstage for both front seat passengers, at the same time (For what it's worth, this ain't me ... I'm lazy, unskilled in fabrication, and selfish). Here's why kickpanels make sense in this case :
1. Left, right pathlength differences cannot be ELECTRONICALLY compensated for both front seat passengers at the same time. Simply not enough degrees of freedom, for the four ears involved. Kickpanels almost always present the best choice for PHYSICALLY equalizing PLD's.
2. Vertical cues can be electronically compensated, by a specialized form of equalization known as head-related transfer function inversion and substitution. A great reference on this topic is :
Creating Source Elevation Illusions by Spectral Manipulation, by P. Jeffrey Bloom, JAES, vol. 25, pp.820-828, September 1977.
So, simply put, the kickpanel optimization suggests that you solve the left/right problem physically, since it can't be solved electronically for both front seat passengers. The price you pay is in stage height ... but this can be solved electronically for both front seat passngers And yes, the best installers and tuners have known all this for many, many years.
Incidentally, there's another interesting conclusion to be drawn : physical separation of drivers in any single channel ... like mid & tweet, for example ... is not necessarily the horror of horrors it's often considered to be, particularly if the separation is vertical. Either the ear can't tell to begin with, since vertical localization cues don't even start until the treble, or spectral manipulation (EQ) can be employed to very convincingly trick the ear. Check the AES reference above. One of the most convincing soundstages I've ever heard ... with no obvious penalties in tonality, or coherence ... had midranges in the kicks and tweets in the A-Pillars. And I'm far, far from alone on this one
Armed with that background, perhaps we can list some kickpanel pros & cons :
- Most convincing lateral soundstage for both front seat passengers at the same time.
- Early reflections ... floor, underdash ... are relatively easy to tame, compared to windshield reflections of higher-mounted drivers, for example.
- Often ugly and intrusive (to me, anyway)
- Establishing vertical stage height requires clever EQ ... spectral manipulation, involving HRTF inversion and substitution.
- Early diffraction caused by feet and legs.
Now it may be, that some users don't care at all about staging ... only tonality. There's plenty of room for everyone in this hobby