Bored.....
So on topic:
Re: Localization of subwoofer
- Given the appropriate lowpass filter, a subwoofer's direct sound is not easy to locate. It's the reflected sound that is a concern: it typically excites resonance in various parts of the vehicle that have considerable storage problems. From this, it can be quite easy to localize a subwoofer. Thus, it is good to cross your mids as low as possible (or bring a subwoofer up front).
Re: Left/Right vs Front/Back
- If there were front/back information available in a stereo recording, I would agree that it was as important. However, there simply isn't. There are circumstances in which it can be important, but never as important as L/R.
Re: Why midbass?
- Typical 6.5" midrange drivers are virtually devoid of quality midbass output. A lot of drivers out there should not even be used below 100 Hz (particularly in those pre-made component sets that so many people use). So in the rare event that someone has designed a quality driver with good midbass, it is worth noting. Let's not forget that there is a lot of fundamental and second or third order harmonic information below 200 Hz.
Re: The use of rear fill
- This is a long one. First, space up front is limited. Is it the easiest location to put an 8" or larger midbass? Definitely not. So how do we achieve that visceral output from the midbass that is so enjoyable? The only option left is to find a spot in the rear (typically rear deck in sedans).
Concerns about localization are valid. But it is a complex issue. In terms of stage width (ie. localization along the horizontal axis), there are two key concerns. They are ITD (differences in arrival times) and IID (differences in SPL). It should be noted that ITD is a dominant effect below 1-2 kHz and IID is a dominant effect above that. There is some overlap, of course.
But there is something very important in this particular argument: it's called the Haas Effect, named after the German Helmut Haas. Basically, it says that if the difference in arrival times between two sources with the same material is less than 25-35 ms, then the perception of the latter of the two sources is suppressed. Here is where Time Alignment comes in really handy: if you can't account for this physically, you can electronically delay the front speakers to make sure the difference between front and rear is less than the Haas Effect....and thus, the rear speakers should be hard to locate.
Of course, the concept is just cut and dry. Since we localize using ITD at low frequencies, everything is fine (for the most part) in the midrange and midbass frequencies. However, at high frequencies, we use IID to localize. The addition of a second source creates a comb filtering effect, ie. dips and nulls are created in SPL. This isn't a big deal for low frequencies, but it directly affects our localization along the horizontal axis at upper frequencies.
So in short, we should draw the conclusion that the addition of a second source in the rear is not a big deal at low frequencies if we manage the Haas Effect appropriately. However, it can be detrimental in localization in the treble. And although it doesn't affect the localization at low frequencies, the creation of peaks and nulls in SPL at low frequencies is still a problem if an even, accurate response is the goal. Of course, the use of an EQ goes a long way here...at least localization isn't a problem.
Re: Is midbass in the rear practically realizable?
- It is important that conclusions we draw in theory are realizable in practice, otherwise the theory is inaccurate. And if you look around, there are a number of installs that either use multiple midbass or midbass in the rear (or both) with great success. Off the top of my head, there are some guys you might have heard of known as SpeakerWorks who have had great success using midbass in the rear.
It isn't a tactic that is used often because it is still widely misunderstood, but offers great potential. Imagine, for a moment, that you have a small 6.5" driver up front. You play it down to the crossover point but obviously lack the output you're looking for. Good news! You may keep it in the current bandwidth, add large midbass drivers in the rear, keep the difference in front/rear arrival times below 30 ms, lowpass below the frequencies at which IID becomes prominent, EQ out any unevenness, and gain output with no effects on localization. Much like the use of a subwoofer in the rear, it is important to minimize resonance of surrounding panels. It's complicated but the result is worthwhile.
