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This is going to be long but essential, gotten it from another forum in another part of the world...introducing some professional knowledge to the forum, of course, not from me...enjoy... ------------------------------------------------------ Much in hifi is the product of folk lore. However, within reason, speaker placement is not. That's not too say that speaker placement has to follow rigid rules, it doesn't, quite the opposite. But there must be a reference point, and thus a reason for that reference point. That reference point is to start with the speakers at a height where the tweeter is at or about ear level. Toe-in and rear wall reinforcement are separate considerations, and are relative to different speakers. The reason for ear height placement is due to in part to the initial localisation of sound, and from then, the definition of a stereo signal. Localisation occurs in the very first instance of a sound, if the sound reaches the right ear first it is to the right, etc. In order to facilitate/maximise the accuracy of this process, initial sonic reflections should be minimised. In practice, therefore, tweeters need to be at or near ear level, and the speaker toe-in adjusted so that the initial sonic event is correctly localised. The level/requirement of toe is very much dependant upon the listening room (the likelihood of reflection, etc), and the speakers themselves, (the radiation pattern from a speaker baffle being a factor of the baffle size, the drive unit layout, and the crossover, etc). Once localisation has taken effect, stereo separation is reliant upon the fact that there is space between the left and right ears, and thus a time delay in sound from the left reaching the right ear, enabling the brain to perceive that the sound is from the left. Room reverberations/reflections then come in to play with regard to spaciousness, soundstaging, and imaging. There is also an incidental benefit of correct localisation in the Haas effect which prioritises the localised incident sound over reflected sound (assuming enough time between the incident and relected sounds, and that the reflections are of low enough order), and reduces the perception of response deviations that may be caused by said reflections. With respect to tweeter beaming and the resultant sound quality from a tweeter, I agree that the sound is hardly pleasant on it's own. It is, however, critical for initial localisation, and subsequent soundstaging and imaging. Yet, whilst it is true that playing a tweeter alone will result in a poor response, that is irrelevant. The response is from the loudspeaker and it's collection of drive units, which sum to give an overall response, rather than considering the response of an individual drive unit. And this is all the more relevant when crossover types are considered and their consequent effect on phase response and radiation pattern. Now, this essentially pertains to domestic audio, and much is unattainable in an in-car environment. However, the rules apply equally, it just means that real soundstaging and imaging in-car is very difficult. The most common approach to soundstaging in-car is the placement of tweeters at dashboard height. Naturally this brings sound upward from the door/kickwell, but it does not generate a soundstage, and is of little benefit with regard to imaging. Why?. Well, the ear is at it's most sensitive at frequencies at the upper part of the vocal range 1kHz - 3kHz, and tweeters play very little of that range, most being filtered at around 3kHz. Now, at this point many people will say that the output from the tweeter is ultra-directional, and yes it is, but that is actually detrimental when the tweeter is disparate from the mid-bass. Why?. Because the tweeter has an ultra-directional output that the ear localises, and uses as a reference for stereo separation (see above), however, the sensitive vocal frequencies are likely coming from the door or the kickwell. This creates multiple point sources (reflection notwithstanding), and naturally multiple path lengths between each source and the ear, which then create vagaries of phase and time alignment as DaVitch mentioned. As I said, this is difficult to circumvent in-car. One solution is the application of mid-range higher in the car, essentially in place of the ubiquitous tweeter. Why?. Well, as mentioned above, the ear is sensitive to vocal frequencies, and their reproduction higher in the car will improve soundstage height and imaging. Bear in mind that treble remains important, and it cannot be forsaken for mid-range. But, treble must be coincident with mid-range for the effect to work satisfactorily. This can be achieved with coaxial type drive units, or closely placed components (of course, the ease of fitting rather depends on the target vehicle). Such a system then leaves the mid-bass, which is absolutely critical. Mid-bass speakers should be used that have maximal reliable low frequency response, so that low bass output is not concentrated at the rear of the car from a subwoofer. Much is stated about the directionality of low bass, or rather the perceived lack of it, and most of it is nonsense. 80Hz is barely acceptable, lower being extremely beneficial. In many respects the upper end response of the mid-bass is determined by the supplied filtering. However, it is better to apply DIY filtering that limits the onset of directionality from the mid-bass. The transistion into directionality starts when the diameter of the speaker equals half the wavelength. The transition frequency (as with much filtering) can, and often needs, to be tweaked. To run such a system successfully it really needs to be multi-amped and actively filtered. It is also likely that the mid-treble combo will need equalisation, but that depends on their location, and their susceptibility to reflection from glass and other hard surfaces. At the end of the day it is important to know that an in-car system will never stage/image as well as a domestic system. And that includes serious installs featuring path length compensated kick builds and horn loaded mid/tweets. So it is a matter of how far you want/need to go. A set of tweets on the dash is far easier, and will lift the sound. In the majority of cases such a system can be made to sound acceptable, and that is often all that is required.