nm2285 said:
A few questions regarding a large, 2-way bookshelf design:
1) When offsetting the two drivers from a vertical alignment in a bookshelf speaker, is there any standard or best practice method of determining the amount of offset?
The offset is used[when it's used] to create an assymetrical re-radiation diffraction behaviour. By using an assymetrical format, for example, you spread the affected frequencies into two lower amplitude bands, as opposed to a single higher amplitude band, thus spreading/distributing the effects. Here is a very good article on the subject:
http://www.birotechnology.com/articles/stuck.html
2)What are the advantages of inverting the woofer and tweeter? When should this be done
It depends on the crossover topology used. If you have a 2nd order L-R crossover, for example, the phase angle at Fc[crossover points] will be 180 degrees out of rotation between the two drivers. This will result in null[opposite phase cancels in phase]. You must reverse the polarity[reversing the relative phase at Fc] of the tweeter in order to prevent the null. In a 4th order L-R, for example, no polarity inversion is required, since the phase angle will of each driver will rotate an opposite 360 degrees[completing a full circle and being in-phase as a result] from each other at Fc. Just remember, that generally speaking, every order[6db/octave] equals a 90 degrees relative phase shift.
3) Is there any method for determining the optimal vertical distance between the woofer and tweeter? I have a tall front baffle and am not sure how far apart to place them.
Yes. Make it as close as possible. The further distance, once 1/2 the Fc frequency wavelength[in air] is exceeded[actually, this assumes an infinite slope rate, which is not feasible, so in reality even less than 1/2 is ideal due to overlapping frequencies past Fc], measuring from one driver center to the other driver center, will result in worse vertical response linearity. So, simply, make them as close as possible to ensure the best performance.
-Chris
