When it comes to speaker design the received wisdom is that you shouldn’t put two drivers next to one another in the horizontal plane because of the inevitable lobing problem and the adverse effect on the off-axis response. The Line Force does obviously have drivers next to one another horizontally so i’m interested to hear what Danny has to say about this. Please note that this is not an attempt to trash this speaker- it is a perfectly reasonable question given that lobing is a well established problem and so I am genuinely interested in Danny’s response.
Yes, this is a problem with a lot of line source designs, but not with mine.
The problem is that as you move left or right there is a time delay that changes between the two lines. One is delayed in time verses the other. And when one starts arriving with enough phase rotation then it starts cancelling the output and leaving a hole in the response. And a 180 degree phase rotation equals a 15db dip in the response and very uneven room responses.
The amount of cancellation per degree of off axis is easily a product of two things. It depends on the acoustic center spacing and crossover point. The greater the distance of the acoustic centers or the higher the crossover point the greater the problem.
So a typical 6.5" woofer to a line or ribbons will have an acoustic center spacing of about 7" and a crossover point in the 2.5kHz range. This will cause out of phase cancellations that can easily get sever. And if you don't know how to shift the phase using the filter design you can get a much deeper hole on one side than the other. Or you can get a pretty good dip in both directions.
And a 2.5kHz wavelength is about 5.5".
But as the crossover points drop much lower it becomes much less of an issue.
A 1kHz crossover point as used in our LS-6 line source was a wavelength of about 13". And the acoustic center spacing is 5.5".
Here are the off axis measurements. Red is on axis and each color going from red, to orange, to yellow, to green, and blue represent another 10 degrees off axis. This one is towards the tweeter side.
And towards the woofer side.
In this direction we are barely seeing a slight dip in the 1.5kHz range. Not bad at all though. It's really good actually.
And with the Serenity Line force design the acoustic centers are only 4" apart. So the off axis cancellation becomes even less of an issue.