Baffle Step Correction & Speaker Efficiency
I've found that different BSC filter simulators model the BS effect as either a loss or gain. Jeff Bagby's worksheet has an option to do either. I assume that modeling the BS effect as a loss is most accurate? For my baffle geometry, and modeling BS effect as a loss, Jeff's worksheet gives me a small 1.5 db increase between 700 Hz and 2000 Hz, then tails off to a -6 db loss at 10 Hz.
I'm designing a 3-way system with a Usher 8945a driver, with an average 87 db/w/m sensitivity. At 100 Hz, the -5 db baffle effect lowers the sensitive to around 82 db. Adding in a low-frequency boost from a port, and from a small peak in the crossover filter, the model gets ~83.5 db sensitivity from the Usher, after box, baffle and crossover effects.
In order to get a flat response, I'm having to put some pretty large resistors on the mid-range and tweeters, bringing their sensitivities down from 90 and 91 db respectively to the 83.5 db of the Usher.
Is this a reasonable approach? Do I really want to be so heavy handed with the resistors in accounting for the baffle step? Is 83 db a reasonable efficiency for the speaker as a whole?
Thanks for the help
Its all a bit relative and depends on your starting point. I like to think of 2 pi as the more "normal" situation, primarily because Thiele/Small calculations will be based on 2pi conditions and then 2pi to 4 pi correction would give the final response in a free field but with the nominal baffle of the cabinet. Simple models apply to 2 pi but diffraction related response must be considered for 4 pi.
83.5 is pretty low with most commercial sytems falling in the 87 +-2 dB range. You can take a philosophical view on whether you must drop the whole system to the lowest 4pi point. Know that you will get some room gain and some boundary effect at low frequency from the floor and walls, even if some feet away. You can always split the difference, aim for 85-86 and fine tune the sound by proximity to the rear wall. The variablility of room acoustics will make more precision than that a bit unjustified.
Do you have tone controls? Keeping the midrange sensitivity up and giving some bass boost is an equally valid approach.
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