That's a very nice improvement :-)
Made possible by the Celestion mids which are better matched - you can see where S1 and S2 with easy to get closed back mids both required a notch filter at the bottom end. You can also see that S2 only gets to 900 Hz before rolling off. S3 allows a higher crossover, which is one of the main design goals. There is more room to play with regarding the crossover point.
What is that sharp dip at 2.3khz? Is this modeled in Akabak or HornResp?
I use Hornresp because I've found closer correlation where it matters most with the measured result. That dip would be a cancellation notch. Given that this is a bandpass arrangement, above tuning you see resonant behaviour. It becomes more clear when you see the other iterations of the design. That notch sits between another peak higher up, but when all the parameters are set right, they combine into a smoother roll-off. If I adjust things differently, then that notch becomes a cliff face where the response just drops suddently, with a peak much lower down, almost 30 dB down.
Why the curve starts to dip after 1.5Khz instead of flatter respose ?
The dip is the cancellation notch that is a characteristic of an offset mid driver with a reflection back from the throat. It is inherent to the design. The trick is being able to use things like this and massage things into shape.
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