An idea to address the dip at approx. 150 Hz (last curve in this post) is to damp the horn at the position where the pressure minimum is for the frequency. Looking at the internal measurment measurements showed in the .pdf document that would be at a position about 120 cm from S1. I figure that where the pressure is low the air speed is high and therefore the right place to apply some damping.
I did drill three holes in the back of the horn, placed 80 g of polyester fiber wool there and sealed it up properly.
The measurements are made in the middle of the mouth of the horn. I have zoomed in quite a bit to get better resolution of the SPL response graphically.
Green curve is the original horn without any damping and red curve is with 80 g och polyester wool damping.
Well that much damping eats a lot of the bass response and makes the dip at 150 Hz even worse!
Reducing the amount of damping by half and a new measurement.
Not much better. The peaks are somewhat lower, so are also the dips but not to the same extent and I am loosing bass.
I am a bit surprised of this result. I really thought that damping at the pressure minimum would have positive effects on the 150 Hz fip.
It does look much better when I zoom out and apply som smoothing… 😉