In this blog post I look at the differences between an open back or sealed rear chamber on the Sabourin Front Horn. No.1300.
This horn uses the GRS PT6825-8 planar transducer. This horn is the HF for System No.1309 which uses Bass Cabinet No.1303. I've been using the Sabourin System for almost a year now on my main level living room where we watch movies and occasionally listen to music. For the past few months I have been listening to the horns with the rear chamber cover removed making it open back. I then fill the chamber with 100% polyfill (pillow stuffing) which is included in this test. Subjectively I prefer open back with the polyfill but I wanted to understand why this is the case in an objective sense.
I decided to conduct a side by side comparison which can be viewed by clicking on the PDF link HERE.
As you can see below the RED line is with the sealed rear chamber and the green line is the cover removed (but still with polyfill). The chamber helps improve sensitivity in the 400Hz region however with the chamber open back we see things smooth out at 3kHz and above. We see treble extension improve from 11kHz increasing to 14kHz.
Shown below is the response of the open back but with the graph overlay removed.
I then attempted to use 30ppi open cell foam in place of the polyfill.
The results are shown in green below overlaid with the results with polyfill (purple).
As you can see there is not much difference between foam or polyfill.
Conclusion
If you review the PDF link above you will see that the removal of the rear cover improves all objective performance metrics such as frequency and step response, burst decay. Oddly harmonic distortion does not improve but we see an improvement in intermodulation distortion. Subjectively the removal of the rear cover improves treble detail, along with a more lively presentation. No pun intended, but the sound 'opens' up by the removal of the cover. I would suggest perforating the rear cover with about 50% material removal. You can then experiment with your choice of damping material.