Test Setup
A 50 litre test box was built as a cube to keep all the box resonances at a single known frequency (447hz). Two 10 inch holes were made - one for the port and one for the driver
The small length of the test port meant that the port resonance was expected at 573hz. This is well above what a subwoofer driver would go to, so a set of computer speakers was butchered to supply a midrange driver and its amplifier. The driver was mounted on a 10inch disk and installed in place of the subwoofer driver.
The whole assembly was relocated to the back deck for testing to cut down on room effects
The SPL meter was mounted close to the port, and then with a signal of 570hz (predicted port resonant frequency), exciting the port, the meter was moved backwards and forward till the highest response was detected. This was to avoid being in a "null" for the very frequencies that were of most interest.
Results
With the standard port installed, SPL readings were taken from 400hz to 680hz in 5hz steps. The concentric port (with damping pads) was then fitted and the measurements taken again.
When graphed, I hoped to see a difference at the port resonant frequency. Unfortunately, the response jumped around so much, it was impossible to clearly identify either the box resonance or the port resonance.
I made some modifications to produce a more discriminating experiment, which included:
| Enclosing
the driver inside a segment of tubing filled with damping material |
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Mounting the ports external to the box so that the different phsical sizes do not disrupt the internal volume |
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This time the results were clearer, with both the box and port resonances being visible where expected.
Resonance testing
The tests showed that strength of the port resonance was not diminished by using the concentric port
The fitting of foam damping to the endcaps did not improve the situaton
Chuff testing
The midrange driver assembly was replaced with a subwoofer driver and powered with the 350w amp used in the "Blast Furnace". The standard port was fitted and a test passage containing 30hz content was played at increasing levels until "Chuffing" became evident.
With the controls left unchanged, the standard port was replaced with the concentric port and the passage played again. The results were immediately apparrent. Total Disaster! The chuffing was so bad that I've renamed the new design to "little piggy" as in the story of the three little pigs. (I'll huff and I'll puff .....)
Conclusion
There appears to be no point in using this design in its current form. It may be possible to decrease the chuffing by using endcaps made on a lathe which could ease the air into a U-turn, rather than slamming it into a the cap at 90 degrees. I'll leave that up to other researchers....oh well, I had some fun!
