Since I built the cabinets for these drivers undersized I knew I could go larger and not be out of specs. I first took frequency plots of each speaker, the first since I rearranged the dirvers. I had matched pairs of drivers in each speaker cabinet, after listening and measuring enough I decided which pair I wanted on the top and which pair I wanted on the bottem of both speakers. So I put the drivers with the smoothest most extented tweeters on top as the main drivers, as these drivers have the tweeters volume level up more than the lower ones, I wanted it to be as smooth a response as possible. Given the fact that both pairs had the same sonic qualities that was not at issue in the decission.
After measuring each speaker at one meter (39"), I then took a measurement of their combined frequency response in the room at 8' 6" from the listening chair. Then I took the bottom off of the left speaker, and put 2 by 4's under the main cabinet to stand it up higher, increasing the pipes length my 3", and the mouth area by about double. At first when I took a measurement at one meter with this difference the frequency response started out the same at 200 hertz, and 160 hertz. But when I measured it at 125 hertz, then the bass response increased by 4 dbs, and uniformly was increased by 4 dbs all the way down the rest of the frequency spectrum. The bass extention was extended the same amount, to my mild disappointment, I thought lengthening the pipe would have a greater effect on the lower bass response, but this did not prove to be true. Next I plan on rasing the cabinet a little bit higher and seeing what the response will be then.
I thought that this adjustment in the cabinet length and vent size would have had a much greater impact on bass response, to my disappointment on one hand, as I wanted greater extension, and to my surprise on the other, as it did not produce any greater peaks or valleys in the frequency response. After lengthening it a bit more and allowing the port to be even larger, I will reduce the internal volume. It appears at this point that what I will need to get the desired results is lengthen the pipe, reduce its internal volume and enlarge the pipes vent. This I hope will increase the depth of bass, and attenuate the bass response a little, as what I have was plenty before I made this small adjustment. We will see as this project progresses what the end product will be. Perhaps at this point I should close all but the front firing vent to see what effect this has. |
