Speaker Stands for the Abrams Planetarium
In 2004, The Abrams Planetarium (Figure 1) installed a new sound system. Mounting the speakers presented a problem. There is limited space behind our projection dome. The inside of the outer copper dome is covered with insulation that is scheduled to be removed at a future time due to asbestos in the insulation.
The speakers could not be hung from chains or cables because there was no way to connect the chains to the outer dome. We didn't want to mount the speakers directly on the dome, due to vibration issues, not to mention our hope to get a new dome someday.
Our solution was to build speaker stands that we could mount on the catwalk around the base of the dome. The catwalk is made of concrete and can easily support the weight of the speakers and stands.
Using a CAD program, (Figures 2 and 3) I drew up some designs and made some measurements. To get the speakers to the needed altitude on the dome, the stands needed to be about 11 feet tall, with a tilt of about 76°. (Figure 4)
The MSU machine shop fabricated the stands. They are made from 4" steel pipe. (Figure 5) The shop has a pipe bending tool that bent them to the needed angles. A base plate (Figure 6) was welded to one end of the pipe. Unistrut was connected to the top with U-bolts. Chains connect to the unistrut to hang the speakers. Of course, the stands were painted flat black. The whole contraption needed to fit through our two foot by two foot access hole to the space behind the dome. (Figures 8 and 9) The speakers were assembled and hoisted to the tops of the poles using ropes and pullies. (Figures 10 and 11)
In addition to speakers behind the dome, we also installed new subwoofer speakers. These were placed under a stage in the front of the chamber. (Figures 12 and 13) The subwoofer cabinet was put into a sandbox. The sandbox is a box made of MDF (Medium Density Fiberboard) (Figure 14) and filled with sand, about 500 pounds of sand! (Figure 15) The heavy mass of the sandbox helps direct the low frequency soundwaves forward to the audience, rather than back through the wall.
We are very pleased with our new sound system. We went from 600 watts on a 1/4" reel to reel tape deck in our old system to 12,000 watts with a Mackie SDR-24 hard drive recorder in the new one. (Figure 16)