N7R's in Custom Steel Enclosures

I redesigned the rear mounting system; I could not 'wrench' down on the sorbothane as much as I wanted without the silicone failing around the magnet so I decided to do a steel approach using what the home depot calls clothing line hooks:

The black ring around the magnet is a vacuum belt cut in half (half the width) which helps keep the clothing line hooks aligned and also aids in aligning the aluminum ring; I used electrical tape to cover the hooks so there would not be metal on metal with the basket, also to keep from shorting anything around the electrical terminals.
 

One change I may make would be a better decoupling on the back side; since there is so much more sorbothane surface area on the front of the speakers, the back side 'squashes' quite a bit (not shown well in this pic, I have since tightened them more)





My computer and USM-810 processor in the closet, the only moving part on this system is the one fan on the computer, soon to be changed to a better solution; the buzzing of the cold cathode tube on my screen is all I can hear at my desk though  I hope to get an LED backlit LCD TV which should produce no noise...


Here is the cut sorbothane sheet I used; since this project I have used the sorbothane to decouple my RTA-M mic from a stand I made out of a broken clothing rack... thats for another post though 


Sometime soon I will post the near-field measurements of the speakers...

I have made a few changes to the speakers; a new sorbothane system in the back utilizing larger diameter (2" dia.) washers, I drilled out the area where the rod comes through in the back more so there is a 1/2" diameter hole for the 1/4" diameter rod to go through (this is to ensure that the rod does not touch the steel enclosure, transferring vibration). I have also done a little more calculation regarding the amount of compression/torque put on the whole sorbothane system. Lastly, I put some guides on the top of the speakers to help with alignment for different seating positions. Using a laser level to aim the speakers at the listening position only takes ~30 sec, and with the guides on the top I am sure they are aimed exactly where I want them now 

The three marks are the three points I measured (and averaged) to find the thickness of the initial and compressed sorbothane states (using a digital calipers) to measure the thicknesses (calipers accurate to .01 mm however I got the average for the 3 points within ~.01 mm also). Using the sorbothane design calculator, I found that the difference of .01mm in the rear disc is equivalent to approximately 0.3 lbs difference in force (when near 20% displacement since it is a non-linear material). The overall force pulling the speaker into the enclosure is 392 lbs of force. The resonant frequency of the rear disc should be 24 hz, given a 100 hz signal (the maximum the sorbothane calculator allows, could use their equations to go higher) there is ~83% isolation (17% signal energy is transmitted), which corrisponds to around a 15 db reduction (used the calculator here:http://www.sengpielaudio.com/calculator-thd.htm) from the isolated speaker side to the enclosure, at 80 hz (x-over frequency) there is a 12 db reduction. The higher the frequency (the further from the resonant frequency of the sorbothane) the greater the isolation.

According to the sorbothane company, it dampens vibrations best under 10-20% compression so I shot for 20% on the rear, the front is a bit more difficult because of the shape of the speaker baffle (the metal rim that is visible on the front) and also the shape of the speaker enclosure (there is a 'high point' where it meets the speaker and then it tapers off to the sides of the enclosure) so I am not quite sure how much surface area is being compressed; a 'worst case scenario' (the entire ring is compressed evenly) given the already calculated force of 392 lbs gives a 12.58% deflection (within range!) and a resonant frequency of 34 hz, at 80 hz, there is a 6.6 db reduction in signal transfer from the speaker to the enclosure (remember, this is the worst case scenario), at 100 hz it is 10 db down... the best case scenario (to which I presume is closer to reality) was determined by slideing a sheet of paper into the gap between the curved speaker enclosure and the sorbothane; I determined that an outer diameter of 6.7" is actually in contact with the enclosure, the inner diameter is 6.23", presuming this number is correct, the deflection becomes 85 percent with the given amount of pressure (which seems unlikely since I have disassembled the system and the sorbothane rings return to normal shape, I would not think 85% deflection would be 'recoverable'). Anyways, it is probably somewhere inbetween, however even with a worst case model, a 10 db reduction in signal transfer at 100 hz is pretty good 

Here is the laser leveler that I use to aim the speakers, this picture was taken before I put the guide tape on the top 


I have also put a blanket on my desk for the time being, till I find a better solution, to help reduce the reflection off the desk when sitting at it and listening to tunes  it is also nice for resting my arms on when typing rather than the glass edge which left marks on my arms...

Here is my method for creating an alignment system, I used a drywall square mounted flush against the frame of the speaker (the outer metal ring of the speaker). Then I laid a strip of masking tape along the edge; against that masking tape (which does not stick well to the Line-X) I placed pieces of electrical tape, the center line (between the two pieces) is the line the laser is pointed down. using the welded seam as a reference, I determined that the right speaker is 0.995 degrees offset from being perfectly aligned and the left is off 1.55 degrees... however since the square should find what is on axis for the speaker, the angle 'correction' is built into the line of electrical tape.



Ideas or suggestions for improvements are more than welcome  I want to do some 'knock on the side of the enclosure' tests to show how acousticly dead they are but other than that I am not sure what else will help others with their projects. I should note that the sorbothane calculations were done using the "Sorbothane Design Guide" software available free at www.sorbothane.com