An Anechoic Chamber for SPCR

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FRAMES FOR THE BATTS

It took some time and effort to devise an inexpensive and efficient way of installing the batts against the walls. My solution looks like a mad carpenter's flimsy, minimalist bookcase about 6' tall, with steel wires to hold the batts in place. Each frame was assembled outside the garage and 10 batts were secured to it. Then the ~55 lb assembly was moved into the room where it was secured to the wall, with about a 2-3" gap between the wall and the batting. Again, that air gap actually helps to reduce sound transmission. A total of 11 frames were employed, 3 for each wall except the one with the entry door. The whole process took many days as I was working on my own, with multiple trips to hardware and lumber stores for supplies. Having a small sedan instead of a pickup truck or van was a real handicap. Plus, I was still working on SPCR editorial content... and trying to enjoy Vancouver's summer as well, in case it turned out to be short. Tennis as often as possible, hanging with friends, etc. There was also a week-long visit from Christoph Derndorfer, our EU correspondent who was travelling around the US and Canada on an extended holiday. Much beer was being consumed daily to help fuel all the yikyak that geeks (and other human beings) are wont to share. Exhaustion was a regular daily experience.


The batt frames look a bit like ridiculous bookcases.


The completed room, lined on 5 sides with blue fill. The wood frames are visible here and there through gaps between the batting. The blue fill on the door is mounted more or less the same way as the walls, but it has to hinge with the door, as the 4' batts are deliberately positioned to cover both the door and part of the wall. It's the worse part of the damping installation which produces a bit of dust every time the door is opened or closed.


View from the doorway. At this point, there were two bags of UltraTouch left in the garage. This means over 600 lbs of the stuff is now in the room.


Working with UltraTouch: The material may be nontoxic and safe to handle, but it is dusty. It rubs off on clothes and carpeting, and some of it is fine enough to clog up your nostrils and make you sneeze. It also smells funny; not terrible but odd. It's treated with an EPA registered borate-based chemical that acts as a fire retardant and provides protection against mold, mildew, fungus and pests. Bonded Logic, the manufacturer, makes extensive claims about its benign qualities and lack of toxic ones. After working with the stuff for weeks, I think breathing in all that dust is still probably not great for the lungs. The good thing is that if you're not handling or moving it, the batts don't release any dust. I found no anecdotal or documented evidence on the web of any toxicity related to UltraTouch.

THE SECOND DOOR

After blocking the window, beefing up the external wall and installing all the blue fill damping, the weakest link was the door. It's a cheap, hollow-core interior door that's probably more cardboard than wood, as is often the case. It swings into the room from the outside. As per the original plan, a second door, a substantial solid-core door, was hung on hinges on the outside of the door frame. It swings outward. An air gap of nearly two centimeters between the floor and doors was filled using pieces of MDF as a door sill. This door sill has a groove cut into its bottom that acts as a in/egress path for the microphone cable. A small frame all around the inside of the door frame was fitted and screwed into place, with foam gasket strips applied to ensure a good seal when the door is closed. A non-locking knob was installed on the door, along with a latch for the door frame.

The above paragraph took just a couple minutes to write, but it was the first time that this amateur carpenter ever installed door — a heavy door not prehung or prepared with holes for the knob or mortised for the hinges — into a preexisting frame; it was no piece of cake.


The outside solid-core door is very heavy. There's an air gap of 3-4" between the two closed doors. Some panel damping from PC case damping kits were applied on the outside of the inner door to reduce vibrations the door and reduce the air resonance in the air cavity; it's probably a complete waste.


The door sill is a rough affair made of MDF. The foam gasket strip goes all around door frame. It makes a tight fit against the outer door to keep sound transmission down. Note the groove cut into the bottom of the sill for the microphone cable: A 25' long cable is held captive in the door sill. It's the signal path from the mic to the mic-preamp and sound card.



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