An Anechoic Chamber for SPCR

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The forum thread 2008 SPCR facility/equipment upgrades: hemi-anechoic chamber begun on May 27 provides a more detailed, day-by-day journal of what's covered below in more summary form. Most of the work was done in July, but there was preparatory work before... and a long fallout after. It's one thing to start with an empty room; it's a whole nuther thing to start with a room chock full of stuff. All the furniture, gear, parts and components had to be moved out and squeezed into the other two main lab rooms which were already full of other stuff. This made June a total grind. August was mostly about cleaning up. The SPCR lab has never been one of those tidy, spotless hospital white places; after the dislocations of moving in June and July, it's in greater shambles than ever, and we still stumble around looking for things.

* * *

The first point of attack was the window in the room. It's an aluminum frame double-pane sliding window that does not fit well. There's no question that it's the most direct route for external noise to get in.

The 4'x3' window which faces the house next door is a sound leak.

The window was blocked up with a 1.2" thick piece of MDF board with heavy weather-stripping gaskets all around to make an acoustic seal. Long, heavy duty plastic straps were used to clamp the board to the bars on the window.

Window boarded up.

Laptop drive on the ledge for scale

There's a ledge shelf across the wall, at the level of the bottom of the window. This is actually the concrete perimeter foundation of the house. It's up around 4 feet high. Sound transmission through lower portion of the wall is very low. Most sound transmission from the outside occurs through the window and the top half of that wall. All the other walls of this room are inside walls; there is at least one room plus an outer wall beyond them, so sound transmission from the outside via the other walls is low.

From the outside. The window was left unlocked to periodically ventilate the airspace so moisture does not build mold. It rains a lot in Vancouver, so this is a potential problem. The pebble rock stucco external wall provides much better sound attenuation than a typical internal wall.

The drop in noise level after this window fix was shocking. It became much quieter. At night past 10PM (when it's quieter everywhere), the new acoustic measurement system reported around 12~13 dBA in the empty room. The old B&K 2203 SLM was unable to show any readings below about 15~16 dBA. This may be directly related to internal electronics noise; ie, it's at its internal noise limit. When a fan was turned on at ~400 RPM a meter away from the mic, the SLM read 18 dBA, and happily, so did the B&K. This tends to confirm my conjecture about the B&K's internal noise.

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