DIY Reflection Filter for Voice Recording

 Finished 2

I have been interested to try a Reflection filter since reading an article in Sound on Sound 2006 I got around to making something of the sort in 2015. Here it is

I chose to mount acoustic foam on a semicircular frame made of my favourite material, wood. At the top and bottom would be hoops made of laminated hardwood, five stretchers between the two and a wooden cross-frame to support the thing. The microphone would attach the the frame and it in turn would attach to a microphone stand. The acoustic foam I had was 420 mm square. Two together would give 820 mm and allow a radius of 261mm so that would set the size of the thing.


I started by sawing three strips of wood 20 x 4 x 900 mm for the bottom hoop from beech scrap. The top hoop would be in a nameless red hardwood, again scrap.  I would leave them un-planed because it seemed unnecessary, time consuming hand work. They were too thin for the thicknesser.

 Gluing up the strips

I had a disk made of planks, it is a sanding board, sandpaper on the other side, So I clamped some blocks on the perimeter and would use this as the caul. I know it is not the way to do it. I should have made a proper caul, a continuous curve to be on the inside of the hoop but I figured it would do. And it did.

 Here it is with the freshly glued strips in place. It took my complete sock of G-cramps to hold it together.

I repeated the job for the top hoop. The hoops sprung a little when released, to top one more than the bottom, perhaps because they were of different materials. The top one was about 25mm wider than the bottom. Ah well!All clamps in place





support StructureA cruciform was made from ash, 40 x 12 mm with a slot for microphone mounting hardware. This slotted arm would prove too long in use. I shortened it later. At the cross is a lap joint, reinforced with 4mm plates each side.

Tapping the clamping washerShouldered blocks were made to mate with the slot, one side is tapped 3/8 inch whitworth, the thread of microphone mountings.

Sawing off the washer



Notching one ringHere is the bottom hoop being notched for vertical stretchCloseup of notchers




Gluing ring to frameCruciform clamped to the bottom hoop.



Gluing up the frame
Gluing up the frame, aided by string loops  tourniquets. The twisting blocks were adjusted, tightened and loosened to pull the assembly square, or something close to square.



Joining foam piecesThe foam I have, left over from treating my studio, measures 420 mm square. I joined pieces using plasterers scrim tape and a small amount of Gorilla glue. I glued the outer one first, this time with super glue.Outer foam glues in place



Inner foam positionedThe inner foam in the process of installation. Followed by the "blocking in" of the space created by the different bending radii of inside and outside foams.Outer foam extended



Here is the finished filter


 Finished 2



Finished rear view



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