This instructable is a direct implementation of the "Mitchell Doughnut" guitar speaker directivity modifier proposed by Jay Mitchell from The Gear Page discussion forum. The most relevant discussion can be found in the Speaker Directivity thread.
A typical guitar amplifier with a 12" speaker will seem to have accentuated higher frequencies when listening directly on-axis with the speaker cone. Listening to the speaker "off-axis" will have the converse effect. This phenomenon is commonly referred to as "speaker beaming". The directivity modifier is an attempt to mitigate this on-axis beaming effect without affecting the off-axis tone. Refer to the above gear page thread for background on the principles involved.
A 3/4" foam "doughnut" that will be affixed behind the grill cloth in front of the speaker cone. See pics.
-Guitar Amplifier (this projects specifies a 12" speaker)
-12"x12" polyurethane foam sheet
-Large sharp razor blade
-3M Super 77 contact adhesive spray from Lowes (optional)
-Black Clothing Dye (optional)
Step 1: Foam Acquisition
These can be found for $5.72+ship - part#85735K72 from McMaster-Carr.
I ordered 4 sheets for good measure and the box arrived the next day!
It may be worthwhile to buy a few sheets of varying thickness or firmness to experiment with. Several people have reported positive results using #3 firmness foam. Using 1/2" thickness foam (McMaster #8643K511) may be better for use with thinner baffles. Note, the 1/2" foam is white in color, and you will probably want to dye it black before installation so that it will not be visible through the grill cloth. Black RIT Dye is likely available at your local supermarket.
Step 2: Marking
Use a ruler to mark the center of the foam sheet and then begin to mark the foam at a half-diameter from the center until the marks form a dotted circle. Perhaps there is a more efficient way to draw a good circle, but this worked for me.
Then, using the same technique, create a smaller circle in the center with a diameter of 3" (1.5" radius).
Step 3: Cutting
Step 4: Installation
If your doughnut needs some assistance to stay in place, Jay recommends spraying one side of the doughnut with a light coat of 3M adhesive, and then applying the doughnut to the back of the grill cloth within about 30 seconds. This adhesive is weak enough to allow you to remove the foam in the future.
Step 5: Testing!
Enjoy your beam-free amp!