Your energy is great, the band is tight, and the crowd is eager--but as soon as you take the stage, a high squeal of feedback stops the show. A homemade sound-hole cover for your acoustic guitar can prevent that buzz-killing hum.
Constructing the Cover
Measure the sound-hole of your guitar. Take several measurements to ensure accuracy.
Using the compass, draw two circles with the same diameter as your sound hole onto your cardboard.
Cut the circles out with scissors. The edges should be neat, but absolute precision isn't necessary.
Cut out a slightly smaller circle of packing foam, and sandwich it between the two cardboard circles
Staple your foam-and-cardboard sandwich together, making sure all the staples face the same direction.
Place tape over the sharp ends of the staples to prevent them from scratching your guitar.
Gently crease your sound-hole cover so it will be easier to install.
Loosen your guitar's strings for easier access.
Press the cover into your sound hole so the wood of the guitar's top fills the gap between the two cardboard pieces. The cover will pop into place without much effort. If it doesn't, trim the inner cardboard circle with scissors.
Re-tune your guitar, and hit the stage. The cardboard and foam will absorb stray sound waves, preventing that annoying feedback.
Use light-colored cardboard, and write your band's name on the sound-hole cover for extra publicity.