Abstract: A sound isolation cap for use with a sound level meter. The cap has a bore in which a plug snugly fits. The plug, in the open position, allows air to travel between the exterior of the cap and the cap's interior cavity. In the closed position, the plug prevents air from traveling between the exterior of the cap and the interior cavity. Air may travel between the interior cavity and the exterior of the cap by way of two passages. One passage connects the interior cavity with the bore while the other passage connects the bore with the exterior of the cap. When in use, the microphone of the sound level meter is inserted into the interior cavity.