I'm setting up a converted garage to record voice. The room dimensions are 22'9" x 23'2", with an eight-foot ceiling. I live near an industrial property that produces noise. In particular, there is the sound of a piece of industrial machinery at around 240Hz, and the back-up warning beeping of construction vehicles. To block outside noise, I made some window and door plugs out of 3/4" MDF. The door plugs have EPDM rubber seals to seal them against the french door surfaces. The window plugs have felt around their perimeters which is compressed into the gaps when in place. In one window, I have an air conditioner which was admitting quite a bit of sound so I built a box out of MDF to put over that, under the full-window panel. To control reflections I put (a lot - 224sf) of OC703 on the walls and onto the plugs when in place, and an additional 4'x4' panel over the desk where I record. I attached a photo that shows these modifications. I made a recording of the room tone while the industrial machinery is in operation. The microphone (Rode NT1A) recording level was set to the most sensitive setting which I'm likely to use (for quiet voice). I attached an image of the spectrum analysis of this file, indicating the spikes of known origin - the industrial machinery and the back-up warning tones from construction vehicles. I believe I have these sources of noise under control. What I'm curious about is the low-end, which seems focused around 60Hz. Could this be related to line current somehow? Or perhaps resonance within the room (dimensions already given)?