I recently visited a very well equipped home studio (more like a professional studio built on a home!) which consisted of a control room,a drum room, an iso booth and a vocal booth. The client had noticed that low frequencies were an issue in the vocal booth. That is, certain frequencies taking off, and an overall sense of “boxiness” in the room. When the vocalist backed off the mic, the take was virtually unusable. The dimensions were 1 x 1.5 x 2.4m. The booth was mostly brick and glass, and about 75% of the walls had been treated with 50mm Sonex acoustic foam. Sonex is good quality foam, but like all relatively thin acoustic foam, the Sonex was absorbing virtually none of the low frequencies. This is a very common scenario, and when you buy an acoustic product make sure to look at the absorption coefficients across the spectrum. Total NRC rating is pretty much useless for audio applications. A 50mm product will typically absorb around 25% (.25 AC) at 200Hz and 10% (.1 AC) at 100hz, which is pretty much ineffective. We tried a few thicker panels, but ended up installing the SA600/150 (600 x 600 x 150mm) acoustic panel. We only needed to install three of these panels on the door to effectively control the low frequency resonance. The SA600/150 absorbs at 100 % at 200Hz and 60% at 100hz. This is a good example of how bass absorption should be considered a necessary part of vocal booth acoustic treatment. A brick or concrete voiceover booth will be more reflective than a plasterboard booth, but both situations require bass control.