7dB is totally obtainable. I was able to go much lower than that even. Remember, dB is a log scale of pressure differentials. We like to think of it as measuring sound pressure level when really it is measuring pressure wave differentials. Sound is a wave of compression followed by rarefaction. High pressure followed by low pressure. This is the key concept to keep in mind when trying to lower your dB.
Since what dB really is measuring is the difference between the two then the way to get it lower is to eliminate this difference. This is done most cost-effectively by filling your underground concrete bunker with liquid nitrogen. Within a few hours, a day or two at most, all motion ceases and you have virtually 0 dB. At least I am pretty sure. Every time I try and check my meter freezes over.
Since what dB really is measuring is the difference between the two then the way to get it lower is to eliminate this difference. This is done most cost-effectively by filling your underground concrete bunker with liquid nitrogen. Within a few hours, a day or two at most, all motion ceases and you have virtually 0 dB. At least I am pretty sure. Every time I try and check my meter freezes over.