most mics cannot capture a very low frequency. the lower the frequency the larger the wavelength is. This is why it passes through the walls very easily. The diaphragm on most mics is very small. You can us a large woofer 12 or 15 inches to capture low frequency audio. just wire the leads of the woofer or subwoofer to the mic inputs(btw the impedance of a speaker is usually 4-16ohms mics are much higher in the thousands so you'll get more responce) . this technique is used for recording bass guitars. problem is you wont hear the actual sound through the walls just your stuff rattling. the human ear cant hear below about 20Hz and below 50Hz is a faint sound. it is more felt than heard.