What is a subwoofer? The size, material, and (polypropylene foam;) three-dimensional – What to look for

Introduction Human hearing is capable of hearing 20-20000 Hz sound frequencies on a regular basis. This includes low frequency sounds such as drums and bass instruments, medium frequency sounds such as speech and guitar-type instruments, and high frequency sounds such as birds chirping and cymbals. Typical factory audio systems consist of course the AM/FM/CD player [...]