A 7.1 surround sound system, as opposed to a 5.1 surround sound system, includes two extra side speakers to provide additional mid- and high-frequency sound. For this reason, you will need a more powerful subwoofer to balance the bass frequencies with the louder and more surrounding mid and high frequencies. Many people with 7.1 sound systems actually choose to incorporate two subwoofers to provide the requisite power and to help the bass sound as surrounding as the rest of the sound.