70v system is a constant voltage distributed sound system, often used in commercial settings due to the ease of installation and design. Under the traditional loudspeaker system design, impedance load has to match the operable range of the amplifier. Since most loudspeakers are 8Ω and amplifiers can handle 4-16Ω, matching impedance often become a complex undertaking when more than 4 speakers are required. Some may simply opt to utilize multiple amplifiers but this can become a costly investment.
Under a constant voltage distributed sound system (typically 70v in US and 100v in other parts of the world), there is no need to calculate the impedance load or utilize complicated combination of parallel and series wiring. The installer simply need to add up the wattage of each speaker and make sure the total is under the power output of the 70v amplifier. Installation is a breeze since wiring can be organized in much simpler design, often in a daisy-chain.
However, there are some disadvantages to using a 70v system over a traditional 8Ω system. First, most 70v amplifiers can only output a mono signal, which is often preferred in a big, multi-speakers installation. Second, 70v causes degradation in sound quality, especially in the low frequencies. With good-quality 70v equipment, the difference, however, may not be noticeable except by the most discerning ears.