When purchasing speakers, it is important to consider the ohm rating. Typically, speakers are rated at either eight or four ohms, and choosing the right ohm rating is important to achieving maximum performance from your speakers and audio amplifier.
The ohm rating is another way of writing a speaker's impedance. The impedance refers to how much power will flow through your speaker at a given voltage.
A four ohm speaker has a lower resistance to electrical current than an eight ohm speaker. Therefore, a four ohm speaker will draw more power from the amplifier than an eight ohm speaker.
When choosing four or eight ohm speakers, it is important to consider the minimum ohm rating of the amplifier. Connecting four ohm speakers to an amplifier designed for eight ohms can overdrive the amplifier, damaging both the amp and speakers. However, a four ohm amplifier can handle eight ohm speakers.
All things being equal, four ohm speakers will play louder than eight ohm speakers. This is because the four ohm speakers have less resistance to the flow of voltage from the amplifier.
Four ohm speakers are typically used in car audio applications, because car stereos generally have lower power ratings than home stereos. Four ohm speakers, with their lower electrical resistance, convert more of the amplifier's power into sound.