When you talk about whether an audio loudspeaker is 4 ohm or 8 ohm, you're referring to the speaker's impedance, an electrical property similar to resistance. Speakers come in several different impedance ratings; matching the impedance of your speakers to your amplifier ensures that your audio system will run efficiently and sound its best.
About Impedance
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Like resistance, impedance is a restriction of the flow of electrical current in a circuit. Resistance and impedance are similar, although impedance changes for different audio frequencies and resistance is generally constant. Impedance comes from the interaction of the speaker's voice coil and other electronic components. A speaker's impedance rating is an average taken over a wide range of audible frequencies.
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4 Ohm Vs. 8 Ohm
A 4-ohm speaker requires more power from an amplifier than an 8-ohm speaker to produce the same loudness of sound. This is due to the relationship between voltage, current and resistance in electronic circuits; for a given voltage, a lower impedance means a greater current. Power is a combination of voltage and current, so to provide more current to the speaker, an amplifier must have a higher power rating.
Series and Parallel Connections
When you connect two or more speakers to the same amplifier output, you change the total impedance of the speaker set. You can wire speakers in series or parallel; a series arrangement has the "hot" wire of one connected to the "ground" wire of the next, whereas a parallel wiring connects the "hot" wire of one speaker to the "hot" of the next, and "ground" of one to the "ground" of the other. Speakers connected in a series arrangement combine their impedance; for example, two 4-ohm speakers wired in series add up to 8 ohms total. Connected in parallel, the outcome is more complex: for two speakers, you multiply together each individual impedance, then divide the result by the sum of the impedances. For example, two 8-ohm speakers in parallel are (8 * 8) / (8 + 8) or 4 ohms total.
Impedance Matching
An amplifier's outputs have an impedance rating just as a speaker does; 4-ohm speakers should be used with the 4-ohm speaker outputs of an amplifier, and 8-ohm speakers should be used with 8-ohm amplifier outputs. A mismatch of speaker and amplifier impedances can damage the amplifier's circuits; it also results in distortion and poor sound quality. This issue is most important when you listen to loud music, as the demands on the amplifier for power are greatest at high loudness levels.