When connecting a speaker to an amplifier, the ideal matchup is when the speaker impedance is the same as the output rating of the amplifier. The best sound can be obtained from the speakers when they have a correct impedance match.
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An amplifier with an "150 Watts RMS (root mean square) into 8 ohms" output specification would be ideally matched with an 8-ohm, 150-watt speaker. Amplifiers are designed to provide a specific signal level to an impedance range. A specification that read "100 watts into 8 ohms or 4 ohms" would be an example of a speaker that could match either an 8- or 4-ohm speaker.
The lower the speaker impedance the more power it produces for an equal signal from the amplifier. A 4-ohm speaker will absorb twice the power as an 8-ohm speaker provided they have the same signal level from the amplifier.
2-Ohm vs 4-Ohm
If the amplifier is rated to drive a 2-ohm speaker, the 2-ohm speaker will result in more acoustical power. If the amplifier is rated for 4-ohm speakers only, it runs the risk of damage to the amplifier with 2-ohm speakers. The major application of 2-ohm speakers is in woofers and subwoofers.
Series Parallel Combinations
Speakers can be connected in series--positive connection on one to negative connection on the other--to form a single functional speaker. The resulting impedance is the total of the two speakers. A pair of 2-ohm speakers could be connected in series to make a functional 4-ohm speaker. Two identical speakers can be connected in parallel--negative to negative and positive to positive--to form a single functional speaker. The impedance would be half the value of one of the individual speakers. These wiring approaches make it easier to better the amplifier.