The ohm rating of a loudspeaker is its dynamic impedance with dynamic acoustic program. This value runs higher than electrical resistance when conducting DC current from a volt-ohm meter. It is only one rating among other factors that allows users to classify speakers so they can be matched to amplifiers. The question, "How do I change 4 ohm speakers to 8 ohm?" usually is asked when trying to adapt 4 ohm automotive speaker drivers to 8 ohm home amplifiers, although this is not always the case.
Originally, most speakers were 16 ohms because this worked best with tube amplifiers. Later, speaker drivers with about 8 ohms were the right match for transistor amplifiers because they provided the best balance of output power, volume, fidelity and low distortion. Early automotive stereos necessitated speaker drivers with the much lower 4 ohm impedance to get the needed volume, albeit with some loss of sound quality because the driving voltage was limited to 12-volt DC automotive battery-alternator electrical systems.
Modern automotive amplifiers are able to internally jack their output voltage as evidenced by those annoying thumpers prowling the streets. An amplifier has to apply twice the voltage to an 8 ohm speaker to get it to allow the same amperage (and thus watts) as a 4 ohm speaker. Inversely, an amplifier intended for 8 ohm loads might pass too much current if used at moderate to high levels with 4 ohm speakers, which would melt its output transistors. For that reason, it is imperative to completely understand the output constraints of the amplifier that will be used, as well as the rationale for changing speaker impedance in the first place--either up or down.
Series or Parallel Connected
The best and easiest way to obtain desired impedance for a system is to play with the number of drivers and their configuration. For example, if two 4 ohm speaker drivers are connected in series (amplifier plus to speaker 1 plus, speaker 1 common to speaker 2 plus, speaker 2 common to amplifier common), the system impedance will be 8 ohms. Connected in parallel (amplifier plus to both speakers 1 and 2 plus, and both speakers 1 and 2 common to amplifier common), the total impedance is 2 ohms. Four 4 ohm speakers with two parallel connected pairs connected in series will result in--yes, 4 ohms all over again. Two parallel connected 4 ohm speakers connected in series with one 4 ohm speaker will yield a 6 ohm system. Two series connected 4 ohm speakers connected in parallel with a 4 ohm speaker will yield 2.67 ohms. The formulas are easy: For series connected speakers, just add all the impedance values, period. For parallel connected speakers, break out the calculator, it will be: 1/R total equals 1/R (speaker 1) plus 1/R (speaker 2) plus 1/R speaker 3 ... and so on.
Protecting the Amplifier
While putting a 4 ohm speaker on an 8 ohm amp is risky business, an 8 ohm speaker with an amp designed for 4 ohms is OK, although the highest attainable volume may be lower.