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Speaker Impedance, Power Handling and Wiring

Published
Antique Electronic Supply - based in Tempe, AZ.
The speaker ohm rating is an indication of the speaker's AC impedance, which varies with the frequency of the input signal. This variation of the speaker's impedance can be seen on the speaker's spec sheet impedance curve. This is why the spec sheet indicates this speaker to have an 8 ohm "nominal impedance."
Speaker Impedance Curve - 8 Ohm
Most of the speakers are available in alternative ohm ratings (usually 4, 8 an 16 ohm versions). This variety allows for more flexibility in matching the overall equivalent impedance of your speaker(s) to the output impedance of the amplifier. It is important that the output impedance of your amplifier matches the overall equivalent impedance of your speaker(s) for maximum power transfer and so that you do not damage the amplifier.

When using more than one speaker with your amp the equivalent overall impedance changes depending on how the speakers are wired. You can wire multiple speakers "in series," "in parallel" or in a combination of the two wiring configurations ("series/parallel").

Speakers also have a wattage rating which indicates how much power from the amp they can handle before being damaged. When you use multiple speakers, the output power from the amplifier will be distributed among the speakers. We recommend using speakers with the same ohm rating in multi-speaker cabinets so that the power is evenly distributed to each speaker. (For guitar amplification, we recommend choosing a speaker rated for at least twice the maxiumum power it could experience from the amp).

Example 1: Single Speaker Wiring

Example single speaker wiring
In example 1, we have a 50W amp with an 8 ohm output impedance. It has been matched to one 8 ohm speaker.

Since there is only one speaker, it could experience the entire 50W from the amplifier.

In this case we recommend choosing an 8 ohm speaker with a rated power of at least 100W.

Example 2: Series Wiring

Example multiple speaker series wiring
In example 2, we have a 50W amp with an 8 ohm output impedance. It has been matched to two 4 speakers wired in series.

Since there are two speakers, each speaker could experience 25W (half of the output power from the amp).

In this case we recommend choosing two 4 ohm speakers with rated power of at least 50W each.

ztotal = Equivalent Overall Impedance
zn = Impedance of speaker n
Formula for overall impedance of parallel wiring

Example 3: Parallel Wiring

Example multiple speaker parallel wiring
In example 3, we have a 50W amp with an 8 ohm output impedance. It has been matched to two 16 ohm speakers wired in parallel.

Since there are two speakers, each speaker could experience 25W (half the output power from the amp).

In this case we recommend choosing two 16 ohm speakers with rated power of at least 50W each.

ztotal = Equivalent Overall Impedance
zn = Impedance of speaker n
Formula for overall impedance of series wiring

Example 4: Series/Parallel Wiring

Example multiple speaker series-parallel wiring
In example 4, we have a 50W amp with an 8 ohm output impedance. It has been matched to four 8 ohm speakers wired in series/parallel.

Since there are four speakers, each speaker could experience 12.5 W (one fourth of the output power from the amp).

In this case we recommend choosing four 8 ohm speakers with rated power of at least 25W each.

For this configuration, it is easiest to calculate the equivalent overall impedance in two steps.

Step 1: two 8 ohm speakers wired in series have an equivalent overall impedance of 16 ohm.
ztotal = Equivalent Overall Impedance
zn = Impedance of speaker n
Step 1 of calculating overall impedance in series-parallel wiring
Step 2: two 16 ohm speakers wired in parallel have an equivalent overall impedance of 8 ohm.
ztotal = Equivalent Overall Impedance
zn = Impedance of speaker n
Step 2 of calculating overall impedance in series-parallel wiring
By Kurt Prange (BSEE), Sales Engineer for Antique Electronic Supply - based in Tempe, AZ. Kurt began playing guitar at the age of nine in Kalamazoo, Michigan. He is a guitar DIY'er and tube amplifier designer who enjoys helping other musicians along in the endless pursuit of tone.