How to Measure Speaker Impedance
Check the label for a nominal impedance rating., Set a multimeter to measure resistance., Remove the speaker from its cabinet or open the back of the cabinet., Cut off the power to the speaker., Connect the multimeter leads to the speaker...
Step-by-Step Guide
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Step 1: Check the label for a nominal impedance rating.
Most speaker manufacturers list an impedance rating on the speaker label or the packaging.
This "nominal" impedance rating (usually 4, 8, or 16 ohms) is an estimate of the minimum impedance for typical audio ranges.
This usually occurs at a frequency between 250 and 400 Hz.
The actual impedance is fairly close to this value within this range, and rises slowly as you increase the frequency.
Below this range, the impedance changes rapidly, peaking at the resonant frequency of the speaker and its enclosure.Some speaker labels display an actual, measured impedance for a specific listed impedance.
To give you an idea of what these frequencies mean, most bass tracks fall between 90 and 200 Hz, while "chest thumping" sub bass can be as low as 20 Hz.
The midrange, including the majority of non-percussion instruments and voices, covers 250 Hz to 2kHz. -
Step 2: Set a multimeter to measure resistance.
A multimeter sends out a small DC current to measure resistance.
Since impedance is a quality of AC circuits, this will not measure impedance directly.
However, this approach will get you close enough for most home audio setups. (For example, you can easily distinguish between a 4 ohm and 8 ohm speaker this way.) Use the lowest range resistance setting.
This is 200Ω for many multimeters, but a multimeter with a lower setting (20Ω) may give more accurate results.
If there is only one setting for resistance, your multimeter is auto-ranging, and will find the correct range automatically.
Too much DC current can damage or destroy the voice coil of a speaker.The risk is low here, since most multimeters only produce a small current., If you're dealing with a loose speaker with no connections or speaker box, then there is nothing you need to do here. , Any power running to the speaker will ruin your measurement, and could fry your multimeter.
Turn off the power.
If the wires connected to the terminal are not soldered on, detach them.
Do not remove any wires connected directly to the speaker cone. , Look closely at the terminals and determine which is positive and which is negative.
There is often a "+" and a "-" sign to identify them.
Connect the multimeter's red probe to the positive side, and the black probe to the negative side. , Typically, the resistance reading should be roughly 15% less than the nominal impedance on the label.For example, it's normal for an 8-ohm speaker to have a resistance between 6 or 7 ohms.
The majority of loudspeakers have a nominal impedance of 4, 8, or 16 ohms.
Unless you get a strange result, it's safe to assume your speaker has one of these impedance values for the purpose of pairing it with an amplifier. -
Step 3: Remove the speaker from its cabinet or open the back of the cabinet.
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Step 4: Cut off the power to the speaker.
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Step 5: Connect the multimeter leads to the speaker terminals.
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Step 6: Estimate the impedance from the resistance.
Detailed Guide
Most speaker manufacturers list an impedance rating on the speaker label or the packaging.
This "nominal" impedance rating (usually 4, 8, or 16 ohms) is an estimate of the minimum impedance for typical audio ranges.
This usually occurs at a frequency between 250 and 400 Hz.
The actual impedance is fairly close to this value within this range, and rises slowly as you increase the frequency.
Below this range, the impedance changes rapidly, peaking at the resonant frequency of the speaker and its enclosure.Some speaker labels display an actual, measured impedance for a specific listed impedance.
To give you an idea of what these frequencies mean, most bass tracks fall between 90 and 200 Hz, while "chest thumping" sub bass can be as low as 20 Hz.
The midrange, including the majority of non-percussion instruments and voices, covers 250 Hz to 2kHz.
A multimeter sends out a small DC current to measure resistance.
Since impedance is a quality of AC circuits, this will not measure impedance directly.
However, this approach will get you close enough for most home audio setups. (For example, you can easily distinguish between a 4 ohm and 8 ohm speaker this way.) Use the lowest range resistance setting.
This is 200Ω for many multimeters, but a multimeter with a lower setting (20Ω) may give more accurate results.
If there is only one setting for resistance, your multimeter is auto-ranging, and will find the correct range automatically.
Too much DC current can damage or destroy the voice coil of a speaker.The risk is low here, since most multimeters only produce a small current., If you're dealing with a loose speaker with no connections or speaker box, then there is nothing you need to do here. , Any power running to the speaker will ruin your measurement, and could fry your multimeter.
Turn off the power.
If the wires connected to the terminal are not soldered on, detach them.
Do not remove any wires connected directly to the speaker cone. , Look closely at the terminals and determine which is positive and which is negative.
There is often a "+" and a "-" sign to identify them.
Connect the multimeter's red probe to the positive side, and the black probe to the negative side. , Typically, the resistance reading should be roughly 15% less than the nominal impedance on the label.For example, it's normal for an 8-ohm speaker to have a resistance between 6 or 7 ohms.
The majority of loudspeakers have a nominal impedance of 4, 8, or 16 ohms.
Unless you get a strange result, it's safe to assume your speaker has one of these impedance values for the purpose of pairing it with an amplifier.
About the Author
Sharon Chavez
Enthusiastic about teaching creative arts techniques through clear, step-by-step guides.
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