How to Measure Speaker Impedance

Check the rated impedance on the label. Most speaker manufacturers list the impedance level on the speaker label or packaging. The “nominal” impedance level (approximately 4, 8 or 16 Ω ) is the estimated minimum impedance of most of the typical negative range. The typical tonal range is usually between 250 and 400 Hz. The actual impedance is quite close to the value in this range, and increases slowly as the frequency is increased. Below this range, the impedance changes rapidly and peaks at the speaker’s resonant frequency with the cabinet.

• Some manufacturers use the actual impedance as the rated impedance on the label.
• To make sense of these frequencies, note that most bass tracks are between 90 and 200 Hz, while sub-drum bass can only reach 20 Hz. Midrange tones, including most voices and instruments other than percussion, range in frequency from 250 to 2 kHz. ^[1]

Set the multimeter to resistance measurement mode. A multimeter will pass a small DC current to measure resistance. Since impedance represents the quality of the AC circuit, this method cannot measure impedance directly. However, the multimeter is fairly accurate with most home audio setups (you can easily tell the difference between a 4 Ω and an 8 speaker with this method). Before you proceed, you need to set the multimeter to the lowest range resistance setting (usually 200 Ω with most multimeters). A multimeter with a lower setting (e.g. 20 Ω) will give more accurate results.

• If there is only one level setting for resistance, this is an auto-ranging multimeter, which will find the appropriate range on its own.
• Excessive DC current can damage the speaker coil. This risk is quite low because most multimeters only produce a small amperage. ^[2]

Remove the speaker from the case or open the back of the cabinet. With removable speakers without connections or enclosures, you do not need to perform disassembly.

Turn off the speaker power. Any current passing through the speaker will cause the measurement to be erroneous, even burning the multimeter. You need to disconnect the power of the devices. If the wires connected to the terminals are not soldered, remove them.

• Do not remove any wires directly connected to the speaker cone.

Connect the multimeter’s probe leads to the speaker terminals. Look closely at the poles to determine which is the positive and which is the negative. Usually the poles will be distinguished by the “+” and “-” symbols. Connect the red probe of the multimeter to the positive terminal, the black probe to the negative terminal.

Estimate the impedance from the resistor. Typically, the resistance rating will be 15% lower than the rated impedance on the label. For example, an 8 speaker with a resistance of 6-7 Ω is normal.

• Most loudspeakers have a nominal impedance of 4, 8, or 16 Ω. After estimating, if the impedance falls into one of these values, you can safely connect the speaker to the amplifier (unless you measure something out of the ordinary).