How to Measure Speaker Impedance

Speaker impedance measures the device's ability to resist alternating current. The impedance is inversely proportional to the current the speaker draws from the amplifier. If the impedance is too high compared to the amplifier, it can affect the volume and dynamic range. If it's too low, the amplifier may be damaged as it struggles to provide enough power to the speaker. To measure the general impedance range of a speaker, you can use a multimeter. However, for a more precise check, specialized tools are required.

Check the nominal impedance rating on the label. Most speaker manufacturers list the impedance rating on the label or packaging. The "nominal" impedance (usually 4, 8, or 16 Ω) represents the estimated minimum impedance of the speaker across a typical frequency range. This range generally spans from 250 to 400 Hz. The actual impedance is quite close to this value within that range, and it rises gradually as the frequency increases. Below this range, impedance changes rapidly, reaching its peak at the speaker's resonant frequency within the enclosure.

• Some manufacturers use the actual impedance as the nominal rating on the label.
• To understand these frequencies, note that most low-frequency sounds fall between 90 and 200 Hz, while bass drum sounds may reach down to 20 Hz. Midrange frequencies, including most speech and non-percussion instruments, range from 250 Hz to 2 kHz.

Set the multimeter to resistance measurement mode. The multimeter will apply a small DC current to measure resistance. Since impedance reflects the quality of an AC circuit, this method cannot measure impedance directly. However, a multimeter is quite accurate for most home audio setups (for instance, you can easily distinguish between a 4 Ω and an 8 Ω speaker using this method). Before starting, set the multimeter to the lowest resistance range (usually 200 Ω on most multimeters). A multimeter with a lower range (e.g., 20 Ω) will provide more accurate results.

• If there's only one resistance setting, it’s an auto-ranging multimeter, which will automatically adjust to the appropriate range.
• Excessive DC current could damage the speaker coil. This risk is minimal since most multimeters produce only small currents.

Remove the speaker from the enclosure or open the back of the speaker cabinet. For standalone speakers without connections or enclosures, no disassembly is necessary.

Disconnect the speaker power. Any current passing through the speaker could lead to inaccurate measurements or even burn out the multimeter. Disconnect all power sources from the equipment. If the wires are not soldered to the terminals, remove them.

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

Connect the multimeter probes to the speaker terminals. Carefully observe the terminals to identify the positive and negative poles, usually marked with "+" and "-". Connect the red probe of the multimeter to the positive terminal and the black probe to the negative terminal.

Estimate the impedance based on resistance. Typically, the resistance reading will be about 15% lower than the nominal impedance listed on the label. For example, a typical 8 Ω speaker will show a resistance of 6-7 Ω.

• Most speakers have a nominal impedance of 4, 8, or 16 Ω. After estimation, if the impedance falls within these values, you can safely connect the speaker to the amplifier (unless an abnormal result is detected).

Prepare a sine wave generator. Speaker impedance varies with frequency, so you need a device capable of generating sine waves at multiple frequencies. A sound frequency oscillator (or function generator) is the most accurate choice. Any wave generator or pulse generator that produces sine waves will work, but some models may give inaccurate results due to voltage fluctuations or poor sine wave estimation.

• If you're a beginner or just testing at home, a measurement device connected to a computer is a good option. While these devices aren't highly accurate, the automatically generated graphs and data make it easier for beginners to understand.

Connect the device to the amplifier’s input. Check the label or specifications to find the RMS (Root Mean Squared) power rating in watts. For this method, a high-power amplifier will give more accurate results.

Set the amplifier to low voltage mode. This testing method is part of the Thiele-Small parameter testing standards, which are designed for low voltage testing. As such, reduce the gain on the amplifier and set the voltmeter to AC current measurement mode before connecting to the amplifier’s output terminals. The voltage on the voltmeter should be between 0.5 to 1 V, but if your device is not highly accurate, it’s best to keep it under 10 V.

• Some amplifiers produce inconsistent voltages at low frequencies, which can lead to inaccurate measurements. To achieve optimal results, check the voltmeter to ensure voltage stability as you adjust the frequency with the sine wave generator.
• If possible, use a high-quality multimeter. Lower-cost models often yield inaccurate results in this test. You can also purchase high-quality probes for multimeters at electronics stores.

Choose a good quality resistor. Refer to the list below to find the resistor with the appropriate RMS power rating (in watts) for your amplifier. Choose a resistor with the recommended value and power rating equal to or higher than the listed values. The resistor value doesn't need to be exact, but if it's too high, it could cause issues with the amplifier and disrupt the test. If the resistor value is too low, the measurement may be inaccurate.

• 100W amplifier: 2.7 kΩ resistor with at least 0.5 W power rating
• 90W amplifier: 2.4 kΩ resistor with at least 0.5 W power rating
• 65W amplifier: 2.2 kΩ resistor with at least 0.5 W power rating
• 50W amplifier: 1.8 kΩ resistor with at least 0.5 W power rating
• 40W amplifier: 1.6 kΩ resistor with at least 0.25 W power rating
• 30W amplifier: 1.5 kΩ resistor with at least 0.25 W power rating
• 20W amplifier: 1.2 kΩ resistor with at least 0.25 W power rating

Measure the resistance value. The value might slightly differ from the one printed on the label. Make sure to record the resistance value that you measure.

Connect the resistor and speaker in series. Link the speaker and amplifier with the resistor in between. This ensures that the speaker receives a constant current.

Place the speaker away from obstructions. Wind or reflected sound waves can disrupt the measurement. Ideally, the speaker should be placed in an area without wind, with the magnet facing down and the speaker cone facing up. For high-precision measurements, secure the speaker to an open frame and ensure it's at least 60 cm away from any solid objects.

Calculate the current intensity. Apply Ohm's Law (I = U / R, where current intensity = voltage / resistance) to calculate the current and record the result. Insert the previously measured resistance value into the formula.

• Example: If the resistance is 1230 Ω and the voltage source is 10 V, the current I = 10/1230 = 1/123 amperes (A). You can leave the fraction unchanged to avoid rounding errors.

Adjust the frequency to find the resonance peak. Set the sine wave generator to the mid-range or higher than the typical range used by the speaker (100 Hz is a good starting point for low frequencies). Connect the voltmeter to the speaker in AC current measurement mode. Decrease the frequency by 5 Hz at a time until you notice a significant increase in voltage. Continue adjusting the frequency until you pinpoint the maximum voltage. This frequency represents the speaker's resonance in an 'open space' (the speaker's resonance frequency will shift due to the enclosure and surrounding objects).

• You can use an oscilloscope instead of the voltmeter. In that case, look for the highest amplitude voltage.

Calculate the impedance at resonance. You can replace resistance in Ohm's Law with impedance Z. The formula is Z = U / I, which calculates the impedance at the resonance frequency. This will be the maximum impedance of the speaker within the expected sound range.

• Example: If I = 1/123 A and the voltmeter reads 0.05 V (or 50 mV), then Z = (0.05) / (1/123) = 6.15 Ω.

Determine the impedance at different frequencies. To measure the impedance across the speaker's frequency range, gradually increase the sine wave in small increments. Record the voltage at each frequency and use the same formula (Z = U / I) to calculate the impedance at different frequencies. You may encounter a second peak or find that the impedance stabilizes after moving away from the resonance frequency.