Quick Estimate
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.
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.
Remove the speaker from its cabinet or open the back of the cabinet. If you're dealing with a loose speaker with no connections or speaker box, then there is nothing you need to do here.
Cut off the power to the speaker. 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.
Connect the multimeter leads to the speaker terminals. 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.
Estimate the impedance from the resistance. 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.
Accurate Measurement
Get a tool that generates a sine wave. A speaker's impedance varies with frequency, so you'll need a tool that allows you to send out a sine wave at any given frequency. An audio frequency oscillator is the most accurate option. Any signal generator or function generator with a sine wave or sweep function will work, but some models may give inaccurate results due to changing voltages or poor sine wave approximation. If you are new to audio tests or DIY electronics, consider audio testing tools that connect to a computer. These are often less accurate, but novices may appreciate the auto-generated graphs and data.
Connect the tool to an amplifier input. Look for the power on the amp label or spec sheet in watts RMS. Higher power amplifiers produce more accurate measurements with this test.
Set the amp to a low voltage. This test is part of a standard series of tests to measure the "Thiele-Small parameters." All of these tests were designed for low voltages. Lower the gain on your amp while a voltmeter set to AC voltage is connected to the amp's output terminals. Ideally the voltmeter should read somewhere between 0.5 and 1 V, but if you don't have sensitive tools, just set it to below 10 volts. Some amps produce inconsistent voltage at low frequencies, which is a common source of inaccuracy in this test. For best results, check with the voltmeter to make sure the voltage stays constant as you adjust the frequency using the sine wave generator. Use the highest quality multimeter you can afford. The inexpensive models tend to be less accurate for the measurements later in this test. It may help to purchase higher quality multimeter leads at an electronics store.
Choose a high value resistor. Find the power rating (in watts RMS) closest to your amplifier's on the list below. Choose a resistor with the recommended resistance, and the listed wattage rating or higher. The resistance doesn't need to be exact, but if it's too high, you may clip the amplifier and disrupt the test. Too low, and your results will be less accurate. 100W amp: 2.7k Ω resistor rated to at least 0.50W 90W amp: 2.4k Ω, 0.50W 65W amp: 2.2k Ω, 0.50W 50W amp: 1.8k Ω, 0.50W 40W amp: 1.6k Ω, 0.25W 30W amp: 1.5k Ω, 0.25W 20W amp: 1.2k Ω, 0.25W
Measure the exact resistance of the resistor. This may vary slightly from the printed resistance. Write down the measured value.
Connect the resistor and speaker in series. Hook the speaker up to the amplifier, with the resistor between them. This creates a constant current source powering the speaker.
Keep the speaker away from obstructions. Wind or reflected sound waves could disrupt this sensitive test. At minimum, keep the speaker magnet side down (cone up), in a windless area. If high accuracy is required, bolt the speaker to an open frame, with no solid objects within 2 ft (61 cm) in any direction.
Calculate the current. Using Ohm's Law (I = V / R or current = voltage / resistance), calculate the current and write it down. Use the resistor's measured resistance for R. For example, if the resistor has a measured resistance of 1230 ohms, and the voltage source is 10 volts, the current I = 10/1230 = 1/123 amps. You can leave this as a fraction to avoid rounding errors.
Adjust the frequency to find the resonance peak. Set the sine wave generator to a frequency at the mid or upper range of the speaker's intended use. (100 Hz is a good starting point for bass units.) Place an AC voltmeter across the speaker. Adjust the frequency downward about 5 Hz at a time, until you see the voltage rise sharply. Tweak the frequency back and forth until you find the frequency where the voltage is highest. This is the resonance frequency of the speaker in "free air" (an enclosure and surrounding objects will change this). You can use an oscilloscope instead of a voltmeter. In this case, find the voltage associated with the greatest amplitude.
Calculate the impedance at resonance. You can substitute impedance Z for resistance in Ohm's Law. Calculate Z = V / I to find the impedance at resonance frequency. This should be the maximum impedance your speaker will encounter in the intended audio range. For example, if I = 1/123 amps and the voltmeter measures 0.05V (or 50mV), then Z = (0.05) / (1/123) = 6.15 ohms.
Calculate impedance for other frequencies. To find the impedance across the speaker's intended frequency range, adjust the sine wave in small increments. Record the voltage at each frequency, and use the same calculation (Z = V / I) to find the speaker's impedance at each frequency. You may find a second peak, or the impedance may be fairly steady once you get away from the resonance frequency.
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