Speaker impedance is a measure of the speaker's resistance to alternating current. The lower the impedance, the greater the current drawn from the amplifier. If the impedance is too high, the volume range and dynamics of the speaker will be affected. If the impedance is too low, the amplifier may self-destruct as it struggles to meet the power requirements. If you just want to check the general range of your speakers, all you need is a multimeter. To perform a more accurate test, you will need some more specific tools.
Step
Method 1 of 2: Performing an Express Forecast
Step 1. Check the nominal impedance rating on the label
Most loudspeaker manufacturers list the impedance rating on the label or package. This “nominal” impedance rating (usually 4, 8, or 16 ohms) is the approximate “minimum” impedance for a typical audio range. This range is usually in the 250-400 Hz frequency. The actual impedance is quite close to values in this range, and increases slowly as the frequency is increased. Below this range, the impedance changes rapidly, peaking at the resonant frequency of the speaker and its casing.
- Some loudspeaker labels list the actual rated impedance for the specified impedance listed.
- For a better understanding of frequency, most basses are between 90-200 Hz, while "chest thumping" subbasses have frequencies as low as 20 Hz. Midrange loudspeakers, including most nonpercussion instruments and voices ranging from 250 Hz to 2 kHz.
Step 2. Use a multimeter to measure resistance
The mutlimeter will direct an electric current to measure the resistance of the impedance. Since impedance has the quality of an alternating electric circuit, this method cannot measure impedance directly. However, this approach will yield fairly accurate measurements for most home audio setups (for example, you can easily tell the difference between 4 ohm and 8 ohm speakers using this method). Use the lowest range resistance setting, which is 200 ohms for most multimeters. However, a multimeter with a lower setting (20 ohms) can give more accurate results.
- If the multimeter has only one resistance setting, the device automatically adjusts the range (autoranging), and will find the correct range by itself.
- Excess direct current can damage the voice coil in the loudspeaker. In this project, the risk is quite low because most multimeters only produce a small amperage.
Step 3. Remove the speaker from the case and open the back of the case
If you're dealing with detachable speakers or speaker cases, there's nothing you can do at this step.
Step 4. Disconnect power to the speakers
Any power to the speaker will damage the meter and potentially burn the multimeter so it is best to turn it off. If the wires connected to the terminals are not soldered, disconnect them.
Do not remove any cables that connect directly to the speaker mouthpiece
Step 5. Connect the lead of the multimeter to the speaker terminal
Look carefully and determine the positive and negative terminals. Usually there is a "+" and "-" symbol that distinguishes the two terminals. Connect the probe/red lead of the multimeter to the positive side, and the black lead to the negative side.
Step 6. Estimate the impedance of the resistance
Typically, the resistance is approximately 85% of the nominal impedance on the label. For example, it is normal for an 8 ohm loudspeaker to have a resistance of 6-7 ohms.
Most loudspeakers have a nominal impedance of 4, 8, or 16 ohms. Unless the results are unreasonable, you can assume that the loudspeaker has one of these impedance values to pair with the amplifier
Method 2 of 2: Measuring Accurately
Step 1. Prepare a device that generates a sine wave
Speaker impedance varies with frequency so you need a device that will allow you to transmit sine waves at any frequency. Audio frequency oscillators often give accurate results. You can use any signal generator or function generator with a sine wave or sweep function, but some models may give inaccurate results due to voltage changes or poor sine wave estimation.
If you're new to self-testing audio or electronics, consider using a computer-connected audio test kit. Usually this tool is less accurate, but graphs and data will be generated automatically making it easier for beginners
Step 2. Connect the device to the amplifier input
Look for power on the amplifier label or user manual in watts RMS. The high-power amplifier provides more accurate measurements in this test.
Step 3. Set the amplifier to low voltage
This test is part of a standard series of tests to measure the "Thiele-Small parameter". All of these tests are designed for low voltages. Decrease the gain on the amplifier while using a voltmeter attached to alternating voltage and connected to the amplifier's output terminal. Ideally, the voltmeter should be between 0.5-1 volts, but if you don't have a sensitive device, set it below 10 volts.
- Some amplifiers produce inconsistent voltages at low frequencies, which usually results in inaccurate measurements. For best results, check with a voltmeter to make sure the voltage remains constant when adjusting the frequency using a sine wave generator.
- Use the best quality multimeter you can afford. Cheap multimeter models are usually less accurate for tests that will be carried out later in the test. So, it's a good idea to buy a high-quality multimeter at an electronics store.
Step 4. Choose a high value resistor
Find the power rating (in watts RMS) closest to the amplifier in the list below. Choose a resistor with the recommended resistance, and current rating listed or higher. The resistance doesn't have to be exact, but if it's too high, you could pinch the amplifier and interfere with the test. On the other hand, if the measurement is too low, the results will be inaccurate.
- 100 W amp: 2.7 kΩ resistor rated at least 0.50 W
- 90 W amp: 2.4 kΩ, 0.50 W
- 65 W amp: 2.2 kΩ, 0.50 W
- 50 W amp: 1.8 kΩ, 0.50 W
- 40 W amp: 1.6 kΩ, 0.25 W
- 30 W amp: 1.5 kΩ, 0.25 W
- 20 W amp: 1.2 kΩ, 0.25 W
Step 5. Measure the exact resistance of the resistor
The exact resistance of the resistor may differ slightly from the figure listed on the component. Write down the measured resistance number.
Step 6. Connect the resistor and loudspeaker in series
Connect the speakers to the amplifier with a resistor between them. Thus, a constant electric current will power the loudspeakers.
Step 7. Keep the loudspeaker out of the way
Wind or reflected sound waves can interfere with this sensitive test. At a minimum, keep the magnetic side of the speaker facing down (mouthpiece up) in a windless area. If you want very accurate results, attach the speaker to the exposed frame using screws, and ensure that there are no solid objects within 61 cm of the speaker.
Step 8. Calculate the electric current
Use Ohm's law (I = V/R or current = voltage/resistance) to calculate and record the electric current in a circuit. Use the measured resistance of the resistor to obtain the R value.
For example, if the rated resistance of the resistor is 1,230 ohms, and the source voltage is 10 volts, then the current I = 10/1,230 = 1/123 amperes. You can leave it as a fraction to avoid rounding deviations
Step 9. Adjust the frequency to find the resonant peak
Set the sine wave generator to a frequency in the middle or upper range of the speaker you want to use (100 Hz is a good starting point for a bass unit). Place an AC (alternating current) voltmeter along the speaker. Lower the frequency setting by 5 Hz at a time, until you see the voltage spike sharply. Flick the frequencies back and forth until you find the frequency with the highest voltage. This is the resonant frequency of the loudspeaker in "free air" (the case and objects around the speaker will change this frequency).
You can use an oscilloscope instead of a voltmeter. In this case, find the voltage corresponding to the largest amplitude
Step 10. Calculate the resonant impedance
You can exchange impedance Z for resistance R by Ohm's law. Calculate Z = V/I to find the impedance at the resonant frequency. The result is the maximum impedance that the speaker receives within the desired audio range.
For example, if I = 1/123 ampere and the voltmeter shows 0.05 V (or 50 mV), that means Z = (0.05) / (1/123) = 6.15 ohms
Step 11. Calculate the impedance of the other frequency
To find the impedance across the desired loudspeaker frequency range, adjust the sine wave a little at a time. Record the voltage at each frequency, and use the same calculation (Z = V/I) to find the loudspeaker impedance at each frequency. You could find a second peak, or the impedance might be stable enough once you go through the resonant frequency.
