The impedance of a loudspeaker is the resistance it opposes to alternating current; the lower this value, the greater the current that the loudspeakers absorb from the amplifier. If the impedance is too high, the volume and dynamic range are affected; if it is too low, the speaker can be destroyed by emitting too much power. If you just want to have a confirmation of the general values of the loudspeakers, all you need is a voltmeter; if you intend to perform a more precise test, you need some special tool.
Steps
Method 1 of 2: Quick Estimate
Step 1. Check the speaker label for the impedance rating
Most manufacturers indicate this value on the packaging or on a label applied right on the speaker. This is "nominal" (typically 4, 8, or 16 ohms) and represents an estimate of the minimum impedance of the typical audible range, usually when the frequency is between 250 and 400 Hz. Real impedance it is close enough to the nominal one when the frequency falls within that range and slowly increases as the frequency increases. Below 250 Hz, the impedance changes rapidly, peaking at the resonant frequency of the speaker and its enclosure.
- Some speaker labels show the actual and measured impedance value for a list of different frequencies.
- To get an idea of how frequency data translates into sound, just think that most of the bass tracks fall in a range between 90 and 200 Hz, while a double bass can reach frequencies that are perceived as a " beat in the chest "with a value of 20 Hz. The intermediate range, in which voices and most non-percussion musical instruments fall, is between 250 Hz and 2000 Hz.
Step 2. Set up a multimeter to measure resistance
This instrument sends a small amount of direct current and is unable to measure impedance directly, as this is a characteristic of alternating current circuits. However, with this method you can get a fairly accurate setting for most home audio systems (you can easily distinguish a 4 ohm speaker from an 8 ohm speaker). Use the setting with the minimum resistance range. This corresponds to 200 Ω for most multimeters, but if you can set your meter to lower values (20 Ω), you can get even more accurate readings.
- If your multimeter has only one resistance setting, it means that it automatically adjusts and finds the correct range on its own.
- Excessive direct current can damage or destroy the speaker coil; however, in this case the risk is small, as most multimeters emit a very small current.
Step 3. Remove the speaker from the outer case or open the rear door
If your speaker has no casing and has no connection, you can skip this step.
Step 4. Remove power from the speaker
The presence of current inside the loudspeaker circuits can alter the readings and burn the multimeter; turn off the power supply and, if there are any cables connected but not soldered, disconnect them.
Do not remove any wiring inserted directly into the cone membrane
Step 5. Connect the terminals of the multimeter to those of the speaker
Inspect them closely to distinguish the negative from the positive; typically, they are marked with a "+" and "-" sign. Connect the red probe of the multimeter to the positive pole and the black one to the negative pole.
Step 6. Estimate the impedance using the resistance reading
Usually, the resistance values should be 15% less than the nominal impedance shown on the label; for example, it is normal for an 8 ohm speaker to have a resistance between 6 and 7 ohms.
Most speakers have a nominal impedance of 4; 6 or 16 ohms; Unless you get abnormal results, you can safely assume that the speaker belongs to one of these categories when you need to repair the amplifier
Method 2 of 2: Accurate Measurement
Step 1. Get a sine waveform generator
The impedance of a speaker varies with frequency; consequently, you need a tool that allows you to send a sinusoidal signal at different frequency values. An oscilloscope is the most accurate solution. Any signal generator, sine waveform or sweep signal generator is fine, but some models may provide inaccurate data, due to potential difference oscillations or a poor approximation of the sine wave.
If you don't have much experience in audio testing and amateur electronics, consider purchasing tools that plug into your computer; generally, they are less accurate, but beginners appreciate the automatically generated charts and data
Step 2. Connect the instrument to the amplifier input
Read the value of the amplifier power (expressed in watts RMS) on the label or on the data sheet; those with higher power allow to detect more precise data with this type of test.
Step 3. Set the amplifier to a low electrical potential
This test is part of a standard series of inspections to measure the "Thiele & Small Parameters" and which have been designed to be conducted at a low potential difference. Reduce the gain of the amplifier, while the voltmeter - set to a potential difference for alternating current - is connected to the outputs of the amplifier itself. In theory, the meter should report a reading between 0.5 and 1V, but if yours is not very sensitive, simply set it below 10 volts.
- Some amplifiers emit an inconsistent potential difference at low frequencies and this phenomenon is the main culprit for inaccurate data during the test. If you want the best results, use a voltmeter to make sure the electrical potential is constant as you change the frequency using the waveform generator.
- Use the best multimeter you can afford; cheap models tend to be less accurate when taking measurements that you need to take later when testing. It may be helpful to purchase high-quality multimeter leads from an electronics store.
Step 4. Choose a resistor with a high resistive value
Find the power rating (expressed in watts RMS) closest to that of the amplifier, choose the recommended resistance and the corresponding (or higher) power. The resistance does not have to be exact, but if it is too high, it could cut the amplifier and ruin the test; if it is too low, the results are less accurate.
- 100 W amplifier: 2700 Ω resistor with minimum power of 0.50 W;
- 90W Amplifier: 2400Ω resistor with 0.50W power;
- 65W Amplifier: 2200Ω resistor with 0.50W power;
- 50W Amplifier: 1800 Ω resistor with 0.50W power;
- 40W Amplifier: 1600Ω resistor with 0.25W power;
- 30W Amplifier: 1500Ω resistor with 0.25W power;
- 20W Amplifier: 1200Ω resistor with 0.25W power.
Step 5. Measure the exact resistance of the resistor
This value may be slightly different than the nominal one and you need to write it down.
Step 6. Connect the resistor in series with the speaker
Connect the speaker to the amplifier by placing the resistor between them; by doing so, you create a constant current source that powers the speaker.
Step 7. Keep the speaker out of obstructions
Wind or reflected sound waves can skew the results of this delicate test. At a minimum, place the magnet side down (the conical membrane facing up) in a wind-free area. If maximum accuracy is required, screw the speaker to an open frame in a space free of solid objects for a radius of 60 cm.
Step 8. Calculate the current intensity
Use Ohm's law (I = V / R, ie current intensity = potential difference / resistance) to calculate this value and write it down; remember to enter the measured resistance value (not the nominal one) in the formula.
For example, if you found that the resistor has a resistance of 1230 ohms and the potential difference of the source is 10 volts, the current intensity is: I = 10/1230 = 1/123 A. You can express this as a fraction, to avoid errors due to rounding
Step 9. Change the frequency to find the resonant peak
Set the waveform generator to a medium or high frequency level, based on the intended use of the speaker; a value of 100 Hz is a good starting point for those dedicated to bass. Put the AC voltmeter on the speaker; Lower the frequency by 5 Hz at a time until you notice that the potential difference rises rapidly. Raise and lower the frequency until you find the point where the potential difference reaches its maximum; this corresponds to the resonance frequency of the loudspeaker "in the open" (without the enclosure or other objects that could alter it).
As an alternative to the voltmeter, you can use an oscilloscope; in this case, find the potential difference associated with the maximum amplitude
Step 10. Calculate the impedance at the resonant frequency
To do this, you can replace the resistance with impedance (Z) in Ohm's law, so: Z = V / I. The result should match the maximum speaker impedance you can get within the frequency range used..
For example, if I = 1/123 A and the voltmeter reports 0.05V (or 50mV), then: Z = (0.05) / (1/123) = 6.15 ohms
Step 11. Calculate the impedance for other frequencies
To find the different values within the frequency range in which you want to use the speaker, change the sine wave step by step. Write down the potential difference data for each frequency value and always use the same formula (Z = V / I) to get the corresponding impedance. You may find a second peak value or the impedance may be quite stable once you move away from the resonant frequency.
