Impedance is kind of a “scientific” sounding word, right? At first glance it’s fairly confusing and you might not know how much – if at all – it matters for hooking up speakers.
But what is car speaker impedance? As it turns out, it’s really important and can have some serious consequences on your vehicle’s sound, your car amplifier, and more. Let’s dig in!
- What does speaker impedance mean?
- How to tell the impedance of a car speaker
- Measuring speaker impedance for tweeters & after crossovers
- Car speaker measured Ohms to impedance chart
- Can I hook up 8 ohm speakers to a 4 ohm amplifier or radio?
- What is better: 8 ohm or 4 ohm speakers? Are 2 ohm or 4 ohm speakers better?
What does speaker impedance mean?
Speaker impedance, measured in Ohms, is the voice coil total resistance to the flow of electric current as it operates with a musical signal.
Just like you can’t have a short circuit across a battery, an amp or stereo needs some amount of speaker load impedance to limit how much electrical current the radio or amp tries to supply.
Unlike straight wire that goes from point “a” to point “b” when you hook up power, the voice coil’s wire winding forms a loop that has an electrical property called inductance. Inductance is a bit different from resistance as it changes as the frequency changes. This is called inductive reactance.
For car speakers, this means that the real impedance (the total resistance) actually changes a little bit as music plays! However, the good news is that we can still categorize car speakers according to an Ohms rating since it’s always pretty close.
When we talk about the impedance of a speaker, most of the time people are referring to the category (general range) of the speaker as used to match home or car stereo amplifiers.
How does speaker impedance work?
When a musical signal (made up of alternating current) is applied to a speaker it generates magnetic fields as current flows through the tightly wound wire coil. Interestingly enough, a coil of wire develops magnetic fields that resist the flow of the current (resistance, also called reactance in this case).
Similarly, many other electrical components like motors deal with the same electrical resistance as alternating current (AC) is applied.
How to calculate the total impedance (if you like!)
Because of how inductance works and the physics involved, the speaker “impedance” (total resistance) isn’t the sum of the resistance and the inductive reactance. Instead it’s the “algebraic” sum, meaning it’s the square root of the sum of the squares. You may remember this kind of math from trigonometry class.
Speaker impedance isn’t as simple as just adding the measured DC resistance of the coil wire and the inductive reactance for a given frequency.
Instead, speaker impedance is found from the algebraic sum of the coil’s wire resistance and inductive reactance. You can find this by squaring each and then taking the square root of the two numbers added together.
Inductive reactance is commonly written as “Xl”, pronounced “X sub L” and is measured in units of Ohms just like resistance. Inductance is measured using a unit called the “Henrie” and commonly noted with an “H”: “uH” for microHenries, “mH” for milliHendries, and so on.
There’s also a corresponding value for capacitors called capacitive reactance (Xc) but that doesn’t usually apply for speaker voice coils.
How to tell the impedance of a car speaker
There are a few different ways to tell what a car speaker’s impedance is – even if it’s missing the label or it’s not printed on it anywhere.
Here’s what you need to know:
- A speaker’s impedance is usually listed on the speaker magnet, packaging, and/or box and specifications. Unfortunately, it’s not always the case as some manufacturers might not have printed it on the speaker.
- If the Ohm rating (impedance) is not available on the speaker, you can measure the impedance of a speaker using a test meter set to the Ohms (resistance) function. This will give the resistance of the voice coil which will let you determine the speaker’s impedance range/category such as 2 ohms, 4 ohms, 8 ohms, etc.
- Unlike when a speaker is playing, measuring resistance with test meter won’t give you the total impedance – just the DC resistance of the speaker coil. However, that’s all you need to figure out the Ohms rating of your car speaker.
Long story short, if your speaker doesn’t have the impedance listed anywhere or you can’t find the manufacturer’s specs, the best thing to do is to measure it.
That’s the best way as you can be 100% sure of what you’re dealing with – especially if you need to match the impedance to an amplifier, car stereo, or crossover.
How to measure the impedance of a car speaker
It’s easy to measure car speaker impedance using a test meter set to read resistance (Ohms). Once you get a reading you can tell what Ohms rating your speaker is.
To measure the impedance of a car speaker you’ll need a multimeter (test meter with multiple functions) or a dedicated Ohm (resistance) meter. Digital multimeters are inexpensive and easy to find these days so I recommend using one of those.
- Turn on the meter and set it to measure resistance (Ohms) on the lowest range. This is usually the x1 range, 0-10, 0-20, or auto range setting.
- Disconnect one or both speaker wires from the speaker to avoid a false reading due to other resistance that may be connected to it.
- Hold the probes tightly against the speaker terminals on a clean, bare metal spot. The meter should quickly settle to a reading. The meter will show the resistance of the voice coil inside the speaker.
- Use the meter reading to determine the closest approximate speaker impedance (see my chart below for help).
- For speakers inside a box or enclosure there may be a crossover connected elsewhere which can interfere with your reading, so be sure to disconnect at least one speaker wire if possible. Subwoofers are usually fine to measure while installed in a subwoofer box.
As I mentioned above, the goal here isn’t to try and measure the perfect impedance rating.
Remember that you won’t measure exactly 4 ohms, 8 ohms, etc. You’ll measure an Ohms value that’s close to that and will help you tell the actual Ohms/impedance range of your speaker.
How to set your test meter for measuring car speakers
Shown are some example test meter resistance range settings to use for typical test meters.
As I mentioned earlier, it’s important to use the correct resistance range on your meter when measuring speaker impedance. that’s because the wrong setting may display nothing or give you the wrong idea that perhaps the speaker is blown when it isn’t.
If you’re not sure, check the test meter’s manual. Many modern digital meters often have an auto setting that will automatically adjust for the Ohm measurement it detects and will display the reading & decimal places accordingly. Other meters require you to select the correct range manually.
As a general rule, use the lowest range that includes 0-10 ohms (or similar).
Once you’ve got your measurement, use my speaker impedance chart to find the next closest speaker impedance value listed.
Measuring speaker impedance for tweeters & after crossovers
Tweeters often are supplied with a high-pass crossover in the form of a capacitor. To get a correct reading you’ll need to disconnect it or measure around it. Be sure to disconnect the tweeter from an amp or head unit!
Measuring the speaker impedance where crossovers are in place is a problem. That’s because capacitors, which are commonly on tweeters as a high-pass filter, appear to behave like an open circuit when measuring resistance as the capacitor charges.
You’ll want to measure around the capacitor if used or disconnect one capacitor lead or one tweeter wire.
For 2-way speakers, there may or may not be a crossover used on the woofer. Often there’s an inductor in series with it. The good news is that directly reading resistance across a speaker and an inductor doesn’t make much difference – inductors have a tiny resistance value.
In fact, they’re usually in milliOhms (thousandths of an Ohm) which is almost nothing. However, as a general rule, it’s best to disconnect speakers from their crossovers before measuring Ohms.
Car speaker measured Ohms to impedance chart
|Measured Ohms*||Speaker impedance rating|
|3.1-4.0 ohms||4 ohm|
|6.0-8 ohms||8 ohm|
|1.2-2 ohms||2 ohms|
|4.0-6 ohms||6 ohms|
|0.5-1.0 ohms||1 ohm**|
|12-16 ohms||16 ohms**|
To use this chart, take the speaker resistance measurement you got from your test meter reading and use it to compare to the measurements here. Your car speaker should fall into one of the common ranges you see above.
Can I hook up 8 ohm speakers to a 4 ohm amplifier or radio?
This graph shows what happens when you use an 8 ohm speaker in the place of a 4 ohm one. The 8 ohm speaker will work – however, it comes with a price. Since the 8 ohm speaker isn’t matched to the 4 ohm amp, it can only receive up to 1/2 the power output (and a lower volume) than a 4 ohm speaker would.
Using a speaker that’s not properly matched to an amplifier or car stereo can have minor – or major – consequences.
Using an 8 ohm speaker in place of a 4 ohm won’t hurt anything. However, it can only develop 1/2 the power output of a 4 ohm speaker meaning lower volume. It also won’t work properly with speaker crossovers since it will shift the cutoff frequency.
For example, if you were to use some home stereo 8 ohm speakers or subwoofer instead of 4 ohm speakers, you’d notice the volume would be lower than when using 4 ohm ones. That’s because a speaker needs more and more power output to increase the volume more and more.
Car amplifiers & car head units don’t have much supply voltage to work with unlike home stereo receivers and amps. That means they need a lower impedance speaker to develop the same amount of power by letting more current flow.
I also don’t recommend mixing 8 and 4 ohm car speakers because they won’t have the same volume level once you turn up the volume. That means the sound won’t be right and you’ll be left having to deal with some sound frequencies being poor after a certain point.
What is better: 8 ohm or 4 ohm speakers? Are 2 ohm or 4 ohm speakers better?
8, 4, and 2 ohm speakers aren’t necessarily “better” than one another. The correct answer is that it depends on the application and what stereo or amplifier is being used. The best impedance is the one that matches an amplifier or stereo’s impedance spec correctly.
Traditionally 8 ohms are used for home and some theater speakers. 4 ohm speakers are generally used for car use, with some 2 ohm models used at times (usually subwoofers).
- 8 ohm speakers are used in home stereo systems and require 1/2 the current of a 4 ohm speaker. That means they can use smaller speaker wire as they can take advantage of home electrical systems that have a high voltage supply for driving speaker amplifiers.
- 4 ohm speakers are used because car stereos and amplifiers (particularly car head units) can’t make large amounts of power in speakers as they have a very low 12V power supply. Reducing the speaker impedance from 8 to 4 means we can double the power for the same output voltage.
In fact, car stereos can only put out about a measly 15-18 watts RMS per channel, despite the exaggerated peak power ratings you may see in advertisements. That’s because they can only work with a 12V supply to develop power across a speaker.
Car amplifiers are able to deliver huge amounts of power to 4 and 2 ohm speakers by using an internal power supply that generates higher voltages for amplifying the speaker signal. Without that, it wouldn’t be possible to drive car speakers with tons of power to get boomy bass like many people enjoy.
When are 2 and 1 ohm speakers used?
Factory-installed amps sometimes use 2 or 1 ohm speakers to develop more power without spending the money on amplifier designs using an improved power source.
2 and even 1 ohm (yes, 1 ohm!) car audio speakers are rarely used except for car subwoofers and some special cases for main speakers. Some factory-installed premium amplified car audio systems use lower impedance speakers to “cheat” using a “real” amplifier and save money.
That’s because they use the 2 or 1 ohm speaker to develop more power at each speaker without having to supply an amplifier with an internal power supply as is normally done. While it does technically work, it’s not a substitute for simply using a proper amplifier.
They introduce other problems, like not being compatible with standard 4 ohm speakers when it’s time to upgrade or replace faulty ones. They also still can’t produce as much power as a decent aftermarket amp can with 4 ohm speakers, meaning you’ll still end up needing to replace them.
Speaker impedance matching
In order to get the most enjoyment (and power) for your dollar – along with avoiding damaging audio electronics – it’s important to match the speaker impedance (impedance load the amp sees at its output).
Here are some simple reasons to help you understand what happens when you don’t:
- Using a speaker properly matched to the amplifier or radio’s minimum Ohms rating allows it to deliver the maximum output power it’s designed for.
- Using a higher than specified speaker impedance will work. However, the speaker won’t be able to develop the full power that you paid for. As I mentioned earlier, a speaker needs more power to produce more volume, meaning you’ll lose volume because of this.
- Using a lower than specified impedance speaker will cause an amp or stereo to run hot and can permanently damage the output transistors. Don’t do it!
While in some cases an amplifier might be able to shut itself off before it becomes damaged when a lower speaker impedance is used, don’t ever assume it will. Sometimes the damage still happens and you’ve just ruined an amp.
Most car stereos don’t have any type of overheating or high-current self-protection circuitry built-in so they’re likely to have their output stages destroyed.
Subwoofer impedance options
It’s a little bit different when we’re talking about car audio subwoofers, but the same rules hold true. Since a subwoofer channel on an amp usually has a lot power output on tap it’s not always an issue when using say a 4 ohm sub vs a 2 ohm sub with a 2 ohm min. amp.
However, as a general rule, it’s best to match the subwoofer impedance to get the power you’re paying for.