It can be confusing when we talk about impedance, Ohms, and all that with regards to car or home audio speakers. Yeah, most people are familiar with speakers, Ohm ratings and more…but what does it really mean?
Exactly why is it bad if speaker impedance is too low? That’s what I’ll answer here and clear up once and for all.
In this article, I’ll cover everything you need to know:
- What exactly is speaker impedance?
- Two big reasons why a low speaker impedance can be bad…and why it matters
- Why you can use higher impedance speakers but not lower ones (and what to expect)
There’s a lot to cover so let’s jump right in!
What does impedance mean for speakers? Speaker Ohms explained
An illustrated view of the parts that make up a speaker including the voice coil. The voice coil is a tightly-wound, long length of wire that has a certain amount of resistance from the electrical conductor used. It creates magnetic fields that drive the speaker cone forward and back, creating sound as it moves air.
In the world of electricity and electronics, we need a few things to do useful work:
- A power source with voltage to move electrical current through a resistor, motor, etc to do something useful. A home or car amplifier or radio provides this.
- Electrical conductors (speaker wire) to create a path for that current to flow
- Some level of resistance to limit how much current can flow (too much current causes things to burn out, get hot, etc)
By that same token, just like other electrical devices, speakers are like little motors that use electricity flowing to turn motion (the cone) into sound we can here – that’s basically all speakers are!
What does speaker impedance mean?
Speaker impedance, measured in units of resistance called Ohms, is the total amount of resistance a speaker has to the flow of electricity.
Speaker impedance comes from two things:
- The resistance of the long winding of wire that makes up the voice coil
- A special property created when wire is wound into a coil called inductance
Just like you can’t have a short circuit across a battery, an amp or stereo needs some amount of speaker resistance to limit how much electrical current the radio or amp tries to supply.
Speaker voice coils use a very long length of wire that’s tightly wound into the voice coil necessary to produce magnetic fields to create motion of the cone. Because of this length, there’s always a certain amount of resistance that is part of what makes up speaker impedance.
The resistance of a given speaker is almost always a few units of resistance, measured in Ohms.
What does inductance mean? Why it matters in speakers
Inductors are very useful electrical parts that take advantage of inductance. Inductance is a property of electrons flowing through a wire loop and the magnetic fields that build up because of it. Similarly, speakers have inductance due to their voice coils, although a small amount.
Coils of wire have an interesting side effect that happens unlike straight sections of wire. The voice coil’s wire winding forms a loop that has an electrical property called inductance. When an alternating frequency like a musical signal is applied to a coil with inductance there’s an opposition to the flow of electrical current due to the magnetic fields present.
This is called inductive reactance and it’s different from resistance as it changes as the frequency changes; resistance stays the same.
For speakers, this matters because it means that the total resistance is made up of the two things I mentioned: wire resistance and inductive reactance. The name used to describe this total is impedance.
For speakers, this means that impedance (the total resistance) changes slightly as music plays because of the changing sound frequencies. However, the good news is that we can still categorize 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 range of the speaker assigned to categories like 2 ohms, 4 ohms, 8 ohms, and so on. This is how we match speakers to a car or home amplifier, radio, and so on.
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 the math works (yeah, it’s a little complicated!)
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. It’s very important for speaker crossovers, however.
Why is it bad if speaker impedance is too low?
Just like any other device connected to an electrical power source, the speaker impedance will determine how much or how little current a home or car receiver, amplifier, etc will produce. The speaker impedance also affects how some speaker components such as speaker crossovers behave too.
What happens if speaker impedance is too low?
You can connect a higher speaker impedance in most cases without any problems (at least not major ones). A radio, home or car amplifier, etc will still produce sound and run at normal or low temperatures. That’s because a speaker with a higher impedance than expected will reduce how much electrical current the audio source tries to produce.
As a side effect, you’ll get sound but with much lower power output than you would with the correct speaker load. Car stereos or amps, for example, have to work with lower voltages than home stereos so they need a lower impedance 4 ohm speaker typically to produce more power.
Home stereos, on the other hand, have higher voltage available and can use a higher speaker impedance (8 ohms, typically).
Internal view of an amplifier. When connected to a speaker impedance load that’s too low, the amp will begin to get very hot and this can burn out the output transistors as they can’t handle the heat caused by trying to supply excessive current to a lower speaker load.
However, using a lower speaker impedance is bad because it causes the radio or amp to attempt to put out twice as much (or more!) current than it’s designed for. Your home or car stereo will get very hot quickly and if you’re lucky will go into a self-protect mode and shut itself off.
However, in my experience, it’s pretty common for the output stage electronics to burn out when connected to a lower speaker load than they should be. The high-power transistors in a home or car amplifier or stereo are only rated for a certain amount of heat & electrical current.
When they’re forced to try and handle an amount outside that range they become super hot and start to break down permanently. It doesn’t take long before the damage is permanent and they no longer produce sound.
Why does speaker impedance matter for crossovers?
Speaker crossovers work to separate the sound sent to certain speakers for improved sound, reducing distortion, and to give you more control over how they’re used. For example, they block bass that tweeters can’t produce and highs that a woofer can’t produce well. However, they’re designed for a specific speaker impedance. Changing the speaker impedance affects the sound.
Speaker crossovers are amazingly helpful for getting better sound with speakers. Even the cheapest, most basic capacitor connected inline with a tweeter working as a high-pass filter makes a big difference in the sound.
The result is cleaner sound and avoiding possibly damaging it when bass sounds are played.
The catch is that because of how crossover components (capacitors and inductors) behave, they’re designed for specific speaker loads and can’t be used with other Ohm loads without affecting the sound output.
Crossover shift when using different impedance speakers
When you change the speaker impedance connected to a speaker crossover it can significantly shift the crossover’s cutoff frequency. As a general rule:
- Halving the speaker impedance (ex.: 8ohms to 4 ohms) doubles the frequency
- Doubling the speaker impedance (ex: 8 ohms to 16 ohms) halves the frequency
That’s bad because it allows the speakers to be sent a sound range they’re not suited for. In the case of tweeters, bass & midrange are bad because they can’t produce it properly. Similarly, many woofers can’t produce high frequency sounds well.
The end result in either case is poor sound that’s a lot worse sounding that it should be. If you change the speaker Ohms load you’ll have to replace the speaker crossover as you’ll need different parts values for it to work the same.
Is 8 or 4 ohm better? Is higher or lower impedance better for speakers?
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.
By industry tradition, 8 ohms are used for home and some theater speakers. 4 ohm speakers are generally used for car and marine audio, with some 2 ohm models also (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.
As a matter of fact, car stereos can only put out about a small 15-18 watts RMS per channel, despite the exaggerated peak power ratings you may see in advertisements. That’s because they only have about 12 volts to work with and have to divide that in half in order to produce AC waves that drive a speaker.
Car amplifiers are able to deliver huge amounts of power to 4 and 2 ohm speakers. They use an internal “inverter” power supply that steps up the +12V supply to higher voltages. This way they’re able to supply much more power to 2 or 4 ohm speakers than would be possible otherwise.
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