How Does Increasing Speaker Impedance Affect dB Volume & Power Output?

Does changing speaker impedance make a difference in the volume (decibels, or dB) you’ll get? And what about power – how does that change?

I’d love to help clear all of this up! In this article I’ll cover:

  • What impedance is all about
  • How the dB output (volume) changes if you increase speaker impedance
  • Car and home stereo power & volume differences related to speaker impedance

…and lots of other great info. Read on to learn more!

What is speaker impedance?

What is speaker impedance diagram

Speaker impedance, measured in Ohms, is the voice coil total resistance to the flow of electric current as it operates with a musical signal.

Any time you can a voltage delivering current flowing through wire you need some amount of resistance to limit how much can flow. Likewise, an amp or stereo needs at least some speaker resistance (a speaker load, if you will) 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 has resistance depending on the frequency of the alternating current (AC) flowing in it.

This is called inductive reactance.

Speaker impedance changes with music frequencies – somewhat

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 give speakers an Ohms rating (speaker impedance rating) as it’s always within a certain range like 2 ohms, 4 ohms, and so on.

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.

In the electrical world, Ohms are sometimes represented by the Greek symbol Omega, or “Ω.”

How does speaker impedance work?

how does speaker impedance work diagram

As I mentioned earlier, a speaker’s impedance is made up of it’s resistance and inductive reactance due to the coil of wire that makes up the voice coil.

Impedance can’t be measured fully with a direct current (DC) test meter. For example:

  • If you use a digital multimeter set to read resistance in Ohms, you’ll measure only the DC resistance of the wire.
  • If you had the right equipment and could apply an AC signal you’d see an additional amount of resistance also.

Adding both of these together would give the actual “impedance” total. Like wise, the same things happen when an musical signal is driving a speaker to make sound: the amp or stereo will “see” a speaker load that’s the sum of both.

Resistance and inductive reactance math explained

Unlike standard resistance, you can’t simply add inductive reactance to it. To compute the total speaker impedance, you’d add them together by finding the square root of the sum of the squares, sometimes called the “trigonometric sum.”

Note: For the sake up general discussion, we don’t need to know the exact speaker impedance when matching speakers, talking about dB output & power, and so on.

I’m offering this as additional info to help your understanding of where it all comes from.

How does increasing speaker impedance affect power?

Speaker impedance vs power & Ohms law diagram

As you can see from the diagram I’ve provided above, increasing a speaker’s impedance doesn’t just affect volume (which I’ll cover in more detail below) but power, too. In fact, it can make a big difference in how much power you’ve got on tap with your home or car stereo or amplifier.

That’s because of Ohm’s Law and how power works:

  • Home & car stereos and amps have a certain amount of voltage they can produce to deliver power to speakers.
  • According to Ohm’s Law, if you change the resistance (speaker impedance, in this case), the power developed changes accordingly.
  • Amps and radios have an upper limit to how much voltage they can put out, so the maximum output doesn’t change if you increase the speaker impedance. It’s a fixed limit.
  • Therefore: if you increase a speaker’s impedance, less power will be developed using the same stereo or amp.

Increasing speaker impedance vs power in watts comparison graph

In this example graph, you can see how much power a 4 ohm speaker will develop with the same amplifier as an 8 ohm speaker. The 4 ohm will not only develop 2x the power of the 8 ohm speaker, but the 8 ohm one will never get the full power available from the amp.

That’s important in some cases and less important in others. For example, for home stereos where you don’t need a huge amount of power, you can use 16 ohm speakers in place of 8 ohm ones without noticing that much of a difference.

However, it’s a different case when we talk about car audio as they often need a lot more power to produce great sound or volume in a vehicle – especially with subwoofers for heavy bass. In that case, switching from 4 ohm to 8 ohm speakers means you’ll have 1/2 the power available that you used to.

Does speaker impedance affect volume? 

Increasing speaker impedance vs dB volume output comparison graph

Diagram showing an example graph comparing the dB output volume of an 8 ohm speaker at the same amplifier or stereo output level as a 4 ohm speaker. An 8 ohm speaker will produce less power and volume at the same volume output setting. When the amp is at its maximum output (maximum output voltage), the 4 ohm speaker will have a higher dB output than the 8 ohm at the same level.

In the graph above, you can see what happens when we use an 8 ohm speaker in the place of a 4 ohm speaker connected to the same amplifier. To help keep things equal I’ll compare 4 and 8 ohm speakers with the same sensitivity of 89dB at 1W/1M.

It might seem confusing at first, but what the graph is showing is:

  • The dB output for the 4 ohm speaker at each amplifier/stereo output voltage that supplies it with 1 watt, 2 watts, 4 watts, and so on. An 8 ohm speaker needs a higher amp or stereo output voltage to get the same power level.
  • The dB output for an 8 ohm speaker at those same amplifier output level when used in place of a 4Ω speaker.
What this helps show is that when you increase the speaker impedance on a stereo or amplifier designed for a lower speaker impedance, the decibel output (dB, volume) will be lower throughout the full power range.

It can also be a lot less than the correct impedance at the maximum output.

You’ll have to increase the output of the radio or amp to get the same dB volume output for the higher impedance speaker.

This means that:

  • For a higher impedance speaker, the overall volume will be a few dB lower typically, depending on the particulars of that speaker.
  • You’ll never get the full capacity out of the stereo or amplifier as you would with a lower impedance speaker like it’s designed for.

For car subwoofers, for example, that’s a big deal. For every doubling of power to a speaker, you’ll get 3dB more volume, not double the volume. 4x the power is 6dB, and so on.

Our ears work in a way such that about 10dB is considered a big difference, which takes about 10x the power. When doubling the speaker impedance, as you can see from the graphs above, you’ll “run out of steam” well before reaching the maximum power your amp could put out like it would with the correct (lower) impedance speaker.

For car stereos with lower power output (most car stereos are limited to about 14-15W or so) that’s especially not ideal.

What happens if I use a higher impedance speaker on a crossover?

Crossover shift due to speaker impedance change explained diagram

What is crossover shift?

Speaker crossovers are designed using predetermined values for the capacitors and inductors they use as filters. When a speaker manufacturer design speaker crossovers, it’s always based on the speaker impedance they’re designed to be used with.

Crossovers behave differently when the speaker load (Ohms load they see) changes. Because of this, when you change the speaker impedance you change the crossover frequency and the sound. The crossover frequency will change – typically a lot.

In other words, changing the speaker impedance will shift the crossover frequency. You may notice several problems after doing this:

  • A “harsh” sound from woofers or midrange speakers. Tweeters may sound distorted and being to “break up” the sound at volume.
  • A “thin”, weak quality to the music.
  • Gaps in the sound ranges you should be hearing.

Speaker crossovers should only be used with the speaker impedance they’re designed for or they won’t sound right.

For example, using a 16 ohm speaker with an 8 Ohm home speaker crossover won’t work correctly. It’ll sound poor and won’t work as designed. You can definitely expect to be disappointed with the sound.

Don’t forget that if your speakers aren’t of the same impedance, the higher one won’t be at the same output level as the correct one, meaning they’re not properly matched.

Do I need to match tweeter and woofer impedances for 2-way speakers?

Do I need to match speaker impedance in 2-way systems? Image man thinking

I don’t recommend mixing speaker impedances in 2-way or 3-way 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.

As you can see from the graphs I provided earlier, as the power increases the higher impedance speaker will always fall short of the also correctly matched speaker.

In 2-way speaker systems, that an even bigger problem because very often tweeters already have a higher volume output than their woofer or midrange counterparts. To make matters worse, most 2-way speakers have at least crossover they depend on.

This means in many cases changing the speaker Ohm load will also change the crossover behavior and affect the sound negatively.

I’ve seen some speakers systems where one speaker (typically the tweeter) has a different Ohms rating than the others, but in that case the designers take that into account.


About the author

Marty is an experienced electrical, electronics, and embedded firmware design engineer passionate about audio and DIY. He worked professionally as an MECP-certified mobile installer for years before moving into the engineering field. Read more »

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