The Complete Guide To Speaker
Impedance (2Ω, 4Ω, 8Ω & More)
Written by Arthur in Speaker Specifications,Speakers
Whether it's on the specification sheet or written as a number of ohms
(Ω) on the back of the speaker, impedance is something we'll see or
hear of at some point when using speakers. The seemingly mysterious
specification of speaker impedance should be understood in order for
us to fully comprehend how speakers work.
What is speaker impedance? Speaker impedance, measured in
ohms (Ω), is the electrical impedance (AC resistance)
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encountered by the audio signal (alternating electrical current) at
the input of the speaker driver. Impedance affects a speaker's
load on an amplifier and is an important spec when matching
speakers and amplifiers.
In this article, we'll discuss the complex topic of speaker impedance in
great detail to understand its effects on speaker performance, how to
optimally match an amplifier and speaker, and the differences between
common nominal speaker impedance values.
Table Of Contents
The Definition Of Electrical Impedance
Source & Load Impedance
Power Matching Vs. Voltage Bridging
Speaker Impedance & Power Demands
Damping Factor
Active Vs. Passive Loudspeakers
Impedance Of Speaker Level Vs. Line Level
Speaker Impedance Specifications
Actual Speaker Impedance
Understanding Phase & Impedance
Speaker Impedance Factors
Speaker Driver Design
Number Of Speaker Drivers
Enclosure
Wiring A Single Speaker Vs. Wiring Multiple Speakers
Wiring Multiple Speakers In Parallel
Wiring Multiple Speakers In Series
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Impedance Of Alternative Speaker Types
Electrostatic Loudspeakers
Magnepan Loudspeakers
Air Motion Transformers
Related Questions
The Definition Of Electrical Impedance
Let's begin this article with a general description of impedance:
Electrical impedance is a measurement of the
opposition/resistance to an alternating current in a circuit
when a voltage is applied.
Impedance is measured in ohms (Ω) just like electrical
resistance and can even be thought of as a type of “AC
resistance” in an AC circuit.
Technically speaking, impedance is the combination of DC resistance
and any reactance in an AC circuit.
Resistance is defined simply as the opposition to the flow of electric
current.
Reactance is the opposition of a circuit element to the flow of current
due to that element's inductance or capacitance.
It's easiest to think of impedance as AC resistance in the context of
audio. However, we'll explain the full impedance of speakers in this
article.
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Because impedance acts on AC circuits rather than DC circuits, there
are frequency and phase components.
As we'll get to shortly, speaker impedance generally varies across the
audible range of frequencies and, thus, a nominal value is typically
used to represent the impedance.
Every electrical device that has AC circuitry has an electrical
impedance. Therefore, audio equipment, which passed AC audio
signals, has impedance.
This is certainly the case with speakers, which have input impedances
(and, in some cases, output impedances).
Speaking of audio devices, microphones and headphones also
have impedance. To learn more, check out the following My New
Microphone articles:
• Microphone Impedance: What Is It And Why Is It Important?
• The Complete Guide To Understanding Headphone Impedance
• What Is Amplifier Impedance? (Actual Vs. Rated Impedance)
A speaker's impedance value is an important specification that helps
us determine which amplifiers will best suit the speakers for optimal
performance. This has to do with the source and load impedances of
the two devices.
Source & Load Impedance
In terms of audio, the source is the device that outputs an audio signal,
and the load is the device that receives the audio signal at its input.
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When connected to a power amplifier, a loudspeaker acts as the load
while the amplifier acts as the source.
As we'll see in the next section, the load impedance should be
magnitudes more than the source impedance for optimal signal
transfer from the source to the load.
Power Matching Vs. Voltage Bridging
We want optimal signal/voltage transfer rather than power transfer
when connecting a speaker to an amplifier.
In other words, we want as much of the amplified signal from the
amplifier to drive the speaker as possible. It's okay if the power
transfer is less than ideal (speakers are notoriously inefficient anyway).
This brings us to a conversation on power matching versus voltage
bridging.
It can be confusing because we're typically tasked with “matching an
amplifier and loudspeaker” when looking for compatible devices.
However, we are not concerned with power matching for maximum
power transfer. Rather, we want optimal voltage transfer, which is
technically referred to as voltage bridging.
To better understand the difference, let's look at a simplified voltage
divider to garner an intuitive comprehension of the connection
between a power amplifier and a loudspeaker:
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As we've mentioned before, the amplifier is the source, and the
loudspeaker is the load. Therefore:
VS is the source voltage or the voltage (signal strength) outputted
by the amplifier
ZS is the source impedance or the output impedance of the
amplifier
ZL is the load impedance or the input impedance of the
loudspeaker
VL is the load voltage or the resulting voltage (signal strength)
that will drive the loudspeaker
We want as much signal transfer (voltage transfer) as possible from
the amplifier to the speaker.
Power matching (impedance matching) is the result of matching the
source and load impedances of two devices. This yields maximum
power transfer between the source and load but with only 50%
efficiency (a 6 dB load loss).
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In other words, the voltage VL will only be half that of VS if ZS = ZL.
Voltage bridging (impedance bridging) is the result of having ZL
much greater than ZS. This yields maximum voltage transfer and much
higher efficiency.
To prove the above points, we look at the source and load circuit
simplified as a voltage divider. Therefore:
VL / VS = ZL / (ZL + ZS)
And: VL = VS • ZL / (ZL + ZS)
Let's say that ZL was equal to ZS. In this scenario, VL would be 1/2 of
VS (the voltage or strength of the connected device's output signal).
Half the signal strength was lost!
Let's now say that ZL was 9 times ZS. In this scenario, VL would be
9/10 of VS. 90% of the signal strength was transferred!
So then, a much higher load impedance is required for optimal signal
transfer. As a general rule, the load Z should be at least 10x that of the
source Z.
Therefore, having the speaker's impedance much higher than the
actual output impedance of the connected amplifier is a sought-after
proposition. It improves signal transfer and efficiency.
Speaker Impedance & Power Demands
Going back to the maximum power transfer for a moment, we can
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state that lower speaker impedances actually demand more power.
We can see this in the power ratings of power amplifiers. For example,
let's look at the Crown Audio XLi 2500 (link to check the price on
Amazon). This stereo power amplifier is designed to drive 8Ω speakers
and 4Ω speakers. As we see below, the amplifier must be able to
provide more power to the 4Ω speaker:
Crown Audio is featured in My New Microphone's Top Best Power
Amplifier Brands In The World.
Crown Audio XLi 2500 Power Specifications:
4Ω Dual: 750W
8Ω Dual: 500W
8Ω Bridged: 1500W
Power can be calculated as voltage squared divided by resistance.
Using this equation, we can substitute resistance for impedance to get
the following:
PL = VL2 / ZL
Intuitively, this tells us that a speaker with a lower impedance (ZL) will
require more power to achieve the same voltage (signal level) across
its driver.
Therefore, we can say that speakers with lower impedances are harder
to drive. They are more taxing on the amplifier and actually require
more powerful amplifiers to drive them properly.
This is critical information to know when “matching” speakers and
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amplifiers.
Note that speaker impedance specifications are typically given as
nominal or “average” impedance values (more on this later).
Amplifier output impedance specs, however, are generally given as
rated values. This means that the amp's “impedance rating” tells us
the compatible speaker impedance ratings the amp will be able to
drive properly. It doesn't actually tell us the real output impedance of
the amplifier.
For more information on speaker power ratings, check out my
article Complete Guide To Speaker Power Handling & Wattage
Ratings.
Damping Factor
Before wrapping our discussion on source and load impedance, it's
important to discuss damping factor.
Damping factor (DF) is technically the ratio of nominal loudspeaker
impedance to the total source impedance that drives the loudspeaker.
This includes the impedance of the amplifier (source) and the speaker
cable.
DF = ZL / ZS
High DFs tell us that the amplifier has more control over the speaker's
moving driver. This is another benefit of having high speaker input
impedance relative to the amplifier's output impedance.
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A higher damping factor improves the transient response of the
amplifier-speaker relationship. Also, it allows the amplifier to damp
(slow down and stop the speaker from moving) when the audio signal
stops.
Lower damping factors yield less amplifier control and can lead to
undefined “loose” speaker sound output. This is particularly true in the
bass frequencies.
So for the sake of signal transfer, system efficiency, and speaker
control, having a high speaker (load) impedance is paramount!
As a rule of thumb, a damping factor of 10 or more is optimal. In other
words, a speaker with an input impedance 10x or more than the
amplifier's output impedance is preferred. Most systems will make this
true.
An Important Note On Active Vs. Passive
Loudspeakers
Before we go any further in our journey to understanding speaker
impedance, let's discuss active and passive loudspeakers.
Passive loudspeakers do not have built-in amplifiers and do not require
power to function. Rather, they rely on external amplifiers to provide
them with signals strong enough to drive them properly. Passive
speaker inputs are designed to expect speaker level signals.
Up until this point in the article, we've been discussing passive
loudspeakers.
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Active loudspeakers, conversely, do have built-in amplifiers and
require power to function.
Active loudspeakers, then, can have line inputs, instrument inputs or
even mic inputs. Their built-in amplifiers will boost these low-level
signals up to a level that can properly drive the speaker drivers.
Know that the voltage bridging and damping factor information listed
above still holds true for active speakers. However, this all happens
inside the speaker rather than between the speaker and a separate
power amplifier, as is the case with passive loudspeakers.
So what about the inputs of active speakers?
Well, as we've discussed, the inputs of active speakers can be
designed to accept a variety of different signal types. These different
signal types actually require different load impedances.
Mic inputs are designed to accept mic level signals and typically have
impedances in the range of 1 kΩ to 10 kΩ.
Line inputs are designed to accept line level signals and typically have
impedances in the range of 10 kΩ to 50 kΩ.
Instrument inputs are less regulated and can have impedances from
47 kΩ and below to 10 MΩ and above.
Therefore, the impedance specifications of an active loudspeaker will
not be in the range of 1Ω to 16Ω like a passive loudspeaker. Rather,
they will be in the ranges stated above, depending on the type of
inputs available in the active loudspeaker.
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As an example, let's look at the input impedance specifications of
the QSC KW153 (link to check the price at B&H Photo/Video): a 3-way
active PA speaker with a 15″ woofer.
QSC KW153
Input Impedance (Ω):
Channel A XLR /¼”:
Mic gain setting:
0 dB: 38 kΩ (Balanced) 19 kΩ (Unbalanced)
+12 dB: 10 kΩ (Balanced) 5 kΩ (Unbalanced)
+24 dB: 2.66 kΩ (Balanced) 1.33 kΩ (Unbalanced)
+36 dB: 660 Ω (Balanced) 330 Ω (Unbalanced)
Channel B XLR /¼”: 38 kΩ balanced / 19 kΩ unbalanced
Channel B RCA: 10 kΩ
In the above example, Channel A is a mic input, and Channel B is a line
input.
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This is all to say that active speakers will not have the typical 1, 2, 4, 6,
8, 12 or 16-ohm input impedance we'll find in passive models.
QSC is featured in the following My New Microphone articles:
• Top Best Subwoofer Brands (Car, PA, Home & Studio)
• Top Best PA Loudspeaker Brands You Should Know And Use
• Top Best Loudspeaker Brands (Overall) On The Market Today
For more information on active and passive loudspeakers, check
out My New Microphone's post titled What Are The Differences
Between Passive & Active Speakers?
Impedance Of Speaker Level Vs. Line Level
Why does speaker level work with lower impedance than line level?
Though there are plenty of reasons (including standardization and
history) for this, electrical current is a main reason.
Remember that impedance is the resistance to electrical current.
Higher impedance means less current, while lower impedance means
more current.
Too much electrical current can be quite destructive to sensitive
electronics and requires more heavy-duty components to handle it
properly. This adds significant cost to audio equipment.
For example, passive speaker crossovers, which deal with speaker
level (high current) signals, are built more robustly than active speaker
crossovers that deal with line level (low current) signals and are built
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less robustly but with greater precision.
Audio recording, processing, mixing, storage and playback all happen
around nominal line level. Electronics (including analog-to-digital and
digital-to-analog converters) are more easily (cost-effectively)
designed at line level due to the low-current nature of line level.
A speaker is responsible for oscillating back and forth to reproduce
audio signals as audible sound. Its motor (made of a voice coil and
magnetic structure) requires speaker level signals with significant
electrical energy to convert into mechanical wave energy (sound
waves).
The relatively robust nature of the speaker transducer means it needs
more current. Lowering the impedance is one way of achieving this.
Note that the voltage is also typically higher at speaker level than at
line level.
The increase in current also causes speaker cable to be relatively thick
(lower gauge) than typical audio (line level or mic level) cable.
That's all a bit of a ramble. I just wanted to state how interconnected
all the amplifier and speaker specifications, including impedance, are.
Speaker Impedance Specifications (Nominal,
Actual & Minimum)
The specification for speaker impedance that we'll find on the
manufacturer's datasheet typically refers to the speaker's nominal
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impedance.
These nominal impedance values are typically given as 2Ω, 4Ω, 6Ω,
8Ω, 12Ω or 16Ω.
The IEC (International Electrotechnical Commission) standard for rated
speaker impedance is as follows: the minimum impedance shall not fall
below 80% of the nominal (rated) impedance over the defined
frequency range of the speaker.
For example:
4 Ω speakers have a minimum impedance no less than 3.2 Ω
8 Ω speakers have a minimum impedance no less than 6.4 Ω
The defined frequency range of the speaker is between the -10 dB low
point and high point across the speaker's average (0 dB) sensitivity.
Some speakers, like the Electro-Voice ZLX-15 (link to check the price
at B&H Photo/Video), give a nominal impedance rating and a minimum
impedance rating to help give us a better idea.
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Electro-Voice ZLX-15
Nominal Impedance: 8 Ω
Minimum Impedance: 7 Ω
Electro-Voice is featured in the following My New Microphone
articles:
• Top Best Subwoofer Brands (Car, PA, Home & Studio)
• Top Best PA Loudspeaker Brands You Should Know And Use
• Top Best Microphone Brands You Should Know And Use
Of course, this does not give us the full picture of the speaker's
frequency-dependent impedance. It only tells us the minimum
impedance at any given point across the speaker's frequency
response and does not set a limit of how high the impedance will be at
other frequencies in the speaker's response.
In addition, this is only if the manufacturer is following the rather loose
standard! The standard is purposely made simple due to the incredibly
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complex nature of speaker impedance and the difficulty of mapping
these complexities with a standard.
The rated impedance values of speakers (and their power amplifiers)
are often a way for manufacturers to state clearly (or unclearly) what
their products are designed to handle appropriately.
It is then the responsibility of the user to follow the “guidelines” laid
out in amplifier and loudspeaker specifications sheets to get the best
results and avoid damage to their equipment.
The main point here is that there's much more to know about speaker
impedance.
Lower impedances mean higher currents. Higher currents mean more
heat dissipation in the amplifier and speaker. This is why power amp
manufacturers specify the lowest load impedance (the lowest safe
impedance value of the connected speaker(s)).
So we know that manufacturer-specified impedance ratings are
typically nominal values.
Actual Speaker Impedance
Is there a way to get information on the actual impedance ratings
across the entire frequency response of a speaker?
Unfortunately, manufacturers do not typically share the impedance
graphs of their speakers. Fortunately, there are third-party testers that
measure and publish impedance graphs of various loudspeakers.
These graphs mark out frequency along the X-axis and impedance
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and phase along the Y-axis.
Stereophile is one such company. Check them out at stereophile.com
Let's have a look at a few examples of speaker impedance graphs:
Aperion Intimus 533-T
The Aperion Intimus 533-T (pictured below) is a 2.5-way floorstanding
speaker with an interesting impedance specification listed as “5-10
Ohms”.
Aperion Intimus 533-T
Its impedance graph is as follows:
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Aperion Intimus 533-T Impedance & Phase Graph
Photo Courtesy Of Stereophile
Dynaudio Excite X12
The Dynaudio X12 (pictured below) is a high-end 2-way bookshelf
speaker/monitor with an impedance specification of 4 ohms.
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Dynaudio Excite X12
Dynaudio is featured in the following My New Microphone articles:
•Top Best Home Speaker Brands You Should Know And Use
• Top Best Loudspeaker Brands (Overall) On The Market Today
• Top Best Studio Monitor Brands You Should Know And Use
Its impedance graph is as follows:
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Dynaudio Excite X12 Impedance & Phase Graph
Photo Courtesy Of Stereophile
Revel Ultima Salon2
The Revel Ultima Salon2 (pictured below) is an audiophile-grade 4way floorstanding speaker with an impedance rating listed as “6 ohms
(nominal) 3.7 ohms (minimum @ 90 Hz)”.
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Revel Ultima Salon2
Its impedance graph is as follows:
Revel Ultima Salon2 Impedance & Phase Graph
Photo Courtesy Of Stereophile
In each of the impedance graphs above, we have graphed lines for
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both impedance and phase.
We should also notice that the speaker impedance graphs show
significant spikes in the speaker's frequency response at one or more
frequencies. This is due to the resonances and reactance of the
driver(s) and the enclosure(s).
Of course, speakers with multiple drivers are wildly complicated to
understand in terms of impedance. Furthering our understanding of
actual speaker impedance will be the focus of the next section.
Understanding Phase & Impedance
The phase of the speaker, put simply, shows us a positive value when
the driver resonance is “pulling” the electrical audio signal up towards
resonance and a negative value when the driver resonance is “pulling”
the electrical audio signal down to the resonance.
At the resonance frequencies (where impedance peaks), the phase is
midway through a flip and is effectively 0°.
Technically, the phase angle determines the degree at which the
current will lead or lag the voltage waveform in a reactive circuit.
Reactance refers to an AC circuit's opposition to the change in
electrical current when a voltage is applied and is a major component
of overall impedance.
In inductive circuits, the current lags behind the voltage, yielding a
positive phase angle. In capacitive circuits, the current will lead the
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voltage, yielding a negative phase angle.
Speakers have both inductive and capacitive properties, and so the
phase angle will alternate.
The phase angles of a speaker actually tell us more about the role of
the amplifier than about the speaker, even though the phase angles
are inherent to the speaker design.
At a phase angle of 45º, the amplifier will have to dissipate twice as
much power (and twice as much heat) than at a phase angle of 0º
(which would mean that the impedance of the load/speaker was purely
resistive.
Here is a table that compares the power dissipated by an amplifier
(and the heat dissipated) to the power transferred through the
loudspeaker at various phase angles:
Phase
Angle
Power Dissipated By
Amplifier
Power Transferred To
Speaker
Power
Factor
0º
1.00
1.00
1.000
15º
1.38
0.94
0.966
30º
1.76
0.75
0.866
45º
2.00
0.50
0.707
60º
1.66
0.24
0.500
75º
1.20
0.08
0.259
90º
4.00
0.00
0.000
Note that 90º is an impossibility in the real world.
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The phase angle will pass through 0º at the regional peaks and
troughs of impedance.
The peaks are produced by resonant frequencies and back EMF, while
the troughs happen when the reactance portion of the speaker
impedance drops to zero.
So, at frequencies with a phase angle of 0º, the speaker's impedance
is purely resistive. This means that any change in voltage has an
immediate effect on the charge in current through the speaker driver.
Understanding Speaker Impedance & The
Factors That Determine It
So far, we have a pretty solid idea of what speaker impedance is.
To recap:
Impedance is the opposition to the flow of alternating current and
audio signals are alternating currents. Impedance, therefore, has
magnitude and phase.
Speaker impedance specifications are generally nominal or
“average”.
The IEC standard for rated speaker impedance states the
minimum impedance shall not fall below 80% of the nominal
(rated) impedance over the defined frequency range of the
speaker.
Speaker impedance is frequency-dependent.
Loudspeakers act as loads and amplifiers act as sources. Optimal
voltage/signal transfer happens when the load impedance
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(speaker impedance) is much greater than the source impedance
(amplifier output impedance).
All else being the same, speakers with lower impedances are
more difficult to drive and demand more power from the
connected amplifier.
A higher speaker impedance means a high damping factor which,
in turn, allows the amplifier more control over the speaker
driver(s).
Impedance is high at the resonance frequencies of the driver(s)
and, enclosure(s).
All speakers have impedance ratings but we're more concerned
with passive speakers that rely on external amplifiers.
Active/powered speakers have built-in amps with mic, line and/or
instrument inputs rather than speaker inputs.
With that knowledge, we have a pretty solid understanding of
loudspeaker impedance.
But this is a complete guide to speaker impedance, and there's a lot
more to know. It's important to know what impedance is, but it's even
better to know the factors that cause speaker impedance.
What factors play a role in determining a speaker's impedance?
The Impedance Of A Speaker Driver Design
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A speaker driver is designed with a conductive voice coil attached to a
moveable diaphragm. The voice coil is suspended inside a gap in a
magnetic structure. As electrical audio signals are passed through the
coil, a changing magnetic field is induced, and the coil (and
diaphragm) oscillate.
Ideally, the diaphragm will move in the exact same waveform as the
audio signal to produce sound that is completely representative of the
audio signal without distortion.
To learn more about speaker drivers, check out my article What
Are Speaker Drivers? (How All Driver Types Work).
The key point here is that speakers have conductive voice coils and
these coils naturally have electrical impedance.
Speaker Driver Resistance
There is a constant DC resistive element to the voice coil (and speaker
driver as a whole). This electrical resistance is the same across all
frequencies and is often at or just below the minimum impedance
value of the speaker driver.
That's the easier part. The more interesting part of the frequencydependent impedance of the speaker driver is the back EMF and the
reactance of the driver.
Impedance Spike Due To Back EMF At The Resonance Frequency
Let's begin with the back EMF (electromotive force).
The speaker driver has a fundamental resonance frequency (Fs). This
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is the frequency at which the speaker driver naturally wants to vibrate.
It is easy to make the driver vibrate at its resonant frequency and more
difficult to make it vibrate at other frequencies.
Simply tapping the speaker diaphragm will cause the driver to vibrate
at its resonant frequency. Subjecting a speaker driver to a sound wave
at its resonance frequency will also cause it to vibrate at this
frequency, similar to a tuning fork.
At this resonance frequency, there is a spike in impedance. This may
seem counterintuitive. The driver moves with the least amount of
physical resistance at its Fs, yet it portrays a sharp increase in its
impedance of electrical current.
This can be explained with back EMF.
As mentioned, applying a voltage across the voice coil will induce a
magnetic field in the coil, which causes it to move. This is how
speakers ultimately work as transducers.
The opposite is also true. Moving the voice coil within a magnetic field
will induce a voltage across the coil. This voltage is in opposition to the
voltage that would cause the coil to move. This is called back
electromotive force. In other words, back EMF opposes the flow of
electricity through the speaker's voice coil (just like impedance).
At the resonant frequency, the speaker driver will want to vibrate
freely, which causes an increase in back EMF and, therefore, an
increase in impedance.
The Fs of a moving-coil speaker driver is often in the range of 20 Hz to
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600 Hz and causes a spike in the speaker driver's impedance.
The fundamental resonance frequency (Fs) is one of the many ThieleSmall parameters that make up a large portion of a speaker driver's
specifications. The impedance at the Fs is noted by another TSP
known as Zmax (“impedance at resonance” or “maximum impedance”).
To learn more about the T/S parameters, check out my article Full
List: Thiele-Small Speaker Parameters W/ Descriptions.
It's important to note that many speakers are designed with multiple
drivers, and each driver will have its own resonance. This may cause
several spikes in the overall impedance of the speaker. Oftentimes
these peaks are damped or tuned in the speaker design to help
achieve a smoother impedance graph.
High-Frequency Rise In Impedance Due To Inductive Reactance
Inductive reactance is a property of an AC circuit (like a voice coil in a
speaker driver) that opposes the change in current.
Reactance is similar to resistance in the fact that it is measured in
ohms. Notice the difference in the definitions: reactance opposes the
change in the electrical current while resistance opposes the current
itself. Both reactance and resistance are factors that make up the
overall impedance of a speaker driver.
As we've discussed, audio signals range in frequency from 20 Hz (or
below) to 20,000 Hz (or above). The hertz values represent cycles per
second.
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We know that the current of higher frequency signals changes
direction more times per second than lower frequency signals. The
reactance of a voice coil, therefore, opposes higher frequencies more
than it opposes lower frequencies.
This is why we see an increasing impedance in the high-end of a
speaker's impedance graph.
The Number Of Speaker Drivers & Their Effect On
Impedance
We've just discussed the variations within a single driver. Now imagine
having multiple drivers within a single speaker unit.
Most loudspeakers are designed with at least 2 drivers (a woofer and
tweeter), and many are designed with more.
As we can imagine, each driver will have its own effect on the overall
impedance of the speaker unit.
This can cause several peaks in the overall impedance that coincide
with the resonance frequency of each driver. Tweeters are often
designed with small Fs impedance peaks (either naturally or
damped/tuned) to lessen the spikes in the overall impedance.
Note that crossovers are used to send specific frequency bands to the
drivers that will best reproduce them. Therefore, the increase in highfrequency impedance due to inductive reactance will likely only be a
result of the tweeter (as no high-frequencies will be sent to the
midrange speakers or woofers).
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For more info on speaker crossovers, check out my article What Is
A Speaker Crossover Network? (Active & Passive).
Note that each driver may also have a different nominal impedance,
which may dramatically alter the overall impedance graph.
The Speaker Enclosure & Its Role On Impedance
Loudspeaker units are practically always built into enclosures.
A speaker enclosure improves the performance of a speaker by
effectively blocking off the rearward out-of-phase sound waves from
the speaker driver. This improves phase coherence and makes for a
stronger/louder output.
Enclosures come in all sorts of shapes and sizes, and each enclosure
has its own resonance(s).
The resonance(s) of a speaker enclosure, like the resonance of the
speaker driver, affects the impedance of the overall speaker unit.
The driver will oscillate more easily at the resonant frequency of the
enclosure and, therefore, more back EMF will be produced in the voice
coil. As we've discussed before, this spikes the impedance of the
speaker unit.
The enclosure resonance is often, but not always, below the driver
resonance. The peaks in impedance due to the enclosure and the
driver resonances coincide with their respective resonant frequencies.
For more information on speaker enclosures, check out my article
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Why Do Loudspeakers Need Enclosures?
Wiring A Single Speaker Vs. Wiring Multiple
Speakers
So far in this article, we've only been describing the impedance of a
single speaker and the load impedance between that speaker and its
connected amplifier.
There are plenty of stereo amplifiers on the market with multiple
channels that can connect to multiple speakers. Generally, these
distinct channels act as multiple single connections between the amp
and a speaker.
Information on these setups can be found in earlier parts of this article.
In this section, I'd like to go into connecting multiple speakers to a
single amplifier channel and the resulting load impedance of such
setups.
There are two methods of connecting multiple speakers to a
single amplifier channel:
In series: speakers connected in series are connected along a
single conductive path. The same current flows through all of the
speakers but voltage is dropped across each of the speakers (due
to the impedance of the speaker).
In parallel: speakers connected in parallel are connected along
multiple paths so that the current is split up but the
same voltage is equal across each speaker.
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As a general rule, parallel wiring should be used when connecting 2 (or
more) speakers with 8Ω impedance or more.
Conversely, series wiring should be used when connecting 2 (or more)
speakers with impedance ratings under 8Ω.
This is because, when connecting multiple speakers to a single
amplifier channel, we must look at the total load impedance of the
circuit.
Let's simplify life by dealing with the resistance of the speakers rather
than the complex impedance. This is not technically correct but will
allow for an easy and intuitive understanding.
Wiring Multiple Speakers In Parallel
Wiring two speakers to a single amplifier channel in parallel would look
something like this:
Two Loudspeakers Wired In Parallel
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To better comprehend the combined load impedance the speakers
produce when wired in parallel, let's have a look at a simplified
schematic:
Two Loudspeakers Wired In Parallel
The combined resistance of the parallel speakers is as follows:
1 / RT = (1 / R1) + (1 / R2) + … + (1 / Rn)
Where n is the number of resistors in parallel.
So, two 8Ω speakers in parallel would produce a total “nominal” load
impedance of 4Ω.
Three 8Ω speakers in parallel would produce a total “nominal” load
impedance of 2.66Ω.
Four 8Ω speakers in parallel would produce a total “nominal” load
impedance of 2Ω.
Wiring Multiple Speakers In Series
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Wiring two speakers to a single amplifier channel in series would look
something like this:
Two Loudspeakers Wired In Series
To better comprehend the combined load impedance the speakers
produce when wired in series, let's have a look at a simplified
schematic:
Two Loudspeakers Wired In Series
The combined resistance of the series speakers is as follows:
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RT = R1 + R2 + … + Rn
Where n is the number of resistors in series.
So, two 4Ω speakers in series would produce a total “nominal” load
impedance of 8Ω.
Three 4Ω speakers in series would produce a total “nominal” load
impedance of 12Ω.
Four 4Ω speakers in series would produce a total “nominal” load
impedance of 16Ω.
Impedance Of Alternative Speaker Types
Thus far, we've only been discussing moving-coil dynamic speakers.
This is for good reason since the overwhelming majority of speakers
utilize these types of drivers.
However, there are certainly other speaker driver types worth
considering when thinking of speaker impedance.
Electrostatic Loudspeakers
Electrostatic loudspeakers tend to have low inputs impedance. The
low impedances are the result of driving a large electrostatic speaker's
inevitably large capacitance over a wide range of frequencies.
However, the “low impedance” is still typically within the range of
normalcy (2 – 4Ω) and so special amplifiers are not necessarily
needed.
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The impedance graphs of electrostatic speakers do change with
frequency due to the capacitive nature of the driver but do not present
overly sharp spikes, as do moving-coil drivers.
The MartinLogan Classic ESL 9 (link to check it out at
MartinLogan.com) is an electrostatic speaker with an input impedance
rating as follows:
Nominal: 4 ohms, 0.8 ohms @ 20 kHz
MartinLogan Classic ESL 9
Magnepan Loudspeakers
Planar magnetic type speakers (commonly known as the eponym
“Magnepan”) work on electromagnetic induction like moving-coil
drivers but do so in a planar fashion.
The impedance of these speakers is typically in the lower range of
what is normal for moving-coil speakers (around 4Ω), but the variance
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in impedance across the frequency response is much, much less.
The Magnepan 1.7i (link to check it out at Magnepan.com) is a planar
magnetic speaker.
Magnepan 1.7i
Air Motion Transformers
Air motion transformers are the ribbon-diaphragm speaker
transducers. These driver types work tremendously well as tweeters.
Their impedance values tend to be in the same range as the typical
moving-coil drivers (most often 8Ω).
The impedance graphs of air motion transformers, however, are much
flatter with significantly less variation. This is due to the lack of an
enclosure and a typical resonant frequency well below the audible
range of sound.
For example, the Dayton Audio AMT Mini-8 (link to check the price on
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Amazon) has an impedance rating of 8 ohms.
Dayton Audio Mini-8
To learn more about the various speaker transducer types, check
out my article What Are Speaker Drivers? (How All Driver Types
Work).
Related Questions
How do audio power amplifiers work? The role of the audio power
amplifier is to amplify line level signals at its input (from audio
players) to a speaker level signal at its output (to drive speakers).
It does so with energy from the power mains that effectively
powers the vacuum tube or transistor-based amplification circuit.
Power amps are different from microphone preamps and
headphone amps. To learn more about these other types of
amplifiers, check out my articles What Is A Microphone
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Preamplifier & Why Does A Mic Need One? and What Is A
Headphone Amplifier & Are Headphone Amps Worth It?
respectively.
How many watts is a good speaker? The best wattage (power
handling rating) of a speaker depends on the power output of the
amplifier that is driving the speaker. It's best to match “big
speakers” with “big amps” and “small speakers” with “small
amps”. Mismatching speakers and amps can lead to poor signal
output, distortion, and even blow-out.
With so many loudspeakers on the market, purchasing the best
speaker(s) for your applications can be rather daunting. For this
reason, I've created My New Microphone's Comprehensive
Loudspeaker Buyer's Guide. Check it out for help in determining your
next speaker acquisition.
This article has been approved in accordance with the My New
Microphone Editorial Policy.
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