Speaker Specifications – My New Microphone

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) encountered by the audio signal (electrical AC current) at the input of the speaker driver. Impedance affects the load a speaker places 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 in order 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 ImpedanceSource & Load ImpedancePower Matching Vs. Voltage BridgingSpeaker Impedance & Power DemandsDamping FactorActive Vs. Passive LoudspeakersImpedance Of Speaker Level Vs. Line LevelSpeaker Impedance SpecificationsActual Speaker ImpedanceUnderstanding Phase & ImpedanceSpeaker Impedance FactorsSpeaker Driver DesignNumber Of Speaker DriversEnclosureWiring A Single Speaker Vs. Wiring Multiple SpeakersWiring Multiple Speakers In ParallelWiring Multiple Speakers In SeriesImpedance Of Alternative Speaker TypesElectrostatic LoudspeakersMagnepan LoudspeakersAir Motion TransformersRelated 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. 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 the audio signal at its input. A loudspeaker, when connected to a power amplifier, 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 we're 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 a simplified voltage divider to garner an intuitive comprehension of the connection between a power amplifier and a loud speaker: 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 amplifierZS is the source impedance or the output impedance of the amplifierZL is the load impedance or the input impedance of the loudspeakerVL 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 transfer of power between the source and load but with only 50% efficiency (a 6 dB load loss). 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 improves efficiency. Speaker Impedance & Power Demands Going back to the maximum power transfer for a moment, we can state that lower speaker impedances actually demand more power. We can see this in the power ratings of power amplifiers. For example, let's have a look at the Crown Audio XLi 2500 (link to compare prices at select retailers). 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 XLi 2500 Crown Audio is featured in My New Microphone's Top 11 Best Power Amplifier Brands In The World. Crown Audio XLi 2500 Power Specifications: 4Ω Dual: 750W8Ω Dual: 500W8Ω 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 This tells us, intuitively, 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 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. A higher damping factor improves the transient response of the amplifier-speaker relationship and also 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 tru 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 pint in the article, we've been discussing passive loudspeakers. 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 bit 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. As an example, let's look at the input impedance specifications of the QSC KW153 (link to compare prices on Amazon and select retailers): 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 unbalancedChannel B RCA: 10 kΩ In the above example, Channel A is a mic input and Channel B is a line input. 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 11 Best Subwoofer Brands (Car, PA, Home & Studio)• Top 11 Best PA Loudspeaker Brands You Should Know And Use• Top 10 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 but a main reason is for electrical current. 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) signal and are built 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 need 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 nominal impedance of the speaker. 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 Ω8 Ω speakers have a minimum impedance no less than 6.4 Ω The defined frequency range of the speaker is the 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 compare prices on Amazon and select retailers) give a nominal impedance rating and a minimum impedance rating to help give us a better idea. Electro-Voice ZLX-15 Nominal Impedance: 8 ΩMinimum Impedance: 7 Ω Electro-Voice is featured in the following My New Microphone articles:• Top 11 Best Subwoofer Brands (Car, PA, Home & Studio)• Top 11 Best PA Loudspeaker 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 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 in order to not only get the best results but to 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 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 intersting impedance specification listed as "5-10 Ohms". Aperion Intimus 533-T Its impedance graph is as follows: Aperion Intimus 533-T Impedance & Phase GraphPhoto 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. Dynaudio Excite X12 Its impedance graph is as follows: Dyneaudio Excite X12 Impedance & Phase GraphPhoto Courtesy Of Stereophile Revel Ultima Salon2 The Revel Ultima Salon2 (pictured below) is an audiophile-grade 4-way floorstanding speaker with an impedance rating listed as "6 ohms (nominal) 3.7 ohms (minimum @ 90 Hz)". Revel Ultima Salon2 Its impedance graph is as follows: Revel Ultima Salon2 Impedance & Phase GraphPhoto Courtesy Of Stereophile In each of the impedance graphs above, we have graphed lines for both impedance and phase. We should also notice that the speaker impedance graphs show significant spikes at one or more frequencies in the speaker's frequency response. 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 out 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 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 of 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 AnglePower Dissipated By AmplifierPower Transferred To SpeakerPower Factor 0º1.001.001.000 15º1.380.940.966 30º1.760.750.866 45º2.000.500.707 60º1.660.240.500 75º1.200.080.259 90º4.000.000.000 Note that 90º is an impossibility in the real world. The phase angle will pass through 0º at the regional peaks and trough 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 the frequencies that have 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 (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? Speaker driver type(s) and designNumber of driversEnclosure The Impedance Of A Speaker Driver Design 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 frequency-dependent 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 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 similarly to a tuning fork. At this resonance frequency, there is a spike in impedance. This may seem counter intuitive. 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 600 Hz and causes a spike in the speaker driver's impedance. The fundamental resonance frequency (Fs) is one of the many Thiele-Small 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 current itself. Both reactance and resistance are factors that make up the overall impedance of a speaker driver. Audio signals, as we've discussed, range in frequency from 20 Hz (or below) to 20,000 Hz (or above). The hertz values represent cycles per second. 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. The increase in high-frequency impedance due to inductive reactance, therefore, will likely only be a result of the tweeter (as no high-frequencies will be sent to the midrange speakers or woofers). For more info on speaker crossovers, check out my article What Is A Speaker Crossover Network? (Active & Passive). Note that each driver may have a different nominal impedance as well that may alter the overall impedance graph dramatically. 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 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. 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 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, then, 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 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: RT = R1 + R2 + ... + Rn Where n is the number of resistors in series. So, then, 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 if for good reason since the overwhelming majority of speakers utilize these types of driver. 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. 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 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, as 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 compare prices on Amazon and select retailers) 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 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.