Why Do Speakers Need Amplifiers? (And How To Match Them)

Speakers are quite remarkable in their ability to convert electrical audio signals into sound waves for our listening pleasure (or displeasure). Speakers of all sizes, from built-in smartphones to live sound subwoofers, require amplifiers.

Why do speakers need amplifiers? Typical analog audio signals are recorded, stored, and played back at line level (nominally 1 volt DC). Line level signals must be amplified (via amplifiers) in order to drive a speaker and produce sound properly. Amplifiers increase signal power and drop impedance to drive speakers appropriately.

In this article, we'll discuss the relationship between speakers and amplifiers to further our knowledge on the subject. I'll be writing about line and signal level signals, matching amps and speakers, important specifications to be aware of, and the differences between active and passive speakers.

Defining Speakers & Amplifiers

Before we get into our full discussion, let's define loudspeakers and amplifiers.

Loudspeakers

A loudspeaker is a transducer of energy. It converts audio signals (electrical energy) into sound waves (mechanical wave energy). The transducer element of a speaker is known as a driver.

Loudspeakers can be manufactured with a single driver or multiple drivers. Each driver has its own frequency response, impedance and overall design characteristics.

Full-range speakers, capable of producing the entire range of audible frequencies (20 Hz – 20,000 Hz), generally require multiple drivers for the various frequency bands of the entire range.

More information on speaker drivers can be found in the following My New Microphone articles:
• What Are Speaker Drivers? (How All Driver Types Work)
• Differences Between Mid-Range Speakers, Tweeters & Woofers

Nearly all loudspeaker drivers have electrodynamic (moving-coil) designs that include a conductive voice coil attached to a moveable diaphragm and a permanent magnet.

As the amplified (speaker level) audio signal passes through a speaker driver's conductive element (voice coil), the speaker driver will react and move a diaphragm according to the audio waveform. This movement results in the production of sound waves that mimic the audio signal.

Where does this speaker level audio signal come from?

Amplifier

An audio amplifier is an electrical device that amplifies an input signal to output a stronger version of that same signal.

There are many different types of audio amplifiers to be aware of, including:

• Power Amplifiers
• Preamplifiers
• Integrated Amplifiers
• Receivers
• Car Amplifiers
• Instrument (Guitar, Bass, Etc.) Amplifiers
• Headphone Amplifiers
• Microphone Preamplifiers
• Impedance Converter Amplifiers
• Distribution Amplifiers

Related article: Are Audio Amplifiers Analog Or Digital Devices?

Of the amplifiers listed above, it is the power amplifier that will effectively output signals capable of properly driving loudspeakers.

Amplifiers effectively use power from a power supply to apply gain to an input signal. Gain, put simply, is the ratio of the output signal level to the input signal level (though it's typically measured with decibels, which are logarithmic rather than linear).

For more information on power and amplifiers, check out my article Why Do Audio Amplifiers & Preamplifiers Need Power To Work?

Signal levels are typically measured in volts, though power amplifiers (and their connected loudspeakers) are generally measured with power ratings.

More gain means a stronger output signal relative to the input signal. It is the amplifier's job to provide this gain and to amplify the signal.

Power amplifiers are also tasked with amplifying the current of an audio signal to drive its connected loudspeaker properly.

Power amplifier circuitry is obviously found within power amplifiers. It's important to note that power amplifier circuitry is also found in the output stages of integrated amplifiers (units that combine a preamplifier and power amplifier); receivers (units that combine an integrated amplifier and a radio receiver, and instrument amplifiers (that combine instrument preamps and, oftentimes, a power amplifier).

It's also important to note that active and powered speakers will have built-in power amplifiers and line inputs. Some active speakers will even have built-in preamplifiers and be able to amplify line level and mic level signals.

Additionally, amplifiers are classified by their circuitry:

• Solid-State Amplifier
• Tube Amplifier
• Digital Amplifier

And by their class:

• Class-A
• Class-AB
• Class-B
• Class-C
• Class-D
• Class-E
• Class-G
• Class-H
• Class-S

We'll save deeper discussion on each amplifier type for other articles.

Recap

Loudspeakers are transducers that convert audio signals into sound waves. They require speaker level signals to be properly driven.

Amplifiers provide gain to a signal to boost its level. Power amplifiers are capable of outputting speaker level signals to drive loudspeakers properly.

Power amplifiers can be standalone units of parts of other amplifiers (integrated amps, receivers and instrument amps).

Separate power amplifiers are required to drive passive loudspeakers. Appropriate speaker and amplifier pairing are necessary for optimal (or even usable) results.

Active loudspeakers have built-in power amplifiers designed specifically for their drivers.

Related article: What Are The Differences Between Passive & Active Speakers?

Line Level To Speaker Level

In the previous section, I mentioned that loudspeakers require speaker level signals to be driven properly.

Power amplifiers are responsible for driving loudspeakers with speaker level signals by amplifying the relatively low-level line (or microphone or instrument) level signals.

Let's quickly define the different audio signal levels.

Speaker Level

Speaker level signals are required to drive speakers. They have the highest voltage and current levels of all the other audio signals.

Because speaker level signals are very strong, they are never stored on playback devices (analog or digital), nor are they used in mixing consoles or other audio processors.

Rather, the speaker level signal is used to drive loudspeakers, and that's about it.

Power amplifiers are responsible for amplifying line level signals up to speaker level. These speaker level signals are generally sent to nominal load impedances (speaker impedances) of 8Ω, 4Ω, or 2Ω.

Speaker level voltage levels range from a few volts up to 100s of volts.

Power amplifiers and loudspeakers are generally rated with power output and power handling capabilities, respectively. These electrical power ratings have to do with the overall strength of the speaker level that is transferred between them.

Line Level

Power amplifiers will amplify line level signals, boosting them to speaker level signals that will properly drive the connected loudspeaker(s).

Audio is generally stored at nominal line level (+4 dBu) or a digital equivalent. Consumer-grade nominal line level exists, confusingly, at -10 dBV.

Audio mixers and other processors typically use line level signals.

A line level signal is meant to drive a load impedance (line input) in the range of 10 kΩ to 50 kΩ. This is much, much higher than the typical 8Ω speaker impedance.

For the sake of proper impedance bridging, line level outputs are generally in the range of 75 to 600 Ω.

A power amplifier, then, is not only tasked with boosting the voltage, current and power of the line level signal. It is also responsible for dropping the impedance of the signal to transfer the audio to the loudspeaker properly.

More on impedance bridging in the following section.

Mic Level

Preamplifiers (typically microphone preamplifiers) are required to boost mic level signals up to line level signals before a power amplifier can bring the signal up to speaker level.

Mic level is generally in the range of -60 dBV (1 mV_RMS) to -20 dBV (100 mV_RMS) and is designed to drive loads of 1.5 to 5 kΩ.

Instrument Level

Instrument level signals are wildcards. Their typical values range depending on the instrument output (guitar pickup, synthesizer output, phono cartridge) etc.

These signals must first pass through an instrument preamplifier, which will generally output a line level signal. A power amplifier can then amplify the output before getting sent to a loudspeaker for reproduction as sound waves.

Headphone Level

Headphone level is rarely discussed. However, headphones also require their own set of signal specifications to be driven properly.

Headphone drivers, like speaker drivers, concert audio signals into sound waves.

To learn more about how headphone and loudspeaker transducers work, check out my article How Do Speakers & Headphones Work As Transducers?

The following table gives us a rough outline of what the various signal level types are:

Matching Amplifiers & Speakers

Now that we understand the roles of the power amplifier and loudspeaker, we can talk about how they work together. It all comes down to finding the right match.

“Matching”, technically, is a somewhat confusing term to use. A better term could be pairing or even selecting.

Why is this? Well, as we'll see shortly, input and output impedance plays a major role in pairing an amplifier and speaker. Impedance matching, as a technical term, means having the same source and load impedance. This is great for maximum power transfer but is terrible for signal transfer. We'll discuss this more later in this section.

That being said, “matching” is the usual terminology used for connecting compatible power amplifiers and loudspeakers.

So even though power amplifiers output speaker level signals, not all power amplifiers will properly drive all loudspeakers. There's a science to choosing amps and speakers that will work well with one another.

This speaker and amplifier “matching” science includes the following factors:

• Impedance: power amplifier output (source) impedance and loudspeaker input (load) impedance.
• Power ratings: the power amplifier power rating and the loudspeaker power handling rating.
• Sensitivity: the sensitivity rating of the speaker.

Of course, there are plenty of other specifications that will tell us about the performance of amplifiers and speakers.

Check out the full list of power amplifier and loudspeaker specifications by clicking the following links:
• Complete Guide To Power Amplifier Specifications & Data
• Full List: Loudspeaker & Monitor Specifications w/ Examples

However, the above three factors are most important for pairing a power amplifier with an appropriate loudspeaker (or vice versa).

Before we get into each of the factors, let's quickly discuss active amplifiers.

Active Amplifiers

Active amplifiers, as we mentioned earlier, have built-in amplifiers.

They will always have at least one power amplifier but may also have preamplifiers.

The types of speakers require power to function properly (in order to power their amplifiers).

We do not need to worry about properly “matching” a power amplifier to an active speaker unless that speaker has a speaker level input that will effectively bypass the inner amplifier.

In fact, connecting a power amplifier to an active speaker runs the risk of overloading and causing damage to the speaker's internal circuitry and may even cause damage to the amplifier.

On top of that, the internal amplifier(s) of an active speaker is generally designed specifically for that speaker's driver(s). So not only do we not have to worry about matching a power amp, but the power amp utilized is likely one of the most compatible amps we could choose!

As an aside, active amplifiers with crossovers (2-way active speakers, 3-way active speakers, etc.) will have individual power amplifiers for each crossover frequency band.

In other words, the crossover will split the incoming audio signal frequencies into distinct bands and send these bands to the driver(s) that will best reproduce them. Each band and its associated driver(s) will be driven by its own power amplifier.

This means the crossover will deal with splitting up line level signals. Therefore, more care can be taken in providing adjustable crossover parameters (rather than providing heavy-duty components that can handle the much higher voltages of speaker level signals).

This is a bit beside the point but worth mentioning. To learn more about crossovers, check out my article What Is A Speaker Crossover Network? (Active & Passive).

All of this is to say that active speakers have built-in power amplifiers, and we need not worry about matching them to external power amplifiers.

Active speakers have line inputs (or potentially mic and/or instrument inputs). Therefore, we'll see much different specification values for impedance, power and sensitivity. These specs are largely based on the input of the speaker.

To learn more about active and powered loudspeakers, check out my What Are The Differences Between Passive & Active Speakers?

Impedance

Properly bridging impedance between a power amplifier and a loudspeaker is of utmost importance.

Generally speaking, power amplifier manufacturers will make things easy by listing rated impedance values on their amplifiers. Simply match the rated impedance of the amplifier with the nominal impedance of the loudspeaker.

Let's dig a bit deeper into the factor that impedance plays in pairing compatible amps and speakers.

To describe a more intuitive understanding, let's simplify to amplifier-loudspeaker connection into a voltage divider. A simplified schematic would look something like this:

In the schematic drawn above, we have the power amplifier output on the left and the loudspeaker input on the right.

• V_S is the source voltage or the voltage (signal strength) outputted by the amplifier
• Z_S is the source impedance or the output impedance of the amplifier
• Z_L is the load impedance or the input impedance of the loudspeaker
• V_L is the load voltage or the resulting voltage (signal strength) that will drive the loudspeaker

In the name of efficiency, want as much signal transfer (voltage transfer) as possible from the amplifier to the speaker.

This happens when Z_L >> Z_S.

Let's prove that with the following equation:

\frac{V_L}{V_S}=\frac{Z_L}{Z_L+Z_S}

Or:

V_L=V_S\frac{Z_L}{Z_L+Z_S}

Power matching (impedance matching) is the result of matching two devices' source and load impedances. 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 V_L will only be half that of V_S if Z_S = Z_L.

This is why “matching” an amplifier and loudspeaker is a confusing term.

Voltage bridging (impedance bridging) is the result of having Z_L much greater than Z_S. 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:

Let’s say that Z_L was equal to Z_S. In this scenario, V_L would be 1/2 of V_S (the voltage or strength of the connected device’s output signal). Half the signal strength was lost!

Let’s now say that Z_L was 9 times Z_S. In this scenario, V_L would be 9/10 of V_S. 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.

This impedance bridging also increases the damping factor of the system.

The damping factor of an amplifier-speaker system is defined as the ratio of the load impedance to source impedance.

DF=\frac{Z_\text{load}}{Z_\text{source}}

The total driving impedance is a combination of the amplifier's source impedance, the impedance inherent in the speaker cable, and even the impedance of the speaker’s crossover.

The load impedance is the impedance of the speaker driver(s).

The purpose of the damping factor is to tell us how much control the amplifier will have over the loudspeaker’s driver(s). A higher DF means the amplifier will move the driver with more precision and accuracy.

So then, impedance is an important factor when matching a power amplifier and loudspeaker.

Issues arise when the load impedance (the nominal impedance of the speaker) is too low.

Some amplifiers will work fine driving an 8Ω speaker but suffer when driving a 4Ω speaker. This is partly due to the bridging and damping factors mentioned above, but it has much more to do with the increased demand in power from the amplifier from a lower-impedance speaker.

See, the voltage (and current) required to drive the speaker at a certain level are pretty well constant and depend on the design factors of the speaker. So, then, all else being equal, lowering the speaker's impedance would demand more power from the amplifier to maintain the same voltage (and drive the same current).

This can be seen in the following equation (derived from the power formula and Ohm's law):

P = \frac{V^2}{R}

As impedance (simplified as resistance R in the above formula) drops, more power is required to maintain the same voltage and push the same current through the speaker driver(s).

Due to amplifier power limitations, as we'll discuss next, some amplifiers are not suited for lower-impedance speakers.

To learn more about speaker impedance, amplifier impedance and damping factor, check out the following My New Microphone articles:
• The Complete Guide To Speaker Impedance (2Ω, 4Ω, 8Ω & More)
• What Is Amplifier Impedance? (Actual Vs. Rated Impedance)
• What Is Damping Factor Between An Amplifier & Loudspeaker?

Note that multiple speakers can be connected to a single amplifier channel. In this scenario, we must calculate overall load impedance.

The total load impedance of multiple loudspeakers depended on the way in which the speakers are wired: in series or parallel?

Wiring two speakers to a single amplifier channel in series would look something like this:

To better comprehend the combined load impedance the speakers produce when wired in series, let’s have a look at a simplified schematic:

The combined resistance of the series speakers is as follows:

R_T = R_1 + R_2 + \ldots + R_n

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.

Wiring two speakers to a single amplifier channel in parallel would look something like this:

To better comprehend the combined load impedance the speakers produce when wired in parallel, let’s have a look at a simplified schematic:

The combined resistance of the parallel speakers is as follows:

\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \ldots + \frac{1}{R_n}

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.

Power Ratings

When checking out power amplifier data sheets, we'll find the power output ratings.

When checking out passive loudspeaker data sheets, we'll find power handling ratings.

Oftentimes, these ratings are manipulated to get the highest numbers possible without technically “lying”. However, the principles remain the same.

For this explanation, let's stick with continuous power specs, which effectively tell us the maximum average power an amplifier can output continuously without failing and the maximum average power a loudspeaker can take in continuously without failing.

I define average power as the product of RMS voltage and RMS current (RMS = root mean square: an average value of alternating quantities).

P_\text{avg}=I_\text{RMS}•V_\text{RMS}

Why do amps and speakers have maximum power limitations? Though there are theoretical limits to the amount of power an amplifier's power supply can provide, it's typically a case of overheating and occasional over-excursion (in subwoofer speakers).

Some of the power used to amplify an audio signal is lost as heat. This depends largely on the amplifier class (Class A runs at about 25% or 50% efficiency while Class C can reach up to 90% and other classes have different efficiencies).

This heat is dissipated via heat sinks and other cooling methods. However, there reaches a point where the heat can no longer be dissipated properly, and the amplifier will either blow a fuse and shut off or have part of its circuit burn/melt.

Similarly, a loudspeaker's voice coil can also overheat to a point where it burns out. Speaker drivers are notoriously inefficient (in the range of 1%, yes 1%!). This means that nearly all the power in a speaker drive is lost as heat!

So then, we don't want an amplifier that will send too much power to a loudspeaker and cause its driver(s) to burn out. This is easy to understand.

Similarly, we do not want a loudspeaker to demand too much power from a power amplifier. As discussed in the previous section, this is a bit trickier to understand and is why some amps are not suitable for low-impedance drivers.

So, it's important first to make sense of the sometimes unhelpful power rating specifications and then to practice safe power transfer between our power amps and speakers.

Does this mean that a high-power amplifier can't drive small speakers? Not necessarily, just ensure that the power amplifier isn't turned up to the point that it sends too much power to the speakers.

Does this mean that a low-power amplifier can't drive large speakers? This is actually arguable worse than the reverse scenario. Large speakers require large signals and are capable of producing large amounts of Back EMF that could potentially damage the amplifier.

On top of that, trying to drive a large speaker with a low-power signal will not sound as clear as a well-amplified signal. Turning the amplifier up may lead to clipping, which sounds awful and runs the risk of damaging the amplifier (and perhaps even the loudspeaker).

For more information on power ratings, check out my article Complete Guide To Speaker Power Handling & Wattage Ratings.

Sensitivity

As mentioned, speakers are incredibly inefficient.

A better specification to look at when determining how loud a speaker will be and, thereby, how much power you'll need from the amplifier is the sensitivity specification.

Speaker sensitivity is the capability of the speaker to convert electric power from an amplifier into sound. It is a measurement that refers to how loud a speaker will be at a given input signal level.

Sensitivity is generally given as a dB SPL / 1 W @ 1 meter rating.

That is the sound pressure level (in decibels) that the speaker will output, measured at a distance of 1 meter from the speaker when 1 watt of power is dissipated across the speaker.

To use this specification productively, we must first decide on the distance at which we'll be listening to the speaker.

This is important because sound pressure level naturally drops by 6 dB for every doubling of distance. Therefore, referencing the sensitivity ratings, we have:

• Listening at 0.5 meters = sensitivity rating + 6dB
• Listening at 1 meter = sensitivity rating
• Listening at 2 meters = sensitivity rating – 6dB
• Listening at 4 meters = sensitivity rating – 12dB

Next, we must understand that there will be an increase of 3 dB in SPL for every doubling in power. So, then, we have the following values at a distance of 1 meter:

• 1 W = sensitivity rating
• 2 W = sensitivity rating + 3dB
• 4 W = sensitivity rating + 6dB
• 8 W = sensitivity rating + 9dB
• 16 W = sensitivity rating + 12dB
• 32 W = sensitivity rating + 15dB
• 64 W = sensitivity rating + 18dB
• 128 W = sensitivity rating + 21dB

So for our calculated listening position, we must find a speaker capable of outputting the necessary SPL and able to handle the amount of power required to produce that SPL.

Then we must find an amplifier capable of supplying that much power to the loudspeaker while also being compatible in terms of impedance bridging.

This may seem a bit vague, so let's use a concrete example:

Let's say we have two speakers:

• Speaker A has a sensitivity rating of 90 dB SPL 1W/1m
• Speaker B has a senisitvity rating of 84 dB SPL 1W/1m

The following picture should illustrate the above points by showing speakers A and B with various amplifier output power and listening distances:

Though the above illustration fails to reach a power level near typical max power ratings and handling ratings, it's a good example with easy numbers to help explain speaker sensitivity.

It's important to note that adding a speaker (or several more speakers) will not double the sound pressure level at the listening position if outputting at the same levels. The overall increase may, in theory, be +3 dB, but acoustics will typically not even allow this much of an increase.

Note, too, that it's important not to overdo it in terms of listening levels. Prolonged exposure to high sound pressure levels can cause hearing damage.

Here is a list showing the recommended safe listening exposure times as defined by NIOSH (National Institute for Occupational Safety and Health) and OSHA (Occupational Safety and Health Administration):

For more information on sensitivity, check out my article Full Guide To Loudspeaker Sensitivity & Efficiency Ratings.

Related Questions

Are bigger speakers better? The term “better” is subjective. Bigger speakers, generally speaking, can move more air, thereby producing higher sound pressure levels and more bass frequencies. That is, by no means, to say that bigger speakers have higher fidelity or perform better than their smaller counterparts. There are plenty of other factors to consider.

Are amplifiers with higher wattage louder? Technically speaking, an amplifier is not “loud,” though it can drive a speaker to be “loud”. Amplifiers with higher wattage/power ratings can supply more gain to the input audio signal and, therefore, output a stronger signal. When turned up and matched with a capable loudspeaker, this may translate to more loudness. It may also result in damage to the amplifier and/or speaker if the two are improperly matched.

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