What Is Damping Factor Between An Amplifier & Loudspeaker?


The damping factor is one of many equations that applies to audio equipment. When discussing the relationship between audio outputs (particularly amplifiers) and audio inputs (particular loudspeakers), the damping factor is somewhat lesser considered rating but important to know nonetheless.

What is the damping factor between an amplifier and a loudspeaker? 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. High DFs tell us that the amplifier has more control over the speaker’s moving driver.

In this article, we’ll go into greater detail about the damping factor and discuss impedance in audio systems with more clarity.


Table Of Contents


What Is Damping Factor?

As mentioned in the answer paragraph above, the damping factor is defined as the ratio of the load impedance to source impedance.

In this way, the damping factor can be written as the following equation:

DF = Zload / Zsource

Damping factor typically refers specifically to the ratio of nominal loudspeaker impedance to total source impedance driving it.

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

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.


The Amplifier Damping Factor Specification

Damping factor is a dimensionless number that should be positive if the amplifier is to control the speaker rather than the other way around.

Once again, the damping factor can be written as the following equation:

DF = Zload / Zsource

The amplifier-speaker connection can be simplified to a voltage divider like the one pictured below:

Simplified Amplifier-Loudspeaker Connection

Where:
ZS (source impedance) is the output impedance of the amplifier.
ZL (load impedance) is the nominal impedance of the loudspeaker.
VS is the voltage of the output of the amplifier.
VL is the voltage across the loudspeaker driver.

To learn more about amplifier and loudspeaker impedance, check out the following in-depth articles from My New Microphone, respectively:
What Is Amplifier Impedance? (Actual Vs. Rated Impedance)
The Complete Guide To Speaker Impedance (2Ω, 4Ω, 8Ω & More)

This simple schematic helps us to simply understand the amplifier-loudspeaker connection. Obviously it’s not perfect but it gets the gist of it.

To garner a basic understanding of the DF spec, we should know the following 3 points:

1. Many would argue that damping factors over 20 should offer more than enough control to the amplifier and these ratings are easily achievable with modern solid-state amplifiers.

2. Tube amps often have damping factors below 20 but are cherished anyway due to their pleasant character.

3. Above a certain point (arguably as low as DF = 100 or lower), increasing the damping factor has little effect on increasing the control of the amplifier. Of course, there is still an increase in control that may be subjectively heard (and argued about) but there is certainly diminishing theoretical and audible returns.

With those three points, we can pretty much understand what to look for in an amplifier’s damping factor specification. As with anything, the story goes deeper and, in the case of DF, quite a bit deeper. We’ll dive into that further in this article.

Remember that, in the previous section, we defined the damping factor. However, amplifier damping factor specifications are calculated a bit differently.

Before we move any further, let’s look at a few examples of real-world power amplifier damping factors.

Examples Of Power Amplifier Damping Factor Specifications

To help illustrate the damping factor specifications we’ll find in power amplifier data sheets and manuals, let’s have a look at a few examples:

Crown Audio XLi 2500

The Crown Audio XLi 2500 (link to compare prices at select retailers) is a popular stereo power amplifier. Its damping factor specification is listed as:

Damping Factor (8ohms), 10 Hz-400 Hz: > 200

Crown Audio XLi 2500

Crown Audio is featured in My New Microphone’s Top 11 Best Power Amplifier Brands In The World.

Anthem STR

The Anthem STR (link to check the price at Crutchfield) is a stereo integrated amplifier with a built-in DAC and Anthem’s proprietary Room Correction technology. Its damping factor is listed as:

Damping Factor (20Hz – 1 kHz): 330

Anthem STR

Hertz Mille ML Power 1

The Hertz Mille ML Power 1 (link to compare prices at select retailers) is a mono subwoofer amplifier. Its damping factor is listed as:

Damping factor (100 Hz @ 4Ω): 100

Hertz Mille ML Power 1

McIntosh MC2152

The McIntosh MC2152 (link to check out the amplifier at the official McIntosh website) is a 2-Channel Vacuum Tube Amplifier. Its damping factor is listed as:

Damping Factor: >18

McIntosh is also featured in My New Microphone’s Top 11 Best Power Amplifier Brands In The World.

Notice that the MC2152, which is the only tube amplifier, has a much lower damping factor than the other 3 solid-state amplifiers.

More on tube amplifier damping factors in the section Tube Vs. Solid-State Amplifier Damping Factors.

I’ll reiterate that the damping factor specification does not tell us the entire story of a system’s actual damping factor. Rather, it gives us an idea as to how well the amplifier will control the speaker.

From the above speakers, we see a few interesting pieces of information that go into testing an amplifier’s damping factor:

  • Specific frequencies or frequency ranges are used in calculation.
  • Specific load impedances are used in calculation.

So the damping factor specification is measured at certain frequencies into a fixed load. This yields a simplified result that really only takes into account the variation in the amplifier’s output impedance.

The calculated amplifier damping factor is always different from the actual system damping factor though it does yield a good representation of the system damping factor.

Still, that’s really all the amplifier manufacturer can test for with any amount of accuracy. The manufacturer doesn’t know what loudspeakers we’ll connect to their amplifiers and which cables we’ll use to do so.

Again, in general, a higher damping factor is better because it means the amplifier will have more control of the speaker’s movement. This control makes for less distortion as the speaker driver(s) oscillate more precisely according to the audio waveform.

High damping factors mean the speaker will stop moving very soon after an audio signal is stopped and very quickly once an audio signal is applied. A high DF yields lower distortion; improved transient response; tighter bass production, and an output sound that more accurately describes the audio signals being outputted by the amplifier into the speaker(s).

The Nominal Load Impedance

The nominal load impedance of the speaker is typically defined as 8Ω unless stated otherwise. Damping factor is typically measured with a simple resistor at the amplifier output that has a resistance equal to the “typical” speaker impedance.

However, speaker impedance is not consistent. It varies (oftentimes by quite a large margin) across the speaker’s frequency response. It spikes at the resonant frequencies of the driver(s) and the enclosure(s) and rises in the high-end due to inductive reactance.

To learn more about speaker impedance, check out my in-depth article The Complete Guide To Speaker Impedance (2Ω, 4Ω, 8Ω & More).

Fortunately, an increase in impedance at the resonant frequencies and in the high-end means a higher damping factor for the connected amplifier.

Another important note on speaker impedance and amplifier damping factor is this: the damping factor is typically understood only through the resistive part of the amplifier, speaker and cable impedances.

The DC resistance of a speaker’s voice coil, as defined by the IEC (International Electrotechnical Commission) is no less than 80% of the speaker’s rated input impedance.

For an 8Ω speaker, then, the DC resistance should be about 6.4Ω. A 4Ω speaker should have a DC resistance of about 3.2Ω.

This is all to say that the actual damping factor of a system will not be significantly lower than any calculated DF using typical measurements.

In fact, at many frequencies, the actual damping factor should be greater than the rated damping factor due to an increase in the speaker impedance. There is only ever a small range (or two) of frequencies that actually drop below a speaker’s nominal impedance.

Test Frequencies Of Damping Factor

We noticed, too, that a signal frequency or a range of frequencies is typically shown in a damping factor specification. This effectively tells us that, even though the damping factor assumes only the resistive parts of impedance, there is a frequency component that manufacturers are weary of.

As discusses previously, damping is most important at low frequencies. Therefore, many damping factor specs are measured at low frequencies and/or low frequency ranges.

Low frequency audio signals cause slower oscillations and, typically, greater excursion in the speaker diaphragm. Proper damping, therefore, is even more important at low frequencies to keep the driver from overshooting and maintain a tight and defined bass response.

For this reason, you’ll often find damping factors specified for lower frequency ranges.

Recap On The Damping Factor Specification

So the damping factor, though perhaps an important concept to understand, is actually a poor specification in terms of the actual damping of the speaker.

Remember that the DF specification is measured at a certain frequency (or band of frequencies) into a purely resistive test load with little to no wiring in between.

It is up to the manufacturer to share these test parameters with us. However, we know that these tests are merely suggestive of what will actually happen in the real world where the speaker impedance varies with frequency; resistive speaker wire is used to connect the amp and speaker, and other circuits (including the speaker crossover) also have an effect on the damping factor.

You’ll find that the numerical specification is not the only rather ambiguous spec on an amplifier’s datasheet.

For more information on power amplifier specifications and datasheets, check out my article The Complete Guide To Power Amplifier Specifications & Data.


How Loudspeakers Work

To really understand what damping factor is, it’s important to understand how speakers work.

As an amplified audio signal (alternative current) passes through the voice coil of a speaker driver, a varying magnetic field is produced through and around the coil.

The voice coil is attached to a diaphragm and sits within a highly-concentrated magnetic field.

So, then, as the audio signal passes through the voice coil, the speaker driver begins oscillating in proportion to audio signal waveform and produces sound waves that mimic the audio.

Moving-Coil Dynamic Speaker Driver Illustration

Speaker diaphragms have mass and their suspensions (surround and spider) have stiffness and mass. These are all factors that go into determining the resonant frequency of the driver.

Left to itself, the speaker will likely overshoot past its equilibrium point, then oscillate back and forth (largely at its resonant frequency) with decreasing amplitude/excursion until all the energy is dissipated into the suspension of the driver.

We do not want this overshooting. We want to dampen the driver so that it effectively does what the amplifier tells it to do rather than what it wants to do naturally.

There are effectively 3 ways in which the speaker driver can be damped:

  • Mechanically: this is the aforementioned loss of energy due to mechanical friction and stiffness. The driver will not oscillate forever. it will eventually dissipate its energy mechanically through the suspension.
  • Acoustically: the acoustic impedance of the medium (typically air molecules) around the driver will dampen it until the driver stops moving.
  • Electrically: the current, voltage and impedance of the circuit that will force dampening upon the driver.

The damping method we’re obviously interested in when it comes to amplifier damping factor is the third: electric damping.

Remember the part about electromagnetic induction in the speaker driver’s voice coil? As an audio signal passes through the voice coil, a varying magnetic field is induced that causes the voice coil to move within the magnetic structure of the driver.

Well, the opposite also holds true: as the voice coil moves within the magnetic field, an electrical voltage is induced across it.

Therefore, as the audio signal flows through the voice coil and causes it to move, an electrical signal is induced through the voice coil in opposition to the audio signal.

So then, the amplifier’s output circuitry actually acts as the main electrical load on the voice coil at the same time that the voice coil acts as the main electrical load on the amplifier’s output.

If the voice coil’s load resistance is low, the current will effectively leave the coil faster and the voice coil will be forced to decelerate at a faster rate than if the load was high and a lower current was produced.

In other words, a higher damping factor means more rapid damping of the voice coil and more control over the voice coil. More control over the voice coil means less distortion; better transients, and better-defined bass response.

In general, having a high damping factor is a good thing.

It’s critical to note, once again, that the actual damping factor will vary from frequency to frequency. This is because voice coils have complex impedances that vary, sometimes greatly, with frequency.

In addition to this, an increase in temperature will increase the resistance and impedance of voice coil.

It’s also important to note that the speaker cable and the speaker’s crossover will also have an effect on the overall load impedance as seen by the amplifier (and the overall load impedance as seen by the voice coil).

For an in-depth read on how speakers work, check out my article How Do Speakers & Headphones Work As Transducers?


Speaker Wire Resistance

As we’ve discussed, the resistance of the speaker wire, though not calculated in an amplifier’s DF spec, is an important element in determining the system damping of the amp-speaker connection.

A general rule of thumb suggests the total resistance of the wire should be less than 5% of the nominal impedance of the speaker.

Speaker cable is typically made of copper so we’ll discuss the typically acceptable lengths of copper speaker cable in this article.

A full discussion on speaker cable could take up several more articles and so it will be saved for other articles. For now, we’ll just talk about the resistance.

The resistivity (ρ) of copper at 20ºC (in ohm-meters) is given as:

ρ = 1.724 • 10-8

Here is a table representing common [copper] speaker wire gauges and their resistance per unit distance:

AWG (Gauge)Wire DiameterResistance Per FootResistance Per Meter
240.0201 in.
0.5105 mm
25.67 mΩ84.2 mΩ
220.0254 in.
0.6452 mm
16.14 mΩ52.7 mΩ
200.0320 in.
0.8128 mm
10.15 mΩ33.2 mΩ
180.0403 in.
1.0236 mm
6.385 mΩ20.9 mΩ
160.0508 in.
1.2903 mm
4.016 mΩ13.2 mΩ
140.0640 in.
1.6256 mm
2.525 mΩ8.28 mΩ
120.0808 in.
2.0523 mm
1.588 mΩ5.21 mΩ
100.1019 in.
2.5883 mm
0.999 mΩ3.28 mΩ

Note that lower gauge wire is actually thicker and, consequently, has lower resistance.

I’ve put together another table to showcase the maximum copper speaker cable length at each of the common gauges that fulfill the <5% rule of thumb at various nominal speaker impedances:

It’s critical to note that most speaker wire uses two conductors and so the resistance is effectively doubled. Therefore, this table makes its calculations with 2x the resistance per unit distance mentioned in the table above.

AWG (Gauge)
*Per Wire In A Two-Conductor Cable*
2Ω Load
Max. Length (by foot)
Total Resistance
4Ω Load
Max. Length (by foot)
Total Resistance
6Ω Load
Max. Length (by foot)
Total Resistance
8Ω Load
Max. Length (by foot)
Total Resistance
241 ft (0.3 m)
0.0513Ω < 0.1Ω
3 ft (0.9 m)
0.1540Ω < 0.2Ω
5 ft (1.5 m)
0.2567Ω < 0.3Ω
7 ft (2.1 m)
0.3594Ω < 0.4Ω
223 ft (0.9 m)
0.0968Ω < 0.1Ω
6 ft (1.8 m)
0.1937Ω < 0.2Ω
9 ft (2.7 m)
0.2905Ω < 0.3Ω
12 ft (3.6 m)
0.3874Ω < 0.4Ω
204 ft (1.2 m)
0.0812Ω < 0.1Ω
9 ft (2.7 m)
0.1827Ω < 0.2Ω
14 ft (4.3 m)
0.2842Ω < 0.3Ω
19 ft (5.8 m)
0.3857Ω < 0.4Ω
187 ft (2.1 m)
0.8939Ω < 0.1Ω
15 ft (4.6 m)
0.1916Ω < 0.2Ω
23 ft (7.0 m)
0.2937Ω < 0.3Ω
31 ft (9.4 m)
0.3959Ω < 0.4Ω
1612 ft (3.6 m)
0.0964Ω < 0.1Ω
24 ft (7.3 m)
0.1928Ω < 0.2Ω
37 ft (11.3 m)
0.2972Ω < 0.3Ω
49 ft (14.9 m)
0.3936Ω < 0.4Ω
1419 ft (5.8 m)
0.0960Ω < 0.1Ω
39 ft (11.9 m)
0.1970Ω < 0.2Ω
*59 ft (18 m)
0.2980Ω < 0.3Ω
*79 ft (24.1 m)
0.3990Ω < 0.4Ω
1231 ft (9.4 m)
0.0985Ω < 0.1Ω
*62 ft (18.9 m)
0.1969Ω < 0.2Ω
*94 ft (28.7 m)
0.2985Ω < 0.3Ω
*125 ft (38.1 m)
0.3970Ω < 0.4Ω
1050 ft (15 m)
0.0999Ω < 0.1Ω
*100 ft (30 m)
0.1998Ω < 0.2Ω
*150 ft (46 m)
0.2997Ω < 0.3Ω
*200 ft (61 m)
0.3996Ω < 0.4Ω

Another important thing to note is that, for optimal results under normal conditions, it’s not advised to run speaker cable runs above 50 feet (even if the math stated above shows no issues).


Impedance Matching Vs. Impedance Bridging

Though rarely talked about when discussing damping factor, I thought it important to add this section on impedance bridging. This is an adjacent topic that I think will benefit us to understand alongside damping factor.

When we talk about selecting loudspeakers for an amplifier (or vice versa), the term we often use in “matching”.

With impedance being such an important value for the amp and speaker, it can be confusing as to whether we should match impedance (choose an amp with equal output impedance to the speaker’s input impedance).

Understanding damping factor, we can see that true impedance matching would yield a DF of 1. This would be rather disastrous and the amplifier would have little control over the voice coil. In fact, the voice coil would affect the amplifier just as much as the amplifier affects the voice coil.

Impedance matching is used for maximum power transfer between two devices.

However, in audio, we’re concerned with optimal voltage (signal) transfer. In addition, when discussing amplifiers and speakers, we’re also concerned with achieving a high damping factor.

Both optimal signal transfer and high damping factors are possible with proper impedance bridging.

Impedance bridging means that the load impedance (impedance of the connected input) is much higher (often magnitudes) than the source impedance (output impedance of the connected output).

Let’s have another look at the aforementioned simplified voltage divider:

Simplified Amplifier-Loudspeaker Connection

• ZS (source impedance) is the output impedance of the amplifier
• ZL (load impedance) is the nominal impedance of the loudspeaker
• VS is the voltage of the output of the amplifier
• VL is the voltage across the loudspeaker driver

Voltage bridging (impedance bridging) is the result of having ZL much greater than ZS. This yields maximum voltage transfer and much higher efficiency.

This is true of all audio connections. For example:

  • Microphone output impedance is much lower than a microphone preamplifier input impedance.
  • Electric guitar pickup output impedance is much lower than a guitar amplifier input impedance.
  • Power amplifier output impedance is much lower than a loudspeaker input impedance.

We’ll discuss impedance bridging and damping factor of other audio connections in the section Does Damping Factor Apply To Other Audio Connections?

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.

This aligns nicely with damping factor. Before I stated that a damping factor above 20 should be strived for. Some would argue that a DF above 10 is more than enough.

The main point, beyond argument, is that a higher load impedance is necessary. Having a load much higher than the source is better for signal transfer and for damping factor. In theory, the higher the better!


Damping Factor & System Q

In this section, we’ll discuss damping factor; the Q values of a system, and the overall damping of a system.

The purpose of this section is to show that, while higher damping factors are generally thought of as better, there is a point at which increasing damping factor has little to do with improving system damping and speaker control and becomes more of a marketing ploy.

So, we already know what damping factor is. What it Q then?

Q is a Thiele-Small parameter (TSP) and stands for quality factor. It is used to describe how well the driver will control its own movement at the resonant frequency. Q is the inverse of damping ratio (of the system) and so a lower Q means more control.

Technically, Q is the ratio of the motional impedance to losses at the resonant frequency of the speaker driver. Since damping is most important at resonance frequency, Q is a good value to use in our calculations of how damping factor affects the amplifier-loudspeaker system.

For an in-depth discussion on Thiele-Small parameters, check out my article Full List: Thiele-Small Speaker Parameters W/ Descriptions.

There are plenty of ways to damp a speaker driver and, therefore, plenty of different Q values. They include:

  • Qa: Q @ Fb Due To Absorption Losses
  • Qec: Q @ Fc
  • Qes: Electrical Q
  • Ql: Q @ Fb Due To Leakage Losses
  • Qmc: Q @ Fc Due To Mechanical Losses
  • Qms: Mechanical Q
  • Qp: Q @ Fb Due To Port Losses
  • Qtc: Pneumatic Q
  • Qts: Total Q

The Total Q (Qts) represents the quality factor of the entire system (the total resistive losses in the system), taking into account the mechanical losses (defined by Qms) and the electrical losses (defined by Qes).

The equation for Qts is as follows:

Qts = (Qms • Qes) / (Qms + Qes)

Qms is defined by losses in the driver’s suspension, leakage losses, absorption losses, and more.

Qes is determined by the electrical resistance of the speaker’s voice coil and crossover components; the amplifier’s output resistance, and the resistance in the cable/leads that connect the amplifier and loudspeaker.

The speaker’s specifications of Qms, Rvc and Rx (mechanical Q, voice coil resistance and crossover resistance, respectively) are unchanging. The speaker cable/lead’s resistance is yet another constant.

For this section, let’s assume the overall load resistance of the speaker is 8Ω (a very common nominal impedance value for a speaker to have). To simplify further, let’s assume negligible wire resistance, though we’ll get back to speaker cable resistance shortly.

With all the above constants, we can continue with the “variable” of the source resistance (the amplifier’s output impedance). Of course, the actual impedance of an amplifier is typically not variable. This is only to set up our calculations.

The effect that the source resistance has on the overall Qes is given as:

Qes‘ = Qes • ((Rvc + Rs) / Rvc)

Where
Qes is the total electrical Q with the amplifier’s output impedance taken into account.
Qes is the total electrical Q assuming a source impedance of zero.
Rvc is the voice coil resistance.
Rs is the source resistance (output impedance of the amplifier).

So then our original Qts equation, when the amplifier is connected to a loudspeaker, becomes:

Qts‘ = (Qms • Qes‘) / (Qms + Qes‘)

As Qes gets smaller, Qts gets smaller. This means an improved damping factor and more control.

Looking again at the Qes‘ equation,

Qes‘ = Qes • ((Rvc + Rs) / Rvc)

We see that as Rs gets smaller, Qes gets smaller.

Therefore, as the amplifier’s output impedance decreases, Q decreases and damping factor increases.

Let’s further our understanding with an example. We’ll use the Dayton Audio RS225-8 (link to compare prices at select retailers) 8-ohm 8″ woofer. as our example here.

Dayton Audio RS225-8

A few of the important T/S parameters of the RS225-8 are:

  • DC Resistance (Qvc): 6.5 ohms
  • Resonant Frequency (Fs): 28.3 Hz
  • Mechanical Q (Qms): 1.46
  • Electromagnetic Q (Qes): 0.51
  • Total Q (Qts): 0.38

We see that the driver does, in fact, has a resonant frequency (at 28.3 Hz).

Let’s test to ensure our Qts = (Qms • Qes) / (Qms + Qes) equation is right:

0.38 = (1.46 • 0.51) / (1.46 + 0.51) checks out!

The following table will show what happens to the overall Q (Qts) of the system (Dayton Audio RS225-8 with connected amplifier) as the damping factor increases. We’ll use the 6.5 Ω Rvc for these calculations. Remember that lower Q values mean more control!

Damping FactorRsQes'Qts'
16.5Ω1.0200.600
23.25Ω0.7650.502
32.166Ω0.6800.464
51.3Ω0.6120.431
100.65Ω0.5610.405
200.325Ω0.5360.392
300.2166Ω0.5270.387
500.13Ω0.5200.384
1000.065Ω0.5150.381
2000.0325Ω0.5130.379
3000.02166Ω0.5120.379
5000.013Ω0.5110.378
10000.0065Ω0.5110.378
20000.00325Ω0.5100.378
30000.002166Ω0.5100.378
50000.0013Ω0.5100.378

As we can see from the table above, at low damping factors (below 20, let’s say), any change in DF will yield a rather significant change in the overall damping of the system. However, at higher damping factors (let’s say 20 or above), any increase in damping factor has comparatively little effect on the system damping.

This is why many would argue that a DF above 20 (or sometimes even 10) is more than enough to control a speaker with ease.

A damping factor of 2000, let’s say, will not provide much more control than a DF of 1000, even though it’s double. A damping factor 20 compared. to10, however, will have a rather significant (and often audible) impact of the system performance.

That being said, mathematically, a higher DF will always provide more control, there’s just a limit to the Qts of any system and a point of diminishing returns.

The Dayton RS225-8 is a single driver without an enclosure. Rest assured that the same type of relationship (though the numbers and curves will be different) occurs between damping factor and overall system Q in full-range speaker units with multiple drivers, enclosures and crossovers. It also holds true at varying gauges and lengths of speaker cable.

To learn more about speaker crossovers, enclosures and drivers, check out the following My New Microphone articles, respectively:
What Is A Speaker Crossover Network? (Active & Passive)
Why Do Loudspeakers Need Enclosures?
What Are Speaker Drivers? (How All Driver Types Work)

The main takeaway I want to drive home is to not be fooled by ultra-high damping factor specifications!


Tube Vs. Solid-State Amplifier Damping Factors

Tube amplifiers are notorious for their relatively low damping factors. A high-quality vacuum-tube amplifier can be expected to have a damping factor between 10 to 20.

Why is it that tube amps have low damping factors while some of the solid-state amplifiers on the market have rated damping factors above 1000?

Well, a big part of the reason for the extremely low output impedances of solid-state power amplifiers is the use of negative feedback in their integrated amplifier circuits.

The negative feedback node, which loops back into the amplifier circuit is at the output stage of the solid-state amplifier and keeps the output impedance of the amplifier incredibly low.

The low output impedances of solid-state amplifiers allow them to act somewhat like voltage sources. They are able to output the steady voltage regardless of the load resistance/impedance.

The output of a tube amplifier, conversely, will generally have an output transformer. The transformer acts to convert the high-voltage low-current signal from the vacuum tube(s) into a high current signal that can drive the speaker. It also acts to block the high DC voltage on the tube side to keep it from reaching the loudspeaker and causing ill effects.

Although the output transformer is a step-down transformer (it drops voltage and impedance), it is limited in how low it can drop the output impedance.

So then, tube amps can be expected to have DF ratings between 10 – 20 though some drop down to single-digits and others have DFs above 20.

Solid-state amplifiers, on the other hand, can easily be designed with very low output impedance and, therefore, very high damping factors.


High Damping Factor Vs. Low Damping Factor

We’ve already discussed that higher damping factors, in theory, give the amplifier more control over the movement (and sound) of the loudspeaker.

However, tube amplifiers, which have low DFs, can sound great.

Conversely, we can see that solid-state amplifiers are easily designed with high DFs and there are plenty of poorly designed solid-state amplifiers that yield lacklustre results.

So the high damping factors, by themselves, are not enough to determine whether and amplifier is of high quality or not.

The truth is that, the closer we are to a damping factor of 1, the more an increase in DF is beneficial to the control the amplifier has over the speaker.

Small changes in an already low amplifier output impedance change overall damping factor by only a small, negligible, amount.

Thus, high damping factor values do not, by themselves, say very much about the quality of a system; most modern amplifiers have them, but vary in quality nonetheless.

Just because a speaker has a high damping factor doesn’t mean it’s a great amplifier. DF is simply one of the factors/specifications to look for when considering the best amplifier for your situation and loudspeakers.


Does Damping Factor Apply To Other Audio Connections?

Damping factor is typically only a concern in the amplifier-to-loudspeaker connection.

This is because the speaker is an electromechanical transducer and we are concern with damping the speaker drivers electrically. As we’ve discussed, a high load impedance to source impedance ratio (damping factor) helps to electrically dampen the speaker driver.

However, there are plenty of other audio connections that require impedance bridging for optimal voltage/signal transfer.

Let’s look at 3 examples:

Microphone To Mic Preamp

For example, a microphone outputs a signal into a microphone preamplifier. The mic output impedance is generally in the range of 150Ω to 300Ω and the mic preamp input impedance is generally 1,200Ω to 10,000Ω.

Let’s say, more specifically, that a microphone with an output impedance of 150Ω connects to a preamp with an input impedance of 1,500Ω. That’s technically a damping facotr of 10.

However, we’re not overly concerned with “damping” the microphone preamplifier input. There’s no transducer to dampen in the mic preamp.

What we are concerned with, in this scenario, is impedance bridging for voltage/signal transfer.

With the above example (damping factor of 10), assuming no losses in the audio cable, there would only be a loss of -0.83 dB between the mic output and the mic preamp input. That’s a signal transfer of 90.9% versus the 50% (-6 dB) that happens with a DF of 1.

For more information on microphone impedance and mic preamps, check out the following My New Microphone articles:
Microphone Impedance: What Is It And Why Is It Important?
What Is A Microphone Preamplifier & Why Does A Mic Need One?

Electric Guitar To Guitar Amp

An electric guitar pickup can range wildly from 3.7 kΩ and below to well above 470 kΩ. The input impedance of a guitar amplifier is typically around 1 MΩ (1,000,000 Ω).

However, we’re once again not concerned with the damping of anything here. The guitar amplifier itself doesn’t have a transducer to damp.

Rather, we’re interested in optimal signal transfer and impedance bridging.

Note that the ouptut of the guitar amplifier should ideally be designed with a proper impedance and damping factor for the guitar cabinet’s speaker.

Let’s say, for an easy example, a guitar pickup with an impedance of 20 kΩ outputted a signal into a guitar amplifier with an input impedance of 1 MΩ.

That yields a damping factor of 50 but damping isn’t what we’re after. Rather, this “damping factor” of 50 (assuming no losses in the patch cord which is an impossibility) would cause only -0.17 dB signal loss. That’s a signal transfer of 98.0%!

Headphone Amplifier To Headphones

Here’s an alternate example where damping factor actually plays a role.

See, headphones are transducers that convert audio signals into sound waves just like speakers do (only much smaller and nearly always is a stereo format without crossovers).

A headphone amplifier’s output impedance can be expected to be in the 0.5 – 50 Ω range. Dynamic headphones generally have impedance ratings in the range of 20 – 300 Ω.

So then, with headphones, we’re concerned with optimal signal transfer and impedance bridging. However, because we’re driving drivers with audio signals, we also want a good damping factor between our headphone amp and headphones.

With headphones, the loose rule of thumb is the “rule of eights” which suggests a damping factor of 8 for optimal headphone results.

There are two reasons for this that reiterate the want for high damping factors between loudspeakers and power amplifiers.

The first reason is that low-end clarity will suffer greatly with low damping factors.

To produce low-end frequencies, the headphone driver is required to oscillate more slowly and with larger excursions. If the amplifier has poor control over the movement of the diaphragm, it will affect the driver’s ability to accurately reproduce the oscillations required of low-end long-wavelength sound waves.

This affects low frequencies more than higher frequencies and the end result is often a boomy and undefined low-end with a poor transient response. This, like in loudspeakers, is unwanted in any headphones.

The second reason is that the impedance of a dynamic headphone driver will spike at its resonant frequency and in its high-end (just like a loudspeaker’s impedance).

A lower damping factor that may seem sufficient to drive the headphone at nominal impedance may fail to do so as accurately at the resonant frequency. This may cause significant distortion and an alteration to the headphone’s frequency response.

To learn more about headphone impedance and headphone amplifiers, check out the following My New Microphone articles, respectively:
The Complete Guide To Understanding Headphone Impedance
What Is A Headphone Amplifier & Are Headphone Amps Worth It?


Are 4-ohm or 8-ohm speakers better? Though the term “better” is subjective, some would argue that 4-ohm speakers, though being more difficult to drive and requiring more powerful amplifiers, do sound better. This may be due to the fact that most consumer-grade speakers are 8-ohm for better compatibility while high-end speakers are more likely to be 4-ohm.

What is amplifier power rating? The power rating/output specification of the power amplifier is a measurement of the maximum electrical power available to be drawn from the amplifier by the loudspeaker. It is typically measured at a continuous (RMS) level at a given frequency and varies depending on the speaker’s input impedance.

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