Damping factor
Encyclopedia
In audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive. The source impedance (that seen by the loudspeaker) includes the connecting cable impedance.
The load impedance
(input impedance) and the source impedance 
(output impedance) are shown in the diagram.
The damping factor
is:
Solving for
:
systems, the value of the damping factor between a particular loudspeaker and a particular amplifier
describes the ability of the amplifier to control
undesirable movement of the speaker cone near the resonant frequency of the speaker system. It is usually used in the context of low-frequency driver behavior, and especially so in the case of electrodynamic drivers, which use a magnetic motor
to generate the forces which move the diaphragm
.
Speaker diaphragms have mass
, and their surrounds have stiffness
. Together, these form a resonant system, and the mechanical cone resonance may be excited by electrical signals (e.g., pulse
s) at audio frequencies. But a driver with a voice coil
is also a current generator, since it has a coil attached to the cone and suspension, and that coil is immersed in a magnetic field. For every motion the coil makes, it will generate a current that will be seen by any electrically attached equipment, such as an amplifier. In fact, the amp's output circuitry will be the main electrical load on the "voice coil current generator". If that load has low resistance, the current will be larger and the voice coil will be more strongly forced to decelerate. A high damping factor (which requires low output impedance at the amplifier output) very rapidly damps unwanted cone movements induced by the mechanical resonance of the speaker, acting as the equivalent of a "brake" on the voice coil motion (just as a short circuit across the terminals of a rotary electrical generator will make it very hard to turn). It is generally (though not universally) thought that tighter control of voice coil motion is desirable, as it is believed to contribute to better-quality sound.
A high damping factor indicates that an amplifier will have greater control over the movement of the speaker cone, particularly in the bass region near the resonant frequency of the driver's mechanical resonance. However, the damping factor at any particular frequency will vary, since driver voice coils are complex impedances whose values vary with frequency. In addition, the electrical characteristics of every voice coil will change with temperature; high power levels will increase coil temperature, and thus resistance. And finally, passive crossovers (made of relatively large inductors, capacitors, and resistors) are between the amplifier and speaker drivers and also affect the damping factor, again in a way that varies with frequency.
For audio
power amplifier
s, this source impedance
(also: output impedance
) is generally smaller than 0.1 ohm (Ω), and from the point of view of the driver voice coil, is a near short-circuit.
The loudspeaker's load impedance (input impedance) of
is usually around 4 to 8Ω, although other impedance speakers are available, sometimes as low as 1Ω.
A loud speaker's flyback current is not dissipated only through the amplifier output circuit, but also through the internal resistance in the speaker itself.
Even if the resistance of the speaker cable and of the amplifier is 0 Ω, the speaker still faces the resistance of its own voice coil wire.
Therefore, damping factor values above 50 or so are not meaningfully different from 50. For higher values of speaker damping than about 50, actual speaker damping no longer increases in any way that can be confirmed by a double-blind A/B listening test, or measured.
.
>> 
.
to control distortion, have extremely low output impedances—one of the many consequences of using feedback—and small changes in an already low value change overall damping factor by only a small, and therefore 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. Given the controversy that has long surrounded the use of feedback, some extend their distaste for negative feedback amplifier designs (and so a high damping factor) as a mark of poor quality. For them, such high values imply a high level of NFB in the amplifier.
Tube
amplifiers typically have much lower feedback ratios, and in any case almost always have output transformers that limit how low the output impedance can be. Their lower damping factors are one of the reasons many audiophiles prefer tube amplifiers. Taken even further, some tube amplifiers are designed to have no negative feedback at all.
Higher electrical damping of the loudspeaker is not necessarily better. Some loudspeakers sound better with lower electrical damping. A lower damping factor helps to greatly enhance the bass response of the loudspeaker, which is useful if only a single amplifier is used for the entire audio range. Therefore, some amplifiers, in particular vintage amplifiers from the 1950's, 60's and 70's, feature controls for varying the damping factor.
One example of a vintage amplifier with a damping control is the Accuphase E-202, which has a three-position switch described by the following excerpt from its owner's manual:
By contrast, in modern high fidelity amplification, the trend is to separate the bass signal and amplify it with a dedicated amplifier. Often, amplifiers for bass are integrated with the speaker cabinet, a configuration known as the powered subwoofer. In a topology that includes a dedicated amplifier for bass, the damping factor of the main amplifier is not relevant, and that of the bass amplifier is also irrelevant if that amplifier is integrated with the speaker and cabinet as a unit, since all those components are designed together and optimized for the reproduction of bass.
Damping is also a concern in guitar amplifiers, an application in which low damping is better. Numerous guitar amplifiers have damping controls, and the trend to include this feature has been increasing since the 1990's. For instance the Marshall Valvestate 8008 rack-mounted stereo amplifier has a switch between "linear" and "Valvestate" mode:
This is actually a damping control based on negative current feedback, which is evident from the schematic, where the same switch is labeled as "Output Power Mode: Current/Voltage". The "Valvestate" mode introduces negative current feedback which raises the output impedance, lowers the damping factor, and alters the frequency response, similarly to what occurs in a tube amplifier. (Contrary to the claim in the handbook, this circuit topology has has appeared in numerous solid-state guitar amplifiers since the 1970's.)
The load impedance
(input impedance) and the source impedance (output impedance) are shown in the diagram.The damping factor
is:Solving for
:Explanation
In loudspeakerLoudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
systems, the value of the damping factor between a particular loudspeaker and a particular amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
describes the ability of the amplifier to control
Control system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...
undesirable movement of the speaker cone near the resonant frequency of the speaker system. It is usually used in the context of low-frequency driver behavior, and especially so in the case of electrodynamic drivers, which use a magnetic motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
to generate the forces which move the diaphragm
Diaphragm (acoustics)
In the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be...
.
Speaker diaphragms have mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
, and their surrounds have stiffness
Stiffness
Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
. Together, these form a resonant system, and the mechanical cone resonance may be excited by electrical signals (e.g., pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...
s) at audio frequencies. But a driver with a voice coil
Voice coil
A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...
is also a current generator, since it has a coil attached to the cone and suspension, and that coil is immersed in a magnetic field. For every motion the coil makes, it will generate a current that will be seen by any electrically attached equipment, such as an amplifier. In fact, the amp's output circuitry will be the main electrical load on the "voice coil current generator". If that load has low resistance, the current will be larger and the voice coil will be more strongly forced to decelerate. A high damping factor (which requires low output impedance at the amplifier output) very rapidly damps unwanted cone movements induced by the mechanical resonance of the speaker, acting as the equivalent of a "brake" on the voice coil motion (just as a short circuit across the terminals of a rotary electrical generator will make it very hard to turn). It is generally (though not universally) thought that tighter control of voice coil motion is desirable, as it is believed to contribute to better-quality sound.
A high damping factor indicates that an amplifier will have greater control over the movement of the speaker cone, particularly in the bass region near the resonant frequency of the driver's mechanical resonance. However, the damping factor at any particular frequency will vary, since driver voice coils are complex impedances whose values vary with frequency. In addition, the electrical characteristics of every voice coil will change with temperature; high power levels will increase coil temperature, and thus resistance. And finally, passive crossovers (made of relatively large inductors, capacitors, and resistors) are between the amplifier and speaker drivers and also affect the damping factor, again in a way that varies with frequency.
For audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
power amplifier
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
s, this source impedance
(also: output impedanceOutput impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
) is generally smaller than 0.1 ohm (Ω), and from the point of view of the driver voice coil, is a near short-circuit.
The loudspeaker's load impedance (input impedance) of
is usually around 4 to 8Ω, although other impedance speakers are available, sometimes as low as 1Ω.The damping circuit
The voltage generated by the moving voice coil forces current through three resistances:- the resistance of the voice coil itself;
- the resistance of the interconnecting cable; and
- the output resistance of the amplifier.
Effect of voice coil resistance
This is the major factor in limiting the amount of damping that can be achieved electrically, because its value is larger (say between 4 and 8 Ω, typically) than any other resistance in the output circuitry of an amplifier that does not use an output transformer (nearly every solid-state amplifier on the mass market).A loud speaker's flyback current is not dissipated only through the amplifier output circuit, but also through the internal resistance in the speaker itself.
Even if the resistance of the speaker cable and of the amplifier is 0 Ω, the speaker still faces the resistance of its own voice coil wire.
Therefore, damping factor values above 50 or so are not meaningfully different from 50. For higher values of speaker damping than about 50, actual speaker damping no longer increases in any way that can be confirmed by a double-blind A/B listening test, or measured.
Effect of cable resistance
The damping factor is affected to some extent by the resistance of the speaker cables. The higher the resistance of the speaker cables, the lower the damping factor. When the effect is small, it is called voltage bridgingImpedance bridging
In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one which maximizes transfer of a voltage signal to the load...
.
>> . Amplifier output impedance
Modern solid state amplifiers, which use relatively high levels of negative feedbackNegative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
to control distortion, have extremely low output impedances—one of the many consequences of using feedback—and small changes in an already low value change overall damping factor by only a small, and therefore 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. Given the controversy that has long surrounded the use of feedback, some extend their distaste for negative feedback amplifier designs (and so a high damping factor) as a mark of poor quality. For them, such high values imply a high level of NFB in the amplifier.
Tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
amplifiers typically have much lower feedback ratios, and in any case almost always have output transformers that limit how low the output impedance can be. Their lower damping factors are one of the reasons many audiophiles prefer tube amplifiers. Taken even further, some tube amplifiers are designed to have no negative feedback at all.
In practice
Over a range from less than 1 to up to around 50, damping factor describes the ability to which the amplifier helps to control the oscillation of the speaker, expressing the degree to which the amplifier does not exert a resistance against the current generated by the speaker. A damping factor of 50 is high, indicating that the output impedance of the amplifier is negligible compared to the internal resistance of the speaker and the speaker cabling.Higher electrical damping of the loudspeaker is not necessarily better. Some loudspeakers sound better with lower electrical damping. A lower damping factor helps to greatly enhance the bass response of the loudspeaker, which is useful if only a single amplifier is used for the entire audio range. Therefore, some amplifiers, in particular vintage amplifiers from the 1950's, 60's and 70's, feature controls for varying the damping factor.
One example of a vintage amplifier with a damping control is the Accuphase E-202, which has a three-position switch described by the following excerpt from its owner's manual:
- "Speaker Damping Control enhances characteristic tonal qualities of speakers. The damping factor of solid state amplifiers is generally very large and ideal for damping the speakers. However, some speakers require an amplifier with a low damping factor to reproduce rich, full-bodied sound. The E-202 has a Speaker Damping Control which permits choice of three damping factors and induces maximum potential performance from any speaker. Damping factor with an 8 ohm load becomes more than 50 when this control is set to NORMAL. Likewise, it is 5 at MEDIUM position, and 1 at SOFT position. It enables choosing the speaker sound that one prefers."
By contrast, in modern high fidelity amplification, the trend is to separate the bass signal and amplify it with a dedicated amplifier. Often, amplifiers for bass are integrated with the speaker cabinet, a configuration known as the powered subwoofer. In a topology that includes a dedicated amplifier for bass, the damping factor of the main amplifier is not relevant, and that of the bass amplifier is also irrelevant if that amplifier is integrated with the speaker and cabinet as a unit, since all those components are designed together and optimized for the reproduction of bass.
Damping is also a concern in guitar amplifiers, an application in which low damping is better. Numerous guitar amplifiers have damping controls, and the trend to include this feature has been increasing since the 1990's. For instance the Marshall Valvestate 8008 rack-mounted stereo amplifier has a switch between "linear" and "Valvestate" mode:
- "Linear/Vstate selector. Slide to select linear or Valvestate performance. The Valvestate mode gives extra warm harmonics plus the richness of tone, which is unique to the Valvestate power stage. Linear mode produces a highly defined hi-fi tone that gives a totally different character to the sound and suits certain modern "metal" styles, or PA applications."
This is actually a damping control based on negative current feedback, which is evident from the schematic, where the same switch is labeled as "Output Power Mode: Current/Voltage". The "Valvestate" mode introduces negative current feedback which raises the output impedance, lowers the damping factor, and alters the frequency response, similarly to what occurs in a tube amplifier. (Contrary to the claim in the handbook, this circuit topology has has appeared in numerous solid-state guitar amplifiers since the 1970's.)
See also
- ImpedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
- Input impedanceInput impedanceThe input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
- Output impedanceOutput impedanceThe output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
- RLC circuitRLC circuitAn RLC circuit is an electrical circuit consisting of a resistor, an inductor, and a capacitor, connected in series or in parallel. The RLC part of the name is due to those letters being the usual electrical symbols for resistance, inductance and capacitance respectively...
- Voltage divider
- Audio system measurementsAudio system measurementsAudio system measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers make them to ensure equipment is still working to specification, or to ensure that the cumulative defects of an audio path are...
- DampingDampingIn physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
External links
- Radio Electronics - December 1954 and January 1955, D. J. Tomcik, Missing Link in Speaker Operation
- Marantz's Legendary Audio Classics. Ben Blish, Damping Factor
- Audioholics. AV University. Amplifier Technology. Dick Pierce, Damping factor: Effects on System Response
- ProSoundWeb. Studyhall. Chuck McGregor, What is Loudspeaker Damping?