Damping factor

The term damping factor can also refer to the amount of damping in an oscillatory system

The damping factor in an electrical circuit gives the ratio of the impedances of two electronic devices, the load impedance $Z_{\mathrm {load} }$ (input impedance) and the source impedance $Z_{\mathrm {source} }$ (output impedance).

The damping factor $DF$ is:

$DF={\frac {Z_{\mathrm {load} }}{Z_{\mathrm {source} }}}$

Explanation

For audio power amplifiers this source impedance $Z_{\mathrm {source} }$ (also: output impedance) is generally smaller than 0.1 Ω (ohms), and can be seen from the point of view of the loudspeaker as a near short-circuit. This will very rapidly absorb any unwanted currents induced by the mechanical resonance of the speaker's voice coil, acting as a very effective 'brake' on the speaker (just as a short circuit across the terminals of a generator will make it very hard to turn), thus keeping it under control.

This is called voltage bridging. $Z_{\mathrm {load} }$ >> $Z_{\mathrm {source} }$.

The loudspeaker's load impedance (input impedance) of $Z_{\mathrm {load} }$ is usually around 4 to 8 Ω although other impedance speakers are available.

Solving for $Z_{\mathrm {source} }$:

$Z_{\mathrm {source} }={\frac {Z_{\mathrm {load} }}{DF}}$

The output impedance of an amplifier can therefore be calculated from the damping factor and the loudspeaker impedance. Note that modern amplifiers, employing relatively high levels of negative feedback, generally exhibit extremely low output impedances — one of the many consequences of using feedback. Thus "damping factor" figures in themselves do not say very much about the quality of a system. Given the controversy that has surrounded the topic of feedback for many years, some may see a high damping factor as a mark of poor quality.

In practice

From a practical standpoint, damping factor describes the ability of the amplifier to control the voltage (the signal amplitude) applied to the speaker. A speaker's impedance varies as the signal frequency and an amplifier must therefore supply a varying amount of current to achieve a particular signal voltage, depending on the circumstances. A loudspeaker can also function as a microphone, generating an electrical signal in response to movement of the speaker diaphragm (or cone). The diaphragm has mass, and therefore, inertia. The cone may resonate in response to short pulses, such as from a bass (kick) drum. A high damping factor indicates that an amplifier will have greater control over the movement of the speakers, particularly the bass. This translates into "tight bass" sound from the sound system.

The damping factor is affected to a small extent by the resistance of the speaker cables. The higher the resistance of the speaker cables, the lower the damping factor. A large damping factor is no advantage beyond a certain point, around 10. Thus provided that the return path of the cables measures less than about 0.8 Ω , thicker or better cables will make no perceptible difference. The difference in damping with a factor of 10 is the difference between 8 Ω and 8.8 Ω, which is unlikely to give more than a fraction of a dB difference in level at the low frequency resonance of the speaker.

See also

• Impedance
• Input impedance
• Output impedance
• Voltage divider
• Audio system measurements
• Matching attenuation

External links

• Understanding Damping Factor
• Calculation of the matching attenuation,the damping factor, and the damping of bridging
• Description of damping factor