Damping factor
From Free net encyclopedia
- 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 <math>Z_\mathrm{load}</math> (input impedance) and the source impedance <math>Z_\mathrm{source}</math> (output impedance).
Image:Source and load circuit Z.png
The damping factor <math>DF</math> is:
- <math>
DF = \frac{Z_\mathrm{load}}{Z_\mathrm{source}} </math>
For audio power amplifiers this source impedance <math>Z_\mathrm{source}</math> (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. <math>Z_\mathrm{load}</math> >> <math>Z_\mathrm{source}</math>.
The loudspeaker's load impedance (input impedance) of <math>Z_\mathrm{load}</math> is usually around 4 to 8 Ω
Solving for <math>Z_\mathrm{source}</math>:
- <math>
Z_\mathrm{source} = \frac{Z_\mathrm{load}}{DF} </math>
One can therefore calculate the small output impedance, hidden in the word damping factor in the manual datas. 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 tell you 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.
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. 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.
Incidentally, maintaining a good damping factor is why speaker cables matter, though they are not as important as some try to claim they are. The damping factor is affected 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, probably around 10. Thus provided that the return path of the cables measures less than 0.8 Ω (0.5 Ω is a good figure to aim for) thicker or better cables will make no perceptible difference because the ratio only tells us how negligible any added resistance is. 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. Moving it by just a few inches in the room will usually make a much bigger difference by changing the way in which Resonant room modes are stimulated. However, this subject brings its own controversies; in general the most gains are made for a modest increase in outlay over the very cheapest of cabling — beyond that is where the controversy lies.
See also
- Impedance
- Input impedance
- Output impedance
- Voltage divider
- Audio system measurements
- Matching attenuation