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Damping Factor

df-diag-2

In this typical loudspeaker, a World Audio Design KLS9, the ferrite cored crossover inductor has a d.c. resistance of 0.66 Ohms. The loudspeaker bass unit 'sees' 0.66+0.3+0.1 = 1.06 Ohms. That results in a real Damping Factor of 5.9, from an output impedance that gives a notional D.F. of 80. There is still sufficient electrical damping on the cone to audibly tighten bass, however.

WHAT IT TELLS US

Damping factor is, indirectly, a measure of output impedance. A receiver with low output impedance grips the bass cone and produces ‘tight’ sounding bass, free of waffle and overhang.


A damping factor figure of around 20 is the break point for audibility with most loudspeakers we find in listening tests. Transistor amplifiers of receivers exceed this figure by a good margin and there is little difference between them in perceived bass control.


In conventional loudspeakers, a bass inductor lies in the signal path between amplifier and bass unit and limits the influence of high damping factors. A resistance of 0.3 Ohms will mean a DF of 27 is the best an 8 Ohm bass unit will see, or just 13 with a 4 Ohm bass unit (which more are nowadays). This makes high damping factors academic.


So Damping Factor does affect bass quality, but there’s little difference between a majority of transistor amplifiers used in AV receivers because their use of feedback results in low output impedance, around 0.1 Ohms, and a notional DF of 40 or more.

HOW WE MEASURE IT

We measure the voltage (pd) driving 8 Ohms and then that across 4 Ohms, with 40Hz sine wave, at around 4V.  Damping factor is calculated from the formula -


We measure the voltage driving an 8 Ohm and then a 4 Ohm load, with 40Hz sine wave at around 4V. Damping Factor is then derived by the formula -


2- V8/V4 / (2xV8/V4)-2  where V8 is voltage across 8 Ohms and V4 voltage across 4 Ohms.



Damping Factor can alternatively be derived by the simple formula -

pd / emf - pd


where emf = open circuit volts;  pd = voltage across a load


Example

If an amplifier's output was 6V with no load and 5V across an 8 Ohm load, Damping Factor = 5/(6-5) = 5

Output impedance = Load Z/DF = 8/5 = 1.6 Ohms

 

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