Power compression is a topic that is rarely discussed, but always important. When you hear someone call various theile/small parameters a “small signal parameter”, the implication is that with larger signals (more power), there will be a shift or change in these theile/small parameters. This is what we call Power compression, and occurs in 3 potential ways.
The first thing to consider is how theile/small parameters are derived. There are 5 basic parameters from which all other data is extracted. These parameters are Re, BL, Mms, Sd, and Cms. So looking at this from a mathematical standpoint, a change in any of these parameters will cause a change in all other parameters as well.
Of those 5 parameters, there are three that can change once power (rather, more power) is applied; they are: Re, BL, and Cms. Now we will review the cause and effects for each one.
Re is the measured DC resistance of the voice coil. When power is applied to the voice coil, the metal becomes hotter, and resistance increases. Heating copper by 200 degrees Celsius will double the resistance, which doubles the measured Re; this results in a 3dB loss in acoustical output. Being that this is also one of the 5 basic parameters, you will see a change in all other parameters as well. For example, a change in Re will cause a large change in your enclosure alignment because doubling Re will also double Qes, as demonstrated below:
Qes = Re*Sqrt(Mms)/(BL^2*Sqrt(Cms))
This is power compression at it’s finest, and the most commonly mentioned one, known as Thermal Power Compression.
The second type of power compression refers to motor force. As power is applied to the voice coil, causing it to progress out of the gap, the motor force (or BL) begins to decrease. This decrease in motor force causes a drop in output and, again, a shift in all other parameters. When the motor force drops by 30%, you've lost 3 dB in output. This is, in part, one of the reasons why more recent definitions (from DLC and Dumax as well as Klippel) of Xmax have defined it as 70% of rest BL. This type of compression is referred to as BL Power Compression.
The third type is related to the compliance of the suspension. As power is applied, the voice coil begins to move out of the gap, causing the cone, spider, and surround to move as well. This movement changes the compliance of the suspension, and is known Cms Power Compression.
Of the three, power compression is a more gradual process. A quick burst of power will heat the voice coil quickly but it will take a while to cool back down and remain fairly constant. Over a 30 second period, the coil will have cooled down and been reheated several times, so that while you thermal power compression occurs, it's not it does not occur nearly as fast as BL and Cms. In other words, thermal power compression is a much less dynamic issue.
BL and Cms power compression are very frequent and occur on every stroke, as a stroke is simply the voice coil and attached assembly moving. For example, playing a 60hz sine wave will cause 60 full strokes in a one second period of time, and since BL and Cms compression occurs at both the positive and negative ends of the stroke, you would actually encounter BL and Cms power compression 120 times a second! Of course, as power is applied, excursion increases, and BL and Cms compression increases as well. This is why it’s so important to find a driver that has very flat BL and Cms curves (measured on Dumax or Klippel) because these curves represent the behaviour of BL and Cms over the stroke; if the curve is relatively flat and extends out fairly far on either side, BL and Cms compression will be of little effect until you reach the extremes.
For most installs, BL and Cms compression will be of the greatest concern. For those who compete or have fairly high power stereos, thermal compression will become a growing concern as more and more power is applied.
To summarize, Thermal, BL, and Cms Power Compression are all issues to consider, but it is important to understand how to work around these issues while knowing which drivers suffer more than others. A driver with adequate cooling and flat BL and Cms curves out to 20+ mm will likely experience very little parameter shift and, as such, is likely a better candidate for your high end stereo.
This mite also explain why in my last system I would have great output at half volume and as I would turn it up more I would loose output. I was running double the wattage to subs. Half volume rocked... And as I went past output dropped noticeably.
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