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General Asylum General audio topics that don't fit into specific categories. |
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In Reply to: A question wtih possibly no answer re: system dynamics posted by TopPop on May 10, 2006 at 08:25:55:
Two main types of dynamic compression occur in a typical home playback system: Clipping and thermal compression.Clipping is most likely to occur in the power amplifier, but it can also occur in the output stage of CD player, phono section, preamp, or other processors. Soft clipping (charactersitic if tubes) is often perceived as a compression of the dynamics before it's heard as an audible distortion, whereas hard clipping (characteristic of solid state) is usually heard as a distortion before it's perceived as a compression of the dynamics.
Thermal compression (also called power compression) occurs in the loudspeaker and is a result of voice coil heating. As the voice coil heats up its resistance rises, lowering both the efficiency and voltage sensitivity of the system. We're used to thinking that a tenfold increase in amplifier power will give us a 10 dB increase in SPL, but in practice the increase is more like 7 to 9 dB depending on the speakers and the input power levels. Obviously this will soften the dynamic impact of transients. I believe that thermal compression is the dominant type of dynamic compression in most sound systems.
Mechanical compression is less common than thermal compression and only occurs when a loudspeaker driver's physical excursion puts the voice coil and/or suspension beyond its linear range. For instance mechanical compression will usually occur before clipping or significant thermal compression when a short-throw high-efficiency fullrange driver (like a Lowther or Fostex) is pushed too hard, but can occur when any driver is pushed too hard at low frequencies.
In my opinion the recipe for good dynamic contrast is to start with high efficiency loudspeakers, or at least with loudspeakers that have negligible thermal compression within the anticipated SPL range (figure based on peak SPL's, which can be 15 or more dB above the average SPL). Then get an amplifier sufficiently powerful to not clip at those anticipated peak SPL's with those speakers.
Unfortunately thermal compression figures are never quoted for home audio loudspeakers, so as a general guideline high efficiency is the primary indicator, and high RMS power handling is the secondary indicator.
I haven't seen data to support this, but I believe that signal smearing can reduce subjective dynamic contrast. I think the effect is more likely to be noticeable through a coherent loudspeaker system. I've had several customers swear to me that a certain speaker cable "made their amp play louder", and the only thing I can think of is that that cable manufacturer places a very high priority on eliminating any signal smearing.
A change that raises or lowers the noise floor will also affect the dynamic range of the system, resulting in the perception of greater or lesser dynamic contrast accordingly.
I have the impression that power supply issues in electronics throughout the chain can effect the dynamic contrast, but I don't know how much of a difference that can make in practice, nor whether or not the result is significantly different from clipping. I'd appreciate clarification from anyone with a better understanding of this.
Finally, subtle frequency response changes due to cabling or component changes can give the impression of increased or decreased dynamic contrast.
But to recap, in my opinion loudspeaker thermal compression is the primary source of compression in most playback systems, and the second most common source of compression, amplifier clipping, can be attributed jointly to louspeaker and amplifier.
All just my opinion, corrections welcome.
Duke
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Follow Ups
- Possibly a partial answer - Duke 13:04:18 05/10/06 (2)
- Re: Possibly a partial answer - Jim Griffin 17:10:55 05/10/06 (1)
- Re: Possibly a partial answer - Duke 22:26:12 05/10/06 (0)
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