Originally Posted by Heinrich S
Originally Posted by arnyk
Most speakers are measured at 90 dB SPL, and with a few ragazines that want to be extra complete, the extra robust speakers might get another shot of measurements at 95 dB SPL.
For the average speaker 90 dB SPL takes 1.00 watts, and 95 dB SPL takes more like 4.00 watts. There are usually measureable differences in performance between those two power levels.
50 watts or 200 watts are over there in the next universe! ;-)
A speaker being mechanically overstressed because not enough power was supplied makes as much sense as a 200 MPH car being overstressed by driving 100 mph.
But that's the thing. What causes mechanical stress?
The two strongest sources of physical stress in speakers is diaphragm displacement and heat build up in the voice coil.
The performance fundamentals are that in order to survice and sound good the diaphragm has to be able to travel back and forth in a linear fashion, and the voice coil has to dissipate heat.
The heat problem is there because the thermodynamic efficiency of most home speakers is around 1% or so. 99% oor so of the amplifier's power that is actually fabricated and delivered to the speaker gets turned into heat and about 1% gets turned into sound. If we look at very large price and size is no object professional speakers, the efficiency is about 10 dB or about 3 times greater. In either case the lack of efficiency is appalling, but its the best we can do right now or have been able to do since Lee DeForest and the turn of the previous century.
So you are saying that a speaker with a power handling of 200W can distort with a 70W amp mechanically and not thermally?
No, the distortion is due to both mechanical and thermal issues.
Tweeter voice coils are tiny and yet they may havve to dissipate a lot of heat for their size. They have to be made out of copper or aluminum or something like them and these materials experience very significant changes in resistance due to normal heating and cooling as they operate. You apply a lot of power to a tweeter, its voice coil resistance might double, and less current than is expected is able to flow through it even though more voltage is applied to it. Voila! Thermal compression.
As I mentioned earlier, a speaker diaphragm has to move in a mechanically linear fashion to have linear response at all relevant SPL levels, but the physics of cone suspensions and magnetic motors fight against this. Linear diaphragm travel is an issue with woofers, midranges and tweeters near the bottom of their frequency range.
Wouldn't the amp be clipping in order to push the speaker into mechanical distortion?
Not necessarily. Please recall the power levels I mentioned in an earlier post - 1 watt and 4 watts. Most speakers are producing or verging on producing say 1% THD or more over part of their operational frequency range by the time amplifier power reaches 4 watts. Distortion in speakers does not follow the same pattern as amplifiers. Instead of being low and then spiking up at the clipping point, their distortion rises more slowly. That means more distortion at lower levels.
Wouldn't mechanical stress be caused by low and loud bass signals? If the issue is in the mids, upper mids or highs, wouldn't it be thermal related, as in I'm not using enough?
I see evidence of mechanically produced distortion at the low end of the operational frequency range of even very expensive and relatively good speakers:
This is the measured THD performance of the Paradigm Studio 100 V3 @ a mere 95 dB SPL:
So what happens at peak reference level (105 dB)? All we know with any certainty is that the THD is significantly higher. Would you be happy with an amplifier that performed like this? ;-)