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RE: Horns and high efficiency . what effect ?

Paul and others mention dynamics and I would underline that word as well. What is very hard to picture for many is why that is.
If one were to take a sound level meter and measure a reproducible event, one finds that on the "fast" setting and C weighting, the meter usually agrees very well with subjective loudness.
One might conclude that the requirements for a loudspeaker to re-produce "that sound" would be what the SLM indicates (accounting for distance etc) and while this often assumed, reality is very different partly because what "we" experience as loudness is NOT very tightly tied to what it takes to reproduce a given signal. The VU meter (Volume Unit) was conceived to indicate loudness but it deliberately has a time constant and that is becase our hearing also has a time constant, loudness is not only related to how loud something actually is but also how long it has lasted, to be perceived as loud, it mush exist for some amount of time. Very short and loud sounds we do not judge as being loud when compared to that sound sustained for a second or so.
The measurement or recording microphone on the other hand converts acoustic pressure into Voltage signal with a fixed proportionality so that one can examine the Voltage and knowing the mic pressure sensitivity, calculate the instantaneous pressure at any point in the waveform.
What one finds is that many common sounds (and musical instruments) have shockingly high instantaneous SPL's which do not show up on a normal sound level meter. For example, I have a large B&K sound level meter which has a "peak hold" reading which captures the loudest peak (over some short time) and simply throwing a metal tea spoon on the tile floor in the kitchen produced an instantaneous peak spl of nearly 140dB. It was so short and no doubt much of it very high frequencies that to me it only sounded like one of the kids threw a spoon on the floor.
Point is, to reproduce anything such that is has the original dynamics, one must produce all the annoying instantaneous peaks not just the "Average" level related to subjective loudness or an spl meter.
This kind of dynamic limiting is more common than one might think and sadly, the ONLY way to see if a system is suffering from this is to get an oscilloscope and look at the amplifier output. Any instantaneous clipping or limiting in any part of the chain, will be visible if preset on the output terminals. In a more complicated sound system, this is one way to verify the gain structure (gain of each element in the chain) is acceptable.
So (looking only through the window of dynamics) if one has high efficiency speakers with a more powerful power amp, you're more likely to be reproducing the instantaneous peaks compared to a low power amp on low efficiency speakers.
Think about it, if one had a good recording that had 20dB or even 30dB difference between the peak and average level, that also means that the instantaneous peaks are 100 to 1000 times louder than the average level that one would read with an integrating meter, like a sound level meter or our sense of loudness. These recordings sound dynamic, not loud because the loud portions are too short.
Funny thing too, being so short, with instantaneous clipping, one cannot hear it as a flaw and is only detectable when compared to the same music without it, or visible on an oscilloscope.

There are many ways to get higher efficiency and like low efficiency systems, all have engineering difficulties

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