Bare in mind that most speakers become LESS efficient as more power is pumped into them. That does not mean that volume goes down as you put more power into them, but that power is lost in greater quantity due to thermal losses as you drive them harder. In effect, you run into what is basically non-linear dynamic compression. Combine this with the fact that many amps have a hard time dealing with increased current demands, their power supplies begin to sag and reflected EMF is increased as you drive a speaker harder. As such, one is typically best off finding the most sensitive speakers they can find that are suitable to the application. By doing so, you'll be pulling less power from the amp, so it runs with less stress and sounds better. At the same time, the speakers aren't being fed as much power, so they don't heat up as much and go into thermal / dynamic compression as rapidly or as often.
Having said that, i haven't found a lot of highly efficient speakers that i really like to listen to on a long term basis. I'm sure that there are others out there that share this dilemma, but then again, there are some folks that find low sensitivity more of a drawback. That is why there are so many different speakers and designs out there : ) Sean
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PS... Driving in a convertible with the top down on the highway, piloting a motorcycle with stock mufflers and no helmet or driving a "Jeep" type vehicle with bigger tires in suburban areas typically clocks in at about 90 dB's. Taking a Jeep on the highway with "off-road" type tires can be amazingly loud due to the amount of air that gets trapped in the treads. The motorcycle doesn't seem as loud as most of the sound is projected behind it, but the wind whipping by your ears definitely has a similar effect.
