Good speakers less efficient?

Accepting low efficiency allows a designer to get deeper bass in a given box size. All else being equal, most people will pick the speaker with deeper bass.

Passive equalization in crossover networks is pretty much limited to "cut" instead of "boost and cut". If the designer needs to reduce the efficiency of the tweeter and/or midrange to mate up with a low efficiency woofer, he can do some response-smoothing while he's at it.

Fairly low impedances (4 ohms nominal) will better utilize the capabilities of high quality solid state amplifiers. A 4-ohm speaker will draw twice as much power as an 8-ohm speaker from a high current solid state amp, so it will play 3 dB louder. All else being equal, most people will choose the extra free 3 dB.

Not being constrained to keep the impedance up around 8 ohms increases the range of drivers the designer can choose from, and gives him more lattitude in what he can do and how he can do it.

With all those advantages, why in the world would any designer not shoot for low efficiency and low impedance?

Diversity in philosophy abounds amongst loudspeaker designers, for the above-mentioned "all else" is seldom "equal". It's all about juggling tradeoffs. Each of the advantages cited above comes at a price. My own designs tend toward higher than average efficiencies and impedances, as I place tube amp compatibility high on my priority list - but I make trade-offs in doing so.

In my opinion, those loudspeakers that really sound good do so for two reasons: First, in combination with the rest of the system the speaker must recreate some aspect of a live performance convincingly enough to allow the listener to suspend disbelief and get lost in the music. That aspect can be timbre, impact, coherence, warmth, sense of rhythm, ambience, inner nuance, sound source localization, liveliness, whatever. Second, the speaker must avoid screwing up some aspect of the sound badly enough to destroy the illusion. Sometimes efficiency and impedance play a significant role in these characteristics for good or for ill, but most often it is other technical attributes that are the dominant factors.

Duke

Sharorne,

My experience with tonal balance shifting as the volume level changes has generally been the opposite of yours, though I don't doubt your observation. My experience with high efficiency speakers has mostly been with horn systems (though usually with a direct-radiator prosound-type woofer).

I have generally attributed tonal balance shift to differing power compression characteristics of the different drivers. Relatively few drivers give a full 3 dB increase in loudness for a doubling of input power - usually it's more like 2.6-2.8 dB. If the woofer compresses more than the tweeter, then the tonal balance will shift towards the tweeter's end of the spectrum at high volume levels. If the tweeter compresses more, then the tonal balance will shift towards the woofer's end of the spectrum at high volume levels.

I recall a couple of years ago listening to a very well known and generally well respected three-way, which sounded recessed and distant at low volume levels, well balanced at medium to borderline high volume levels, and decidedly forward at high volume levels. I think that the midrange (which was an especially high-tech unit) had less power compression than the woofer and tweeter, so the speaker had been "voiced" to sound balanced in the medium to borderline-high volume range. At higher volume levels the midrange left the woofer and tweeter in the dust, and at lower volume levels it withdrew into the background (okay I'm exaggerating a bit with my imagery here, but to my ears that was the trend).

Getting back to your observation, this is just speculation on my part but it's possible that very high efficiency conventional tweeters use ultrathin voice coil wire that heats up quite easily and rapidly, so that in practice such tweeters have more thermal compression than their medium efficiency cousins.

There can be other mechanisms at play, such as the ear's level-dependent perception of certain types of distortion, that may be dominant in some situations. This would tend to shift the perceived tonal balance upward as the volume level increased.

I don't think I've given an satisfactory answer (at least I'm not satisfied with it). I think there's quite a bit more to be known on the subject than what I now know.

Duke