There are good speakers that are also efficient. While "good" is often quite subjective, some speakers that have been praised by some audiophiles that are also efficient include the Klipschorn, Avantgarde Duo, and Wilson Audio WATT/Puppy.
The low impedance characteristic is often the result of crossover design. The Thiel 3.6 is an example of a good speaker whose impedance dips below 3 ohms at certain frequencies due primarily to a design goal to preserve phase coherence.
Inefficiency can arise from crossover design also but is often due to driver and enclosure issues. The Acoustic Research 3a was a well-regarded speaker that employed an "acoustic suspension" design for its woofer. This sealed box design helped extract extended low frequency performance from a speaker of moderate size, something that was considered quite difficult to achieve at the time. The acoustic suspension design was inherently inefficient. (The AR 3a was also a 4 ohm speaker.)
I have not seen any statistical analysis of impedance and efficiency versus speaker quality. Would you share with us the basis for your cited "correlation"?
No stats, just observation. Nor did I say "all", but rather "many". Dali Helicon, Dyanaudio Contour, Harbeth, Totem Mani-2, some of the Tylers, et al.
Not looking for a fight, just information.
The efficiency and impedance of the most popular dynamic speakers solely depend on Faraday's Law i.e. on the driver itself.
The impedance of the coil rises with the freequency growth
For DC current the coil impedance is zero(w=0)or same may apply for freequencies close to zero.
In order to reproduce lower freequencies the coil should be quite massive to hold a large current hence the impedance of such will drop. Hence the full range speakers require powerfull amplification with good current drive.
Larger drivers can reach higher SPL with smaller current and smaller amplification hence more efficient.
Small drivers need larger amplification power to move the air hence less efficient. The advantage of the small driver full range speakers (Dynaudio,Totem) is WAF, compact soundstage and imaging.
Not being a speaker designer, I have a fairly crude impression of how crossovers and drivers are used. My impression is that drivers function naturally over characteristic frequency bands, e.g., woofers only go up so far and start to drop off below a frequency inherent to their configuration and loading. Their performance over their bands is typically not flat. Designers use crossovers to boost and/or cut to smooth the speakers' frequency responses, as well as to extend performance beyond the speakers' natural bands, like coaxing a woofer to go lower. Beyond frequency response, designers also have goals for phase coherence (like Thiel), dynamic range, and low level performance, among others. This often results in unusual impedance loads and inefficient speakers. Nowadays, the availability of high powered solid state amplifiers allows the speaker designer generally to ignore such issues. The utilization of new speaker materials, like beryllium, diamond, or sandwich composites, allows the designer to achieve specific goals without as much complexity in the crossover and hopefully with less penalty in impedance and efficiency. I am hopeful other posters will have more specific responses to your question.
I agree most mid to large size speakers suck most amps dry.
I just looked at the Tyler Linbrook Signature System, a WMTMW design. Ty rates it at 4 ohms/92 db. I demoed my small Jadis OR with them and had no trouble at all.
Which testifies to the super high quality of trans on the Jadis and the fact that Ty knows how to design a crossover to make this impressive and imposing speaker work fine with a 40 watt tube amp. Classical Orchestra sounded rich and beautiful sound sound stage.
Good point for a topic.
Big deal the speaker is large and full range. What sort of power requirements does it need?
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.
It's not a "high performance attribute", nor is there a relation of performance to sensitivity. It has to do mostly with the evolving of amplifiers producing enough current to drive bigger cones and achieve lower frequencies in smaller boxes.
Tube amps, which came first, produce little current output. This meant that speakers needed drivers to be relatively light and the magnets small in order for the low current to move the cone. Lighter cones limits their size (to prevent cone deformation), which limits the low frequency response. One way to get lower frequencies out of relatively small cones is to horn load the cabinet, however; the lower the frequency the bigger the cabinet. Another limitation is the number of drivers as a crossover splits the already small current further. So by keeping the impedance and sesitivity high, a tube amp can be practical.
With transistors, amplifiers are able to generate many times the output current of tube amps. This makes it possible for larger, stiffer drivers with bigger voice coils and magnets to be installed in smaller, closed boxes producing tighter bass response. Also, lower impedances will not over-tax a SS amp they way it does a tube amp.
Since solid state amps are more prevalent, speaker designers are more free to mix and match cabinets, drivers and crossovers because, for one thing, it makes economic sense. Sometimes lower sensitivity and impedance gets them where they want to go (like the ones you mention) or sometimes it's in the other direction (e.g. von schweikert, Fried).
Thank you all for your responses. I now have a better high level understanding of the design issues and tradoffs.
My experience has been that high efficiency speakers are more likely to compress than low efficiency speakers...even at modest SPL levels. A low efficiency speaker often remains more balanced at various SPL levels whereas a high efficiency design often sounds balanced only within a much narrower SPL range.
I don't know why this is....but it is certainly my general observation...perhaps Duke can explain? (I am referring to regular box speakers here)
dipolar radiators have efficiencies in the 85 to 88db range.
i happen to prefer this type of speaker. depending upon the room and the impedance curve, they can pose a challenge fot a tube amplifier.
if one prefers this design to other designs... well, draw your own conclusion.
That's a good point Bartok.
Most mid-range to large models from manufacturers such as Merlin, Altec, Klipsch, Zu, Avantgarde, Cain and Cain, Silverline, Acoustic Zen, Gallo, Wharfdale, Coincident, Tannoy, Fostex, Oris, Duevel, Green Mountain Audio, Maxxhorn, Devore, etc. really do suck most amps dry.
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.
Thanks, I am sure you are right about the causes or shifting tonal balance (from different driver sensitivities). My experience is probably not representative as it does not involve more than a few dozen designs.
However there is another angle. The Fletcher-Munson equal loudness curves. These also have impact on the perception of sound at various levels;
130 db at 20Hz sounds as loud as 100 db at 2 Khz.
110 db at 20Hz sounds as loud as 60 db at 2 Khz.
Notice that this implies that our ears compress the mid range relative to the bass as you go to higher SPL's (a 20 db increase in ultra LF requires a massive 40 db increase in mid range to maintain equal terms of loudness perception)
I guess this explains why rock mixes that are designed to play loud are usually mixed thin in the bass. It implies that to maintain consistency to our ears at various sound levels then the bass should be compressed as you increase SPL in order not to have too much shift in the balance.
Does this imply that relative compression between drivers might actually be desirable in a speaker?
Does this imply that there is fundamentally a SPL sweetspot for particular forms of music that is independent of the speaker used?
Thanks for your insights.
Shadorne says.... "Does this imply that there is fundamentally a SPL sweetspot for particular forms of music that is independent of the speaker used?"
Of course. Just yesterday I listened to my CD of Kings College Chior Christmas Carols, and with all those voices, and the organ thundering away, I got to thinking about why I have six large subwoofer drivers. No way that music would sound right coming out of a 6 inch bookshelf speaker. On the other hand, I have violin recordings that must the played at a much lower SPL. A real violin can play only so loud, and if you crank it up louder than that it sounds lousy.
I agree with Eldartford above in that I noticed there seems to be a level on a number of recordings that seem to "come to life" at a given point and then as it gets louder, the magic tends to go away.
On some recordings there are multiple "magic" SPL points (and I noticed that in a number of cases the other magic points seem to be a multiple of 3 db, give or take, from the initial magic point - go figure).
Sensitivity is the proper term. There's no correlation between high sensitivity and low sensitivity as it relates to quality. Only the speaker designers idea of what works best.
You'll also note most speakers are 86 to 92db sensitive. I'd guess that may cover 90%.
Wait until you count to ten before you throw rocks at me. Klipschorns are the most efficient speakers on the market with a sensitivity of 105 dB, and one of the best sounding speakers as well once modifications have been made. Modifications needed are replacement of the Klipsch crossovers with ALK extreme slope crossovers, replacement of the midrange horn with Martinelli wooden horns, and the replacement of the Klipsch tweeter with the Beyma CP25 tweeter... nine, ten. OK I'm at a safe distance. Hit me if you can.