Speaker efficiency or sensitivity refers to ability to produce a certain sound pressure level with a certain input signal at a certain distance, typically measured in #dB/1watt signal/1 meter distance. A high eff speaker would typically be considered 90 dB or above, but that is a subjective interpretation. In other words, with a 1 watt input signal, that speaker would produce 90dB measured at 1 meter distance from the driver.
Others will probably chime in on the difference between efficiency and sensitivity (there is one, its pretty technical, and the terms are often used interchangably, even if not entirely accurately).
Sensitivity is more often quoted these days, and for solid state amplifiers it is more appropriate. Sensitivity is dB per 2.86 volts, which is one watt for an 8 ohm speaker and 2 watts for a 4 ohm speaker. The reason it is appropriate is that for 8 ohm speakers it is the same, and for 4 ohm speakers most solid state amplifiers will put out the same voltage loaded with 4 ohms or 8 ohms so the dB numbers can be compared directly. A tube amp will not output the same voltage into 4 or 8 ohms, (the 4 ohm output tap provides half the voltage of the 8 ohm tap) so if you use a tube amp of a given 8 ohm power rating, your maximum SPL will be lower.
Efficiency = how much work it takes to give a certain result.
Example a more "Standard" efficiency speaker at around 90 db rating with 100 watt amp, will give a certain output DB at certain Gain level(volume)
Now a 93 db speaker will Give you the same output(theoretically) as the 90 db speaker, but will not take 100 watts it will take 50 watts... Less Work for the same results.
Go up to 96 db and essentially it will take 25 watts to do the same as the 93 db with 50 watts...
every 3 db increase in efficency should basically take half the work for similar results.. of course with everything else being equal, so its not like a 50 watt Threshold is not going to sound as powerfull as a 100 watt SANYO, as a matter of fact it would probably get slaugtered, but thats getting into a whole other issue.
But more watts is not necessarily better, however bigger amps regardless of the efficiency of the speakers can have more Headroom, another subject.
so don't go off thinking that a 100 db speaker will sound its best with a 10 watt amp, and be equal to a 90 db with a 200 watt amp in the sense of final results. its all about combination of equipment and really HOW good an amp is regardless of specs.
In a general sense more efficiency designed speaker could be better at lower level volume sound wise, due to they are more sensitive as well. And sure some big time wilson's or something still need some big time power regardless of the efficiency due to the larger woofers and desired SPL's. And they can work fine with smaller more refined amps as well depending on the rest of the requirments of one's listening environment, preference, and pocket book.
Sorry if I confused matters at all and correct any of the above as seen fit.
Nowadays, many of the "high efficiency" speakers are made (or I should say Marketed) by sticking a dubious quality high power amplifier inside the woofer cabinet. Out of Sight, Out of Mind.
Of course, these speakers aren't really high efficiency at all - they're "high hyperbole". But since you can "run" these speakers with your flea-powered tube amp (which is actually just acting partially as a preamp now), the manufacturers seem to have people bamboozled. There's nothing at all wrong with this concept as an amplification solution - it's just that it can be done by anyone with ANY pair of speakers. It's called BIAMPING!
The only true high efficiency speakers are horn loaded - or headphones.
". . .The only true high efficiency speakers are horn loaded. . ."
Untrue. Fostex and Lowther make hi-efficiency drivers that do not have to be hornloaded. Zu makes speakers over 100 db efficient without hornloading. Dedaelus makes the DA-1 at around 96 db efficiency and it's a multi-way!
True - I should have said "full range speaker". I wasn't talking about individual drivers outside of a cabinet.
Sidebar - I don't want to start an argument about ZU's efficiency claims, because there have been plenty already, and it is a reasonably high efficiency driver - but I am very skeptical of their claims for 101db in a 1 cu ft. sealed box - and the only third party measurements I've seen indicate about an average sensitivity of 95db in the midrange (which ain't bad) by my eyeballing:
(Also note a dramatic roll off in the lows well below the ZU advertised claims.)
In addition, the ZU super-tweeter IS (and needs to be) horn loaded.
Just as a point of reference, my Coincident Total Eclipse are 94db 1Watt @ 1m, 14ohms, 24hz-25khz. No horns here except on the vinyl! Happy Listening!
As you've probably noticed, Punkuk, there's a wide diversity of opinion as to what exactly consitutes a "high efficiency" speaker.
One thing you should keep your eye on is exactly what's being specified by the manufacturer. Typically manufacturers of speakers with an impedance below 8 ohms specify the 2.83 volt sensitivity, as that gives more impressive looking numbers. And typically manufacturers of speakers with an impedance higher than 8 ohms specify the 1 watt efficiency, again because it looks better. Here's how it goes: 2.83 volts into 8 ohms is 1 watt, but 2.83 volts into 4 ohms is 2 watts, and 2.83 volts into 2 ohms is 4 watts. Going the other way, 2.83 volts into 16 ohms is 1/2 watt, and 2.83 volts into 32 ohms is 1/4 watt. When you turn the volume control knob on your preamp, you're changing the voltage output (and the wattage output changes along with it). When you leave the volume control setting the same and switch to lower impedance speakers, the voltage output stays the same but the wattage output increases (assuming the amp doesn't clip).
Let's do an example of converting sensitivity to efficiency. Suppose you have a "93 dB sensitive, 4 ohm" loudspeaker. Sensitivity refers to 2.83 volts input which into a 4 ohm load is 2 watts, so the speaker is really doing that 93 dB with TWO watts input, so the efficiency is only 90 dB with ONE watt.
Unfortunately, some manufacturers use the word "efficiency" when "sensitivity" would be more appropriate, and vice versa. And unfortunately some manufacturers use an "in-room" rather than "anechoic" or "simulated anechoic" measurement, which inflates the specification by 2-3 dB due to the added reverberant energy. Using an "in-room" measurement is not without justification, but does complicate things when you're trying to make an apples-to-apples comparison.
In my opinion efficiency is a more useful specification than sensitivity because wattage input (not voltage input) is what relates directly to voice coil heating and thermal compression. Then if you want to you can take into account how much power the amplifier you have in mind puts out into that particular speaker's impedance. Most solid state amps increase their power output into lower impedance loads, but often there's a sonic trade-off, as many amps distort less into a high impedance load.
I'm not qualified to say where anyone else should draw the line between "high efficiency" and "not high efficiency". Personally, I consider 91-94 dB as moderately high efficiency; 95-98 dB as high efficiency, and 99+ dB (all with a 1 watt input) as very high efficiency. But in my book those are only ballparks, and the lines are blurry.
I've focused on measurements, but your question could be approached from other angles as well (how is a high efficiency driver constructed, what are the techniques commonly used to build a high-efficiency loudspeaker system, what are the challenges involved in building a good one, why would you want one in the first place, etc.) But I'll save the muddying of those waters for another day.
Great discussion. But note that an eight ohm speaker that is 1% efficient in converting electical energy into mechanical energy will output a sound pressure level of 94db. Boy are we waisting a lot of energy!!!
Duke- you have said it all. Thanks for taking the time to put it all together so clearly for all of us. That is one posts that should be put into an FAQ. Anyone from Audigon listening/monitoring?
Opalchip, assuming these tests are accurate, could the speakers tested, being that it was 4 years ago, be different? It seems incongruous with the Zu boys work ethic, that their 101db would not be correct. At least it seems that way to me. With my 18 SET watts my Definitions produce some very serious spl levels in a large room. Granted this is not a scientific method, but my tympanics, from experience tell me they are pretty darn close to 101db. fwiw, warren :-)
Not really, Etby. If the speaker only needs 1 watt to output 94db, the you are 'wasting' 0.9 watts per hour! The parasitic losses on your microwave are about 5w/h.
And at 84db, the loss is 0.09w/h. Don't sweat it!
Great discussion on HIGH efficiency but WHAT MAKES A SPEAKER Efficient? PARTS PARTS PARTS!! Excellent Design, low weight material for cone and suspension, Excellent magnets, proper wiring and design. Oh and Size of the speaker makes a difference also, generally larger drivers are more expensive as the weight requires better design and parts to keep efficiency. Everything has exceptions however.
Horns have become very popular as a high efficiency design but any design can have a high efficiency rating, however the more speakers you have in a design the more energy that is wasted from crossovers and additional components, this one of the reasons why single driver systems have continued in popularity.
Lot's of talk about dB/watt vs dB/volt, but let's not forget that this spec is measured at a particular distance, one meter I think. SPL decreases with distance from the source, and different speakers distribute their sound differently. In particular, sound from a planar or line source speaker system falls off much less than sound from a point source. For example, with my Maggies, I can walk from the far end of my room right up to put my ear to the speaker and hear almost no change in SPL. Maybe I'll do that with my RS meter and post numeric results. Someone else can do a PS speaker. Bottom line is that for two speakers with the same sensitivity or efficiency spec, one planar and one point source, from a normal listening distance the planar will sound louder.
Warrenh - As Duke pointed out - the way the measurements are performed and the room environment can swing the results substantially. Not to slight the "Zu boys" who are apparently nice guys that really enjoy what they do, or the Zu speakers, which some people love - but nobody in history has designed a true 101db conventional cone-based speaker in a 2 cu. ft. cabinet and they never will. It's a marketing claim that undermines their credibility in my eyes.
Here's why - and not to bash Zu (although that IS fun), but because it's an important issue about the original topic here, lets talk about "Hoffman's Iron Law". Here's a simple definition from hometheatermag.com:
"Hoffman's Iron Law, described by Henry Kloss in the mid-1950s and later turned into an exact mathematical formula by engineers Thiele and Small, governs the behavior of woofers. Essentially, it says that a woofer's efficiency is proportional to the volume of its cabinet and the cube of the lowest frequency it can produce before losing relative level (aka the cutoff frequency). Take, for example, a woofer whose response is flat down to 40 hertz in a 2-cubic-foot enclosure. To make its response flat down to 20 Hz, you must either increase the cabinet volume by eight times (to 16 cubic feet) or use eight times the amount of amplifier power to achieve the same listening volume. Given these requirements, you can see how difficult it can be to get respectable low-frequency response from small "full-range" speakers."
To get more technical -
Hoffman's Law relates efficiency, the desired low-frequency -3dB point, and speaker box volume by way of a constant; for efficiency in percent and size in cubic feet the Law is stated as follows:
%eff = k * Vb * f3
where the efficiency constant k ~= 1.4x10^-4 for electrodynamic radiators in vented boxes.
Now, from the Loudspeaker Designer's Selection Guide:
"Hoffman's Iron Law states that the efficiency of a woofer system is directly proportional to its cabinet volume and the cube of its cutoff frequency (the lowest frequency it can usefully reproduce). The obvious implication is that to reduce the cutoff frequency by a factor of two, e.g. from 40 Hz to 20 Hz, while still retaining the same system efficiency, you need to increase the enclosure volume by 23=8 times! In other words, to reproduce ever lower frequencies at the same output level you need an extremely large box!
However, box size isn't the only variable… You can continue to use a small box by accepting a much lower efficiency. In order to retain the same sound pressure level (SPL, meaured in dB's), though, this requires both a very large amplifier and a driver that can handle a lot of power and move a lot of air (requiring high excursions). Furthermore, it must be able to do so with minimal distortion. This is exacerbated by power compression, a phenomenon where the power heating of the driver's voice coil results in a non-linear relationship (read "distortion") between the electrical power in and the acoustical power out."
"Another variable not often mentioned is bandwidth. You can provide the perception of violating Hoffman's Iron Law by using a bandpass design, which can provide a lot of bass primarily across a very limited bandwidth. In all too many bandpass designs, the impressive bass is produced around a single frequency.
This is often referred to as "one-note bass". It can rattle the furniture and impress your friends, and may even be OK for sound effects in action movies, but don't expect too much accuracy when listening to music with a lot of low bass content."
(Back to the Zu's - Looking at the measurement chart link in my previous post, this EXACTLY what is going on - centered around 100hz. All other bass under about 180hz is extremely variable. A few Zu fans have tried to discredit these independent measurements, but I don't buy that for a second. They were performed at the National Research Council of Canada, a serious government lab, in a proper standardized anechoic chamber. And they actually show precisely what any designer would predict from a similar vented design.)
"Summarizing, Low-frequency capability, box size, and efficiency form the three key aspects of system design. To increase any of the three, you HAVE to give up something from the other two, with box size being the most sensitive."
Here a quote from a similar discussion over at the High Efficiency Speaker Asylum website:
"I have heard (literally) tons of loudspeakers that will make lots of sound using test tones. You put the tone on, and read the dB meter, or your ears, and think, "my but that's loud and efficient". Then you check it out with a real time analyzer, and notice that the 40HZ tone is actually the same, or sometimes many dB down from the 80 and 160 HZ harmonic that the speaker is creating, rather than reproducing, making for the high spl level. If a speaker happens to produce even order harmonics (distortion) it will still tend to sound musical, even though it is grossly innacurate."
Now for a quick industry bashing - By plugging the relevant variables into Hoffman's Iron Law, you'll find that true (reasonably flat) 100db music would be achievable with a vented enclosure about 20 times the volume of the Druid's. The Zu's may hit 101db at certain freq.'s, but a coil inside a magnet, connected to a cone of a certain mass and stuck into a box has predictable characteristics. There is nothing different enough about the rather conventional Zu mid/woofer to suggest they could have circumvented physics to such a large degree, especially when faced with objective measurements indicating no such effect.
The fact that various "glowing" reviewers don't trust basic design theory enough to at least intelligently question the claims - or are ready to believe it's all been changed by back venting through a motorcycle muffler because an enthusiastic 30 year old tells them so - is well, typical.
BTW - The only patent (issued or pending) attributable to an inventor named Griewe since 1976 in a search of the United States Patent and Trademark Office database is a pending "Corn Toss Game".
Clearly, it's been a slow day at work. Cheers!
Eldartford- Is that because of the much larger radiating area? If so, then the low efficiency/sensitivity specs for planars may be deceptive. Not that that would necessarily change their appetite for lots of current, but amp matching might be a little easier than some of us have suppossed.
An exception to this would be highly directional Low bandwidth horns...and I am thinking of a Bullhorn, such as used by police and fire departments for crowd control. These must be well over 100 dB.
Swampwalker - thanks for your kind words! Regarding line source loudspeakers, maybe I can offer a few thoughts.
From a point source, radiation intensity falls off at 6 dB for every doubling of distance, and from a line source radiation intensity falls off by 3 dB for every doubling of distance. With a point source the radiation is expanding in all three dimensions, but with a true line source (which would extend infinitely in one dimension) the radiation is only expanding in two dimensions - hence the more gradual falloff with distance.
In practice how well a speaker approximates a true line source depends on the height of the radiating line or line array, the height of the room, the wavelength being reproduced, and the distance from the line-source-approximating speaker to the listener. As a general rule of thumb, line source characteristics will hold up quite well out to about four times the height of the line-source-approximating radiating element(s), then begin transitioning to point-source characteristics.
There is yet another type of source - a planar source. With a true planar source (infinite extension in two dimensions), there is NO falloff in SPL with distance! Up close to a large elecctrostatic panel, planar source characteristics dominate - but we're talking distances of only a few feet at the most.
Getting back to line sources, I once measured a point source speaker and a line source speaker at 1 meter and again back at 8 meters (practical limit in my room). Anechoic theory predicts the point source speaker's radiation would fall off by 18 dB over that distance, and it fell off by 11 dB. That extra 7 dB came from the reverberant sound field. Anechoic theory predicts that the line source speaker's radiation would fall off by 9 dB over that distance, and it fell off by 4 dB, with the reverberant field contribution making up the difference. As you can see in a real-world room the line source speaker's radiation fell off by 7 dB less over that distance (1 meter to 8 meters) than the point source speaker's did. That's a significant difference. But in this case the point source speaker was still more efficient than the line source speaker even measured back at 8 meters.
Opalchip - very good explanation of Hoffman's Iron Law. Having scratched my head about the Druids quite a bit, I think that they are exploiting undamped pipe resonance to extend the bass deeper than it would normally go, but the tradeoff is the deep 150-hz notch revealed in the SoundStage measurements (indicative of a roughly 90 inch long pipe). Having built undamped pipe speakers myself I'll say that they measure worse than they sound - the ear is surprisingly forgiving of that deep notch.
Let's look a little bit more closely at the Zu specs though, and give 'em the benefit of the doubt just for kicks. The Druid is a 12 ohm speaker, so what if when they claimed 101 dB "sensitivity" (implying 2.83 volts input) they really meant "efficiency" (implying 1 watt input)? If so, then translating that to 2.83 volt sensitivity we'd come up with about 99 dB. SoundStage came up with 97 dB/2.83 volt sensitivity. That's pretty close, especially if we let Zu use an "in-room" rather than "anechoic" or "simulated anechoic" measurement.
Next let's look at the claimed bass extension. Zu claims a "bandwidth" of 38 Hz to 25 kHz. We're used to thinking of the bandwidth as the -3 dB points or maybe -6 dB points, but that might be a mistake on our part. In prosound use, the -10 dB point are often given as the limits of a speaker's bandwidth.
Now let's go back and look at the SoundStage measurements. See that little response bump at 38 Hz? I think that's the 1/4 wavelength pipe resonance (though can't correlate it with the impedance curve). Relative to the 97 dB sensitivity determined by SoundStage in a free-air measurement, that 38 Hz bump is 19 dB down. With the reinforcement of three room boundaries, we'd add 9 dB and then we'd be only 10 dB down. And -10 dB fits the prosound definition of bandwidth.
I don't know if this is how Zu arrived at their specifications, but it might be. I welcome correction from anyone who knows the real story.
One final comment on the perceived loudness of the Druid's bass. If indeed the Druid's enclosure is an undamped or minimally damped resonant pipe, then it will sound louder in the bass than it measures. This has to do with human hearing - if two fairly short-duration sounds are exactly the same frequency and sound pressure level, but one lasts longer than the other, the longer-lasting sound will be perceived as louder. So if we have relatively slowly-decaying bass resonances in the Druid's enclosure, the bass will sound louder than it measures. As to why the Druids don't sound resonant and boomy, I think it's because the shape of the frequency response curve keeps us from perceiving it that way. If so, then the Druid's bass loading system (based on a patented automotive muffler design) would not work well with a speaker that measures "flatter" in the bass region. The height-off-the-floor tuning, which apparently is critical to getting the bass to sound right, mght be adjusting the decay of the muffler-like pipe resonances. It may well be that Sean and Adam and the crew of Zu are really pretty darn advanced in their design work if they've taken advantage of psychoacoustics to this extent.
The above paragraph is purely speculation on my part, and once again correction is welcomed.
What Zu has done is build a speaker whose perceived bass extension is much better than I ever would have thought possible in that box size and efficiency combination. I don't think the Druid is without sonic issues, but it is at the very least an intriguing design.
Thanks Duke your was the easiest to unnderstand after that it became more like Quantum physics to me.
The reason I asked was I am getting a pair of sixpacs and my speaker is the Gallo Ref 3's, which as I understand now is far from a High Efficiency speaker at 88db at 1w
But if I am not mistaken its 34hz-50khz is prety good in the sensitivity area
The Gentlman I am getting them from said he ran a High Efficiency speaker (klipsh cornwalls) with horns my concern is am I getting the wrong Mono Blocks for my speaker.
The room is small 18X13 and with kids and the wife I dont get to "crank" them up.
But I would hate to contine to make expensive mistakes in my journey to find that sweet sound I have yet to discover.
Very interesting. Great thread, one and all. Thank you for taking the time to provide all of this info. To this I would only add that having never heard the Zu speakers, if they sound good, then they are good, regardless of how they measure or how much hype or creative "accounting" their marketing department uses. And if they are only 97 dB thats still a pretty darn efficient loudspeaker and should be capable of being driven by small SET amps. To quote Duke "What Zu has done is build a speaker whose perceived bass extension is much better than I ever would have thought possible in that box size and efficiency combination." And that is quite an accomplishment, in my (technically ignorant, but interested) opinion.
Here are some SPL measurements.
Planar is a MG1.6. PS is a Dynaudio Gemini (small MTM).
Signal is pink noise generated by Prepro (intended for setting levels).
Distance is feet.
The first PS column shows the SPL leveling out at 77 dB. I think this reflects room effect. For the second PS column I increased the volume so that the SPL at 12 feet was the same as the planar, 90dB. This required the near field SPL to be a lot higher, 108 dB at 1 foot.
I don't think anyone listens at a distance of 3 feet, although that is close to the 1 meter distance used for the spec. At a more reasonable listening distance, between 8 and 12 feet, the Planar SPL falls off by 1 dB over this distance range, while the PS falls off by 3 dB. To produce equal SPL at 12 feet the PS speaker would need to be about 3dB more efficient at 1 meter. I think that these results are generally in agreement with theory.