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.
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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."
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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.
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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."
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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."
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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!
