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In Reply to: RE: Semantics posted by Satie on September 28, 2014 at 09:12:24
The inherent sensitivity of all the Magnepan models is the same. This is primarily the result of the motor structure.
The magnets are the same, the distance from xducer to magnets is the same, the pitch of the transducer wires is the same, etc, etc. Thus, you have virtually the same amount of conductors cutting the magnetic lines of force in all the models. There's just no way to create significant transducer sensitivity differences in these models.
That's why the 6db relative difference measurement caught my eye in the German testing. Obviously it's an error in their testing, or arithmetic, or something else with the 20.7 system.
I think your subjective evaluation of the sensitivity differences is probably pretty good.
Cheers,
Dave.
Follow Ups:
I don't know that the distance between the voice coil and the magnets is the same on all maggies. It is definitely greater in the 20.x models. I suspect that was the case with the T-4 bass panels and midrange vs the III/A. It isn't easy to measure with accuracy with simple tools (micrometer) but it appeared so with the T-4 and 4A vs MGIII and later 3.3. the SMG appeared to have a really small offset in comparison. But these are finger touch observations, not measurements. It is easy enough to do in manufacture so it would not be an issue to implement.
Bottom line is that I don't know either way but strongly suspect that they are different. Definitely so in the MG20.x.
I think we're beating a dead horse. :)
The Tympani's are a long obsolete model, different scheme, and have a considerably different sensitivity rating from Magnepan. Those are spec'd at 87db at 1 watt. That's fully 4db higher than the X.7 models or most other "standard" models.
The push/pull 20.7 is quoted from Magnepan as exactly the same sensitivity as the 1.7/3.7 at 83db. Your subjective evaluation says otherwise. I'm fine with that. :)
However, it seems the 77db sensitivity quoted by the German test is way out in left field and incorrect?
Dave.
Yes, the horse is long dead but I am very curious to get a definitive answer on the standoffs for the various models.
The T IVa sensitivity spec is, shall we say, "in doubt". I don't have actual measurements but the T IV was not an 87 db speaker. The bass panels are 85 db raw (without the XO) so long as you don't reach levels where they compress. 85 db would be a very generous figure for the whole speaker (or actually the mids).
Stereoplay measured the Tympani IVa. http://www.forumbilder.se/DE2MF/tympani-20iva.jpg
It was found to be 3 dB more efficient than the Infinity RS1. 84 dB/W/m. Compression begins at 103 dB for both in the range 40-400 Hz.
Thanks Roger, also thanks for the scattered details on the differences in magnets wire/foil and spacer dimensions. If it isn't too much of an effort could you point to what models had which structural aspect?
I am thankful that someone stepped in with his observations of the differences, for a minute there I was thinking I may have been "seeing things" for 30+ years.
I think Magnepan speakers have sensitivity rating calculated "backwards" to 1 m. It is not a very good idea to measure them at 1 m, far better at 3 m or even more. When I compared with a conventional pair of speakers at my listening position (as mentioned earlier), I recorded a difference of 5 dB at a distance of 4 m to the 3.6. My conventional speakers are rated for 86-87 dB/m (real world numbers) and impedance is the same as the 3.6. The low rating for the 20.7 is really strangre but most reviews have no measurements at all.
I have also looked at the various Magnepan I have. There are partly different magnets and the distance to the conductors on the Mylar is also different. The differencies are small but they are there. Some speakers use heavier conductors, the old Tympani IIIA and the 3-series. The Tympani IVa has lighter conductors but as far as I can measure thicker magnets and spacers. Tympani IVa mids has curved back plates (allow for greater excursion), thinner Mylar and the diaphragm is partioned in smaller areas, low moving mass of these for sure.
Yeah, I can understand that the Magnepan sensitivity specifications would be extrapolated from a longer distance measurement. But you transition from the far-field to the near-field when doing that for most of the models. :)
I would feel better if Magnepan didn't publish sensitivity ratings for their speakers. It just creates more confusion than it solves for many users. :)
Cheers,
Dave.
Considering the difficulty of doing a "real" standard measurement, I think what they are doing is fine. The figure is a useful representation of output vs. power in the standard form we expect to see in a test or spec, even if it is a bit of an embellishment.
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