Why do Maggies need a lot of power?

Due to the dynamic nature of music, recorded peaks above the average power level required may climb past 10 dB's. In terms of amplifier power, 10 dB's is equivalent to 10 times the amount of power. As such, your calculations ( which i did not verify ) would mean that you would need AT LEAST 640 wpc in order to sustain said listening level and not have the amp go into compression i.e. clipping off the peaks.

Another variable is that sound does not travel uniformly through the air. Depending on the type of driver used ( horn, planar, etc.. ), the physical array ( front firing, dipole, bipole, line array, etc.. ), size of room and your listening distance, SPL's will vary accordingly. If you also factor in that most drivers DO go into compression when being pushed, you might be able to see that any "formula" for SPL's is "generic" unless you have a LOT of specific info about the installation.

If you look at the design of a Maggie ( or any other panel type speaker ), it is obvious that they can't produce a lot of driver "throw" or "excursion". Since it takes large excursions to move a lot of air ( both required for volume and / or low frequencies ), this type of design is limited in several aspects. Once you hit a given drive level, the power is converted more efficiently into heat than it is sound. As such, many engineers try to work within the "efficiency window" with these types of designs and limit low frequency response by using a dynamic driver for the bass and / or increasing the radiating surface area of the panel. Trying to get deep bass, high spl's and minimize compression / distortion is a very tough juggling act to do. That is why all but the largest speakers using a LOT of drivers are the only ones that can do it and do it well. Sean
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I had very good luck driving a pair of Magnepan MGIIIAs with two different versions of the B&K ST140 over the years.
The "original" ST140 was, as it's number implied, a 70 WPC amp with an iron core transformer. Not a lot of these were produced. Also used until recently, a later ST140, in which production swapped the transformer for a Toroid, increasing it's horsepower to 105 WPC.

I was able to achieve very satisfying listening levels in both large and small rooms with this setup. I do think that the combination of a lower impedence and sensitivity make amp choices of less than 200 WPC limited, but there are amps out there.

Make no mistake, The entire Magnepan line loves lots of good, CLEAN power. I never realized what the MGIIIA was capable of until I started feeding them a high power/ current amp. (Sunfire Signature).

Just my 2 cents...

Dear Ake,
here is an assessment from my practical and first hand experiences.

I bought the Audio refinement Complete (50 watts into 8) for my Maggie 1.6's. Good clear sound, but full orchestral works at moderate levels were lacking, and low organ notes definitely pooped out.

I sold that, bought the McIntosh MA-6450 integrated amp (100 watts into 4 ohms). Better sound, better bass.

I sold that & upgraded to the new McIntosh MA-6500, (200 watts into 4). The speakers immediately opened up. Much better bass, great sound on full orchestral works, but most of all superior resolution on low to mid level small acoustical and percussion works.

Quality being the same, Bigger Power Supplies, and more output, seems to be in favor of better sound, especially on Magnepans.

Let me see...86db at 1 watt at 1 meter into 2.83volts at 8 ohms or 83 dB at 4ohms. What is the amp rating at 4 ohms???
So a 200/400 watt 8/4 ohms would translate into 93dB SPL with 10 watts , 20watts for 96dB which is concert level sound (not rock!!!) 200watts should give 106dB SPL, continuous, and 2000watts 116dB for dynamic swings. However ifyou sit at 2 meters or 80 inches from the speakers, then
double everything!!! 3 meters double again! This is for an average size room <3000 sf; 6000 sq. ft double it again!!!!
Of course, I do not think Maggies can go over 110dB anyway.
The Maggies are a relatively benign load being resistive rather than Electrostats which are highly reactive (combo resistive and capacitive) with wicked swings in load at different frequencies.

Excellent detective work, Schubertmaniac.

Yup, Magnepan plays the numbers game a bit in quoting 86 dB for a 2.83 volt input. You see, 2.83 volts into 4 ohms is 2 watts, not 1 watt.

One additional factor is compression. I don't have access to dynamic compression measurements on Maggies, but let's assume they compress about as much as the average 83-dB effienct movng-coil speaker (all moving coil speakers are subject to thermal compression, and in general the lower the efficiency the worse the compression). Suppose we have a 200-watt peak musical impulse, which we would expect to give us 106 dB from our 83-db/1 watt efficient Maggies. But taking compression into account, I would estimate that 200-watt peak input to give us about 101 dB output, and possibly less.

Note that electrostats also suffer from dynamic compression, but it's mainly determined by the transformer's internal properties and has a less direct correlation to overall electro-acoustic efficiency.

Schubert, the power requirements for the Maggies doesn't follow conventional point-source theory as you increase the listening distance. Maggies approximate line sources, and benefit from the significantly more gradual fall-off with increasing distance.

Theoretically, in anechoic space, sound pressure level falls off by 6 dB with each doubling of distance from a point source, but only 3 dB with each doubling of distance from a line source.

I took some measurements in my room to get a handle on the real-world implications of line source vs point source radiation. I measured a conventional point source speaker and a line source speaker at 1 meter and then at 8 meters (the practical limit of my room), using pink noise. The point source speaker's output fell off by 11 dB going from 1 meter back to 8 meters, but the line source speaker's output only fell off by 4 dB!

In my mind, this is one of the things Maggies do very well - you see, the kind of sound field set up by a line source speaker more closely approximates the sound field you encounter at normal listening distances at a live performance. I think this is one reason the Maggies generate such a lifelike "feel" (along with their utter lack of distracting boxiness).