Magnepan MG-20 loudspeaker Planar Imaging

Sidebar 2: Planar Imaging

I find planars to be the only speakers inherently capable of properly reproducing image size. I'm just as impressed as the average card-carrying audiophile with the soundstaging capabilities of a good minimonitor. Such a point-source speaker can be adept at fleshing out a convincing soundstage with excellent width and depth. However, its portrayal of image outlines tends to be too specific. Thus was born the cliché of "pinpoint imaging." Ironically, this is bandied about as a positive attribute, when in fact it's totally artificial.

[Actually, DO is only right in the broader sense, in that real-life images are larger than pinpoint ones. But those images have to be squeezed through the distorting lens of the microphones and recording process. When you consider what imaging information is carried on the two channels of a stereo recording—amplitude and timing differences—the "pinpoint" imaging produced by point-source loudspeakers is actually an accurate representation of that information. What is lost by all current stereo recording systems is the intensity characteristics of the original soundsources; as DO points out next, large planar speakers synthesize reproduced intensities that are closer to the originals.—Ed.]

Looking at the issue from a technical viewpoint, conventional speakers may be said to distort the radiation pattern and surface loudness density of many large musical instruments. If you divide a piano's significant acoustic power output by its radiating area, you arrive at the instrument's surface loudness density. This figure is considerably different for a trumpet, where you have a large output concentrated over a small radiating surface. These differences in loudness density provide important clues as to image size, and help define the timbres of particular instruments—from the piercing cry of a trumpet to the stately demeanor of a piano. The impossible problem for a conventional speaker, then, is this: how to convince you it's a grand piano when the instrument's full acoustic power is being squeezed through an 8" woofer.

By its very nature, a loud instrument possessing a low surface loudness density implies a planar acoustic wave launch. A theoretical point source of sound emits a spherical wave, meaning that, in free space, its intensity falls off as the square of the distance: double the distance, and intensity is reduced fourfold. In real life, musical instruments emit sound from a well-defined surface, such as a sounding board, or from the resonating body of the instrument itself. In the instrument's nearfield, the wave launch is planar (with little curvature), meaning that intensity falls off much more slowly with distance. Small head movements near a planar sound source do not produce as sharp a difference in intensity as they do near a point source. The practical meaning of all of this is that the transducer needs to possess these very characteristics if it's to successfully evoke realistic image outlines.

Some readers have questioned the premise that planars are in fact needed to realistically reproduce a piano. Yip Mang Meng of Singapore wrote me to ask whether "5'-diameter microphones [are] in use," his point being that, after the signal is squeezed through a 1" mike capsule, later reproduction through an 8" woofer should have no effect on image size. It's important to realize that a mike samples only a single point in the soundfield of an instrument. The stereo mike feed contains localization cues that allow us to map the soundstage, but lacks the sort of cues that allow us to determine image size.

Listening live, small and involuntary head movements are used to gauge the spatial extent of a sound source. These are the types of cues that planar speakers synthesize for us in the listening room. Information lost during the recording process is re-created for us by virtue of a planar's radiation pattern and surface loudness density. When I close my eyes and try to conjure up an image size, a planar speaker reproduces piano, double-bass, cello, and human voice with much more spatial conviction than is possible with a conventional dynamic speaker. It's true—a planar makes a recorded piano sound more like a real piano.

Wendell Diller captured the essence of these concepts with the remark that the MG-20s continue in the listening room the wavefront launched originally by the musical instrument. Because the MG-20 is a three-way design, it roughly re-creates the proper range of surface loudness densities. The sounds of higher-pitched instruments are radiated from a small surface area, while large-bodied instruments get the full treatment from the planar woofer and midrange unit.—Dick Olsher

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COMMENTS
yooperaudio's picture

A TWENTY-FOUR year old article is posted here, with some kind of assumption that it will be valued? Jesus god - get a grip.

John Atkinson's picture
yooperaudio wrote:
A TWENTY-FOUR year old article is posted here, with some kind of assumption that it will be valued?

This review is of what was at that time Magnepan's flagship loudspeaker and will be of interest to many. When we launched our website, our plan was to post every review that had been published in Stereophile to our on-line archives and we are well on the way to achieving that goal.

John Atkinson
Technical Editor, Stereophile

Bogolu Haranath's picture

The latest version is 20.7 .......May be Stereophile could review them? :-) .........

Bogolu Haranath's picture

"Get a Grip" ......... Aerosmith :-) ........

JRT's picture

I thank you and/or others responsible for posting this interesting content.

My comment here is intended as counterbalance to complaints about posting older articles.

Respectfully,
JRT

prerich45's picture

Bravo!!!!!! I enjoy reading blast from the past!!!! Especially the Infinity Composition Prelude PFR and the MTS reviews!!!!

Jim Austin's picture

We respect tradition, and history, and our elders.

Plus, whatever you might think, these historic articles are widely and enthusiastically read.

Jim Austin, Editor
Stereophile

yooperaudio's picture

Oh please. What a load of BS. Elders? Fuck your pomposity and your lawn.

Jim Austin's picture

You cannot make him drink.

I'm thinking our site is a poor match for you. Do you agree?

Jim Austin, Editor
Stereophile

yooperaudio's picture

Happily, yes. Carry on with your self-aggrandizing myth-making. You’re grand!

Jim Austin's picture

Excellent. I'm glad we agree. Goodbye.

Jim Austin, Editor
Stereophile

scottsol's picture

Unlike printed magazines where editorial page count is limited by ad revenues, posting old reviews has no effect on the number of posts with new information.

Moreover, in the many years that Stereophile has been posting old material they have typically received more reader comments than new material.

So, your comment was not only of no possible significance, but completely wrongheaded in its premise.

Bogolu Haranath's picture

DO's description of 'Planar Imaging', with JA1's comments, are an interesting read :-) .........

gcvanwinkle's picture

The $650 LRS OWNS the older Model 20 in terms of time coherent performance:

https://www.stereophile.com/content/magnepan-lrs-loudspeaker-measurements

The use of a first-order crossover pays off - along with a little care in speaker placement. Now test a newer 3-way Magnepan and see if they can do the same trick with a more complex design.

And thanks for the look back in time!

volvic's picture

Back in 1989 I auditioned what I thought was going to be my final turntable purchase, it was between the Linn LP-12 and Roksan Xerxes. The speakers used were the large Maggies, I believe they may have been the 20's or at the very least the precursors to the 20's reviewed above. The speakers and Bryston electronics were revealing enough to let me make my decision. I can still remember the addictive sound they had and if I had space in my Manhattan apt I would have swapped my Kans for a pair of Maggies. Love that Stereophile releases these old reviews. Takes me back to a time when physical media was king and when retail hi-fi stores were more plentiful.

BDP24's picture

yooperaudio's comments certainly mirror the unnecessary nastiness I see in so much social interaction these days. Dick Olsher has long been amongst my two or three favorite hi-fi critics of all time, and much of what he had to say about the MG20 is timeless, and still relevant today. I happen to own a pair of Tympani T-IVa, which can be viewed as the basic blueprint for the current Maggie flagship, the MG30.7. Thanks for the reprint, though I still have my copy of the issue it appeared in. A Stereophile subscriber since 1972!

dial's picture

Interesting, I owned a pair of III A (bought second-hand), sold them to a friend cos the WAF you know. They still sound great, with Audio Analyse pre & amp and a Goldmund studio one (extremely rare, only seen it once ! I have the 2)/Linn Basik+/Sumiko Blue Point 2 ! They were not as fragile as the Apogées (see the AS H-E Audio site).

Big eugene's picture

Re: yooperaudio. Wow....what an absolutely pathological, colossal, jerk. No real arguments, just pejoratives.
If this is what passes for civility these days, we're finished as a society.

David Harper's picture

another interesting read would be a review of the original Acoustat monitor, the one that resembled the black monolith in "2001". My friend had a pair of these and they sounded amazing.

scottsol's picture

The Acoustat model that most closely resembled the 2001 monolith was the 2+2 which arrived five years after the first Acoustat. Ironically, they bore a far greater resemblance to that monolith than the first Martin Logan speaker, the Monolith.
https://www.stereophile.com/content/acoustat-22-loudspeaker

David Harper's picture

maybe those were the ones my friend had. I just ordered the new maggie LRS.

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