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2ax design goals: on-axis vs, far-field

Steve F

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A point was raised in the 10Pi voicing thread about the audibility of an on-axis dip in the AR-2ax’s frequency response, as measured in High Fidelity magazine’s test report from 1971. Since this really opens up the discussion into measurement technique, audibility, and design philosophy, I thought I’d start a new thread.

The dip at the crossover point shown in High Fidelity’s on-axis curve of the AR-2ax is a result of the side-by-side placement of those drivers, when the speaker is measured standing vertically. That on-axis dip disappears when the speaker is flipped around onto its side, and the "on-axis" measurement now sees a vertically-oriented midrange and tweeter. This kind of response interference was not recognized in 1970 by AR (or anyone else, for that matter), and not until the AR-9 ushered in AR’s Vertical Series in 1978 did intentional driver orientation become a widely-accepted design issue.

What’s really in question here is the age-old controversy of what is the prime determinant of a speaker’s sound: first-arrival on-axis response, or far-field energy response? There are well-known, highly-respected proponents on both sides of this issue. The "energy people" say that a specific, arbitrary point on any particular curve is not what you hear. They say you hear the total shape of the far-field response. The first-arrival on-axis crowd says that you respond first to the speaker’s initial direct anechoic response, before the sound gets influenced by room reflections, etc. There are numerous articles on the subject, well-researched and well-documented, in the AES journals and other places. Take your pick and choose your side. One thing is for sure—no sweeping generalizations by me or anyone else will conclusively solve this argument.

The instructive thing to keep in mind when evaluating a product is what the designer had in mind. Roy Allison was an ardent supporter of the "energy response" side. (See his letter to me, dated November 27, 1972, in the documents section of the AR Classic speakers section on this site, entitled "Letters from AR to Steve F" for a clear example of Roy’s philosophy.) The 2ax is clearly a product of that thinking. While High Fidelity’s 2ax on-axis curve has some irregularities, caused in part by the arbitrary way it’s measured, the "front hemispheric" response (which is the energy response) is actually quite good, especially for a 1970’s-era speaker. Energy-response proponents will tell you that a slightly downward-sloping direction to the energy curve as the frequency increases—as long as it’s smooth, without any huge peaks or dips or other surprises along the way—is what they look for. Roy adhered to a specific formula for the amount of falloff, something like 1dB or 1.5dB per octave above 1kHz. He felt this would translate into a convincingly-satisfying and realistic speaker. The 2ax handily meets those goals. Ah, but whether or not you agree with those goals is another matter altogether. It’s analogous to the philosophy that drove the development of the Bose 901. The 901 unquestionably meets its design aim of delivering most of its sound into the listening area by reflection instead of direct sound, but whether you AGREE with that design aim is a completely different question.

The 2ax’s main marketplace rival, the original Advent speaker, was designed by Henry Kloss and Andy Kotsatos (nee Petite) to have a flatter on-axis response; therefore a smooth far-field energy response was not their primary design objective.

To another point raised in that same 10Pi thread—one quick aside: I have owned the following AR’s: Classics, Truth in Listening, Verticals, Connoisseurs, and TSW’s. No new AR speaker out of the box that I’ve owned has ever had a frequency response curve stapled onto the rear panel, although some of the LS/LSi models may have had some regrettable front panel graphics to that effect.

Steve F.

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