Just how close IS the "near field"?

[head_unit]

I recall reading a paper by Mark Gander at JBL where the concept first came up, the zone where if you stick a microphone close to the cone and move it around a bit you'll get the same SPL reading. That's not how he expressed it, rather the essence of it. Back when I measured subs more frequently it seemed this need to be within a few inches...however logic dictates it would vary with frequency...

P.S. I mean "near field" specifically as defined above, not "near field monitors" and all that.

 

[weltersys]

Back when I measured subs more frequently it seemed this need to be within a few inches...however logic dictates it would vary with frequency...

Yes, the boundary between near-field and far-field does vary with frequency.

Pat & Brenda Brown have lead SynAudCon (Synergetic Audio Concepts) since 1995, taking over from Don and Carolyn Davis who started the company in 1973, providing seminars, books, and articles with the "real deal" on audio concepts.
In regard to full range frequency measurements, Pat Brown wrote:
"A working “rule-of-thumb” for determining the boundary between near-field and far-field is to make the minimum measurement distance the longest dimension of the loudspeaker multiplied by 3.
He then writes:

"It is often thought that a remote measurement position is necessary for low frequencies since their wavelengths are long. Actually the opposite is true. It is more difficult to get into the far-field of a device at high frequencies, since the shorter wavelengths make the criteria in Item 4 more difficult to satisfy.

Item 4:
4. The distance from the source where the path length difference for wave arrivals from points on the device on the surface plane perpendicular to the point of observation are within one-quarter wavelength at the highest frequency of interest ."


This is an important distinction between high frequency and low frequency measurement, criteria #4 can be satisfied at 95 Hz for a subwoofer of one square meter measured at one meter.

Art