Updated: March 25, 1998.
Struck and Temme [1] have recently provided the speaker builder with conceptual tools to determine the minimum-sized room in which simulated free-field frequency response measurement might be made given a speaker system's physical dimensions. They define the near-field, diffuse-field and far-field in terms of loudspeaker size. The user of time-gated measuring tools attempts to use a combination of far-field and near-field measurements to simulate an anechoic measurement and/or free-field conditions. The object is to obtain a frequency response from a loudspeaker independent of the measuring environment. Given the great size of the dipole baffles tested in this study, the caveat "simulated free-field on a ground plane" is operative.
After much math, here are several of the authors' statements in relation to MLS or Gated-Sine instruments and simulated free-field measurements:
"Therefore the lowest measurable frequency using ANY time-selective technique is a function of the room size, where 'h' is the smallest room dimension (usually floor to ceiling height), and the TEST OBJECT SIZE, which determines the distance to the measurement microphone 'd' according to Eq. (15)."
d > 3M (M = largest linear dimension of source) Eq. (15)"
"...in order to show the dependence of Delta f on both room size and the
test object size...
( Delta f = lower limiting frequency )
( c = speed of sound (344 m/s) )
( h = smallest room dimension, usually floor to ceiling )
Delta f = c / SQRT( h^2 - d^2 - d ) Eq. (16)"
The microphone must be positioned AT LEAST 18 feet away from the BG RD75 in order to simulate a far field response. For an 8' ceiling and the 6' BG75 height EQ.(16) will not evaluate due the the negative number appearing within the SQRT bracket.
Evaluating EQ.(16) for 75Hz, which is one octave below the BG RD75 specified 150Hz highpass cutoff, computes a MINIMUM room dimension of 24.7 feet. Simulated free-field measurement of the BG RD75 will not be possible in any realizable home venue. So at best, any measurements made at home on the RD75, will probably be in the diffuse-field over a major portion of it's range, and near-field in the remainder. This explains much of the variance between large planar speaker measurements, the data are relative to the measuring environment.
In the case of near-field measurements being used to supplement far- field measurements in the low frequencies, the other half of the bad news from Struck and Temme [1] regarding the validity of near-field measurements and large loudspeakers is:
"The near-field measurement will progressively under estimate the true free field response at higher frequencies. Because this limit is governed by the overall size of the test object, rather than the driver radius, the entire enclosure must be considered when comparing the wavelength to the size of the source. In practice we have seen that the near-field response can be used with errors of less than 1 dB (compared to anechoic measurements) at frequencies where the wavelength is GREATER than about 3 times the major dimension of the source M.
M = largest linear dimension of source"
Baffle dimesions of about 20" x 76" (1.996m diagonal), as for typical RD75 usage, computes to an upper valid limit of 57.4 Hz for errors less than 1dB for near-field measurements. This is very far below the cutoff of the RD75s, at approximately 150Hz, and means that near-field measurements in typically sized rooms will contain room effect errors.
Considering both the far-field and near-field problems for large loudspeakers, a simulated free-field measurement of a large dipole line-source in a average sized room is not possible without the introduction of room effects. What we are getting as measurements are relative to the measuring environment.
Simulated 'free-field' measurements are now used throughout the speaker building industry as a standard design tool. Amateurs are also now using such methodologies for speaker design and documentation. Free- field measurements are used to compare loudspeakers independent of environmental effects; and, also to compare loudspeakers separated in time and space from each other.
Clearly, for such large loudspeakers some mention of the size/shape of the measuring environment is necessary, since by the definitions of Struck and Temme they cannot be measured in home-sized envirnoments to a reasonable tolerance of that of free-field and/or anechoic measurements.
The basic assumptions behind "time-gated" measurement techniques for the simulation of free field and/or anechoic response are extremely difficult to meet for physically large loudspeakers. Data obtained is not a close tolerance simulation of free-field response, i.e. that response which would be obtained in an anechoic chamber sized for the desired low frequency limit. The large dipoles, in typically sized rooms, place the listener in some portion of the near-field.
The data of this study comes in three grades of 'purity' in regards Struck's and Temme's criteria:
1. The outdoor data, although containing some wind contamination, seems to meet their dimensional criterion of a minimum 'room' dimension of 24.7 feet.
2. All data acquired in the listening room does not meet their criterion for minimum room dimension.
3. Data from the new gymnasium measurements seems to also meet their criteria for minimum dimension for simulated free field measurement. The gym is square at 93 feet per side, and the ceiling is scalloped, with the lowest portions at approximately 22 feet high and the highest portions at 28 feet high.
Data from the gymnasium measurements documents that the 'near field' for the RD75 planar-magnetic drivers extends at least as far out as 8 meters, based upon the criterion of -3dB attenuation of amplitude per doubling of distance from the driver/baffle. It is understood that a line-source driver will exhibit -3dB attenuation per doubling of distance in it's near field. That the near field for this loudspeaker extends so far out is a very unusual finding.
[1] "Simulated Free Field Measurements", C.Struck & S.Temme, J.Audio Eng. Soc., V.42, N.6, June 1994.
The RD75 Dipole Baffle Study - Table of Contents.
Measurement Issues - Reflections in Impulse Responses.
Acoustic Line Source Research - Table of Contents.