In "Simulated Free Field Measurements" by C.Struck & S.Temme (J.Audio Eng. Soc., V.42, N.6, June 1994) they posit:
In "Simulated Free Field Measurements" by C.Struck & S.Temme (J.Audio Eng. Soc., V.42, N.6, June 1994) they posit:
"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."
Those frequencies at which the wavelengths are greater than 3 times the ~76" height of the baffles under study are below 60Hz. Clearly we will gain no useful approximation to far field/anechoic response above 60Hz - a not particularly useful limit since the RD75 is limited to a low frequency response that ends at 125Hz!
We might ask where does the transition from the near field to the diffuse field to the far field begin with a line-source driver the size of the RD75? It is generally understood that a line-source driver will radiate a cylindrical wavefront whose amplitude decreases at -3dB for each doubling of distance from the driver. At some distance the line-source will begin to behave increasingly like a point-source and will transition to the expected -6dB amplitude decrease per doubling of distance from the driver.
In the three overlayed plots below are shown the results of frequency response measurements on the 1STR baffle for three distance doublings referenced to a 1 meter measurement at three different heights. It can be seen in all cases that the final distance measurement at 8 meters is to all intents -9dB less amplitude than the 1 meter measurement. This is exactly as is expected theoretically for a line-source driver - which means that even at 8 meters one listens to such a driver/baffle combination in the near field. It is a reasonable question, but unanswered at this time, 'Where does the near field end for the RD75 driver?".
Finally, the phenomena of 'moving dips' is seen quite clearly in all three overlays in the responses below 800Hz. Starting at a low frequency at 1 meter note the dip progresses upwards in frequency as the distance increases.
0.75 meter height. 1/3 Octave smoothed.
1.00 meter height. 1/3 Octave smoothed.
1.25 meter height. 1/3 Octave smoothed.
The Gymnasium Measurements - Let's Get Foamed.
The Gymnasium Measurements - Theory: Tapering and Shading the Line Source.
The RD75 Dipole Baffle Study - Table
of Contents
Acoustic Line Source Research - Table of Contents.
