Many thanks to Rudi Blondia for assisting and providing moral support on the day of measurements.
The Gym venue being unavailable, the new Auditorium venue was used to acquire suspended, or free air, measurements of a bare RD75 driver and two different dipole baffles. Selected ground plane measurements were also performed to compliment the free air data. The purpose was to document the interaction of a large line-source driver with both one-sided and two-sided baffles, since this would be of primary relevance to those intending construction of a dipole baffle using the RD75 line-source planar-magnetic driver.
Experimental setup lacked much of the rigor and all of the elegance of a government-funded project. There were no catered lunches, in fact there was no lunch at all. Creation of a 16' high microphone stand was achieved with found materials in the garage. Nothing was perfect, all was compromise. Data obtained with the tall microphone stand contains a very small, but discernable reflection from it. We will note it further later.
The bottom of the driver and baffles were ~62" from the floor, and the microphone was aimed at the center of the 76" tall driver. Distance measurements were made at 1m, 2m and 3m. Baffle sizes and details will be elaborated later. The driver used is the same that has appeared in the majority of my measurements. The required passive notch filter was used, but no high pass filter was in circuit during the tests.
With several exceptions, all data are the result of the merging of high resolution (0.5Hz) data (to ~630Hz) with full-range data (15Hz resolution). Data times were generally in the 30-35mS range, but did range from as low as 24mS to over 40mS. Both unsmoothed and 1/12 octave smoothed data are presented as tutorial material. The Liberty Instruments IMP/MLS was used in acquiring the data, with an D'Appelito calibrated Mitey Mic.
With the exception noted re the microphone stand, the suspended driver data is being presented as free field semi-anechoic.
RD75 driver S/N...54 in free air at 1m, 2m, and 3m - unsmoothed responses.
RD75 driver S/N...54 in free air at 1m, 2m, and 3m - 1/12 octave smoothed responses.
Analysis:
A quick note on a visual data analysis technique. Load the two files into a GIF viewer that allows for a manual control slideshow. You can then flip back-and-forth between the two plots quickly to note the differences that 1/12 octave smoothing make. 'WinJPG' does a good job at this.
The data is very clean. The familiar cavity resonance remnant with the driver, centered at ~6KHz, is evident. It lacks the amplitude and shape seen when the driver is used in dipole baffles, hence a conclusion that a baffle affects the cavity resonance.
Above 2KHz, above the dipole effects, there is almost a perfect -3dB displacement between the 1m and 2m frequency response plots. Thereby confirming the line-source nature of the driver. Previously noted at the Dipole Baffle Study is that this behavior extends at least as far as 8m in ground plane measurements. Line-sources stay louder on-axis farther than point-source loudspeakers do, which have a fallrate of -6dB per doubling of distance.
Of great interest in these three plots is the information below 1000Hz. The dipole behavior of the driver alone can be seen to start at ~850Hz. By implication then, the path distance from front to rear of the driver is ~8 inches.
The high-Q nature of the RD75 is seen when the downward slope transitions into a well-defined bump centered between 100-200Hz. This is what gives the driver it's low frequency capability in a dipole baffle.
The Proximity Effect is seen quite dramatically in the 1-3m data. At 1m the amplitude fall over the two octave span, 200-800Hz, is -2.5dB. At 2m, -7.38dB; and, at 3m, -9.25dB. Dipole cancellation becomes more effective as distance increases from the driver. It would be an interesting fact to know at what distance the cancellation limit would be reached?
Welcome to all who would add to this analysis, based upon the data provided.
Next time: a 'standard' two-sided baffle with the RD75 centered in it's middle.
John Whittaker
Dipole Baffle Study Report#16.
Dipole Baffle Study Report#18.
The RD75 Dipole Baffle Study - Table
of Contents
Acoustic Line Source Research -
Table of Contents.
