Date: Fri, 29 May 1998 16:58:53 -0700
To: bass@mcfeeley.cc.utexas.edu
Presentation of the matrix of free air frequency responses is complete at this point.
If this were pornography then the data/plots presented will constitute the 'money shots'. As such, it is going to make some who 'know' dipole theory sick or at least discomforted! On second thought, they may consider it pornography since it will challenge their belief systems.
Comparison of the ground plane and free air measurements of the bare driver and baffles gives a precise indication of the magnitude of difference when the line-source dipole baffle system is placed on the floor. That is, what effects occur when the line-source dipole operates in a 2-pi space as opposed to a 4-pi space? Would a 'typical' prediction be of an increase in low frequency output? Let us see?
The ground plane matrix of measurements was not 'filled-in', and only the bare driver was measured at 1m, 2m, and 3m. The one-sided and two-sided baffles were measured only at 3m each due to time limitations. Only comparison plot overlays of the free air/ground plane responses are presented. A reminder that only the high-resolution low frequency measurement was recorded for the 3m bare driver suspended condition. Also, careful attention to the color values of the plots should be given as the colors were alternated to best display any reduction in the frequency response envelope in the unsmoothed data.
RD-75, bare driver on floor.
Frequency Response Plot Comparisons:
Bare RD7, suspended and groundplane.
Two-sided Baffle, 18" wide, suspended.
Frequency Response Plot Comparisons:
Two-sided Baffle, suspended and groundplane.
One-sided Baffle, 20" wide, suspended.
Frequency Response Plot Comparisons:
One-sided Baffle, suspended and groundplane.
Analysis:
It is seen in all cases that placing the driver and/or baffles on the floor slightly depresses the frequency response between ~200Hz and 1000Hz! Only below 200Hz does there appear a slight increase in output of the driver/baffles when situated on the floor.
Since the required minimum crossover point for the RD75 is either 125Hz with a 4th order highpass, or 150Hz with a second order highpass filter that slight increase in output below 200Hz will not have much appreciable effect on the overall response of the system.
The data confirms that the RD75 driver emits a very well defined cylindrical 'beam' of sound, which interacts very little with the floor. Hence there will be effectively no 2-pi enhancement of lower frequencies when the driver is floor-mounted in a dipole baffle.
In speaking with Mr. David Graebener today, he confirmed to me that aspects of the performance of these line-source drivers has troubled him for some time. In particular, what is causing the ~1KHz suckout seen with the baffles? Other questions I've posed in past posts remain generally unanswered: Such as why does the RD75 have a depressed output in the 850Hz - 3000Hz range when used in the baffles of these recent posts.
A singular overriding question will remain unanswered until a second- source, large floor-standing line source driver appears in the marketplace, to wit:
Are these dipole effects generally line-source related or unique to the particular line-source driver being used in the measurements. I believe, given the simplicity of the drive mechanism of the RD75, that we must conclude until further evidence is seen, that floor-standing line-sources operate significantly differently from existing cone-based dipole systems to deserve separate analysis and treatment.
Next time, a brief closer look at the low frequency changes when the driver is on the floor, and waterfall plots (which aren't pretty). A brief foray into the realm of the Virtual Diffraction Line Sources will follow after that; and finally, the end of this sequence of posts will conclude with a presentation of the preliminary data on the One-sided Compound Shape (Airfoil) baffle - my best to date.
John Whittaker
Dipole Baffle Study Report#19.
Dipole Baffle Study Report#21.
The RD75 Dipole Baffle Study - Table
of Contents
Acoustic Line Source Research -
Table of Contents.
