A literature search for information regarding Bessel Array loudspeakers prior to the March 1, 1998 gym measurement resulted in discovery of the information on tapering and shading of line-source loudspeakers. References to Augspurger, Keele, and Smith are to be found in the Reference section.
With Discrete Element Line Arrays (DELA) which use independent drivers in a column to fashion a line-source, each element may have an independent frequency response and/or an independent amplitude level. The general term for these techniques is 'tapering'.
Augpurger distinguishes between frequency independent tapering as "shading" (i.e. amplitude variation), and frequency-dependent tapering (frequency response variation). Of course, it is possible to have both effects simultaneously. The question is, why would one want to do such a thing?
Apparently shading and tapering can effectively alter a line-source's dispersion characteristics and create smoother polar response. In a previous section wherein foam on the floor was shown to have some beneficial effects on the output of the RD75, I concluded that there is some detrimental dispersion of the beam and a subsequent floor bounce that serves to muddy the sound.
So based upon the ideas in the references I sought to experiment with the technique. Shown at left are two photos of the 1STR baffle with and without the two foam 'tapering plugs' at the top and bottom of the RD75 driver. Each piece of foam is approximately 3/4" thick x 3" x 10"; and is the grey foam used as packing material for instruments (being the only foam on hand suitable).
The results of the experiment are so intreguing they seem to beg for further study. As always time is the limiting factor. I am currently using the tapering plugs at home to increase the clarity and imaging capability of the system.
Shown below is a comparison of the first 5mS of the impulse responses from the baffle at 3m distance and 1m height with and without the tapering plugs. The initial impulses are obscured due to the necessity to increase the gain of the impulses to show the detail in the impulse tail affected by the tapering plugs. One obvious change is less 'grit' at the end of the 5mS period in the 'with' condition.
Careful inspection will show that there is substantial change in the response, and that the 'with' condition has a smoother 'tail'. Note the Virtual Line-source Diffraction signal at about 1.8mS in the signals. Pretty darn large isn't it?
1st 5mS of impulse response, with and without tapering/shadowing foam.
Below are three frequency response comparisons at different heights, 0.75m, 1.00m and 1.25m at a 3m distance. Each comparison is presented in two versions, 1/12 octave smoothing and NO smoothing. In addition to the tutorial aspect of presenting the comparisons in this manner, it is imperative to consider the concept of 'Envelope reduction' for such modifications as the tapering plugs.
As an aside, the approximately 625Hz merge point for the 1.9KHz sampling rate high resolution data and the 61KHz sampling rate full-range data is clearly seen in the unsmoothed data.
Note the greater amplitude excursions of the WHITE trace compared to the BROWN trace, and the generalized reduction in the 'envelope' of the curve. We see trivial effect upon the frequency response below 1KHz and above 10KHz. Within the range 1KHz to 10KHz we see large effects relative to the casual agent - the two little pieces of foam.
This is a technique that must be investigated further as it's subjective effect on the quality of the sound is very beneficial. Clearly the foam is affecting frequency response in an area in which the ear is most sensitive.
3m distance & 0.75m height. 1/12 Octave smoothing.
3m distance & 0.75m height. NO Octave smoothing.
3m distance & 1.00m height. 1/12 Octave smoothing.
3m distance & 1.00m height. NO Octave smoothing.
3m distance & 1.25m height. 1/12 Octave smoothing.
3m distance & 1.25m height. NO Octave smoothing.
The Gymnasium Measurements - Theory: You are in the Near Field!
The Gymnasium Measurements - Woofer Integration
The RD75 Dipole Baffle Study - Table
of Contents
Acoustic Line Source Research - Table of Contents.
