The 3-way line array/line source being prototyped.
Shown is the 'left' new line array propped up against a RD-75 attached my true ribbon
driver. The RD-75 is quite dark in the photo, hope it can be seen? I'm sure you're
admiring the ever popular white-and-black decor scheme - always a WAF/SAF balm.
This system is far from being completed, since the line arrays are due for a trip
out to the backyard for an attempt at semi-anechoic polar response testing, in addition
to other good measurement intentions.
Measured in place against the RD-75/ribbon baffle the following parameters resulted:
Left Array: Rdc = 9.12 ohm, Fs = 37.20 Hz, Qt = 0.787, Qe = 1.056, Qm = 3.091. Le = 2.38 mH.
Right Array: Rdc = 9.19 ohm, Fs = 37.56 Hz, Qt = 0.753, Qe = 1.025, Qm = 2.839, Le = 2.47 mH.
It should be noted that the month of January 2003 in SoCal was the warmest on record. Temperatures reached into the high 80s (F) and a record of 91F was seen late in the month. The Fs value is affected by temperature, since the compliance of the surround presumably increases with the warmth. What is unclear is whether the slightly lower Fs for the array, compared to the individual drivers, is wholely accounted for by the temperatures?
In no way will this dipole baffle configuration be optimal in any sense. It will ultimately be about 30" wide, and there will be a 3" or 4" radius half-round at the line array edge. What defines or limits the degrees of freedom of the baffle design is the pre-existing red oak framework of the true ribbon driver (built in 1988 or so). It is 9.5" wide.
So much for the virtues of a 3-D shaped dipole baffle. One works with the situation at hand.
In the abstract, what the line arrays may provide in semi-anechoic testing is another look at the issue of dipole frequency response dependencies on the shape of an "Equal Area Baffle." That is, working from the findings of "The RD-75 Dipole Baffle Study" in regards equal area dipole baffles, what influence will the shape and/or orientation of different baffles of the same surface area have upon the frequency response of the array?
In the lower frequencies, does a "one-sided" baffle offer such a dramatic change in frequency response as compared with an equal area "two-sided" baffle as seen with the RD-75 driver? An intriguing possible dipole baffle orientation for such a line array combined with a RD-75 type driver is to extend the baffle straight back, in the Z axis, between both drivers.
It seems quite clear that the best dipole baffle for imaging is that which has the narrowest frontal profile. Reducing the time delay of baffle edge diffractions can only be a good thing I believe. In that regard, the baffle at left is very far from optimal at an intended 30" width. Hopefully the large radius edges on both sides will mitigate the edge diffractions? The true ribbon edge has an approximately 2" radius half-round edge as shown in the photograph.
A new computer arrived today (February 4, 2003) for use with the "Praxis" measurement
system from "Liberty Instruments". "Praxis" is much advanced over the amateur "IMP/MLS", and provides for
many possiblies of measurement. Not in the least convenient polar frequency response measurement
- which is rather important in the dipole realm. See a photo of my DIY polar response
table below.
Once a semi-anechoic profile of the line array is obtained, then can come the
opportunity for close comparison to actual in-room or reverberant response. In that
regard I have two very 'quick and dirty' frequency resonse plots to present below.
At the end of the zobel measurements I decided to just play a bit and informally
obtain a set of casual in-room measurements in the general location of my listening
position.
One third octave in-room/reverberant response around listening postion.
The plot at left shows five frequency response curves, smoothed to 1/3 octave, of the
in-room or reverberant response of the pictured left array above measured around an
about the listening couch. The frequency range of the plot is 20 - 650 Hz, and measured
resolution was 0.47Hz (1.92KHz sampling rate, ~2S MLS stimulus). The measurement
distances varied between approximately 8 and 10 feet, and the width of the microphone
spread was about 8 feet. [The amplitude scale is 6 dB/divison, and is obscured by the plot
lines; and the 18 ohm/20uF zobel was in place for all measurements.]
Four the five plots show the frequency response falling rapidly below ~35Hz - the responses below about 25Hz are presumably room rumble 'junk.' Above about 180 Hz the frequency response spread is roughly +/- 6dB which convirges to about +/- 3dB at 650 Hz. There is a suckout approximately 7-9 dB deep between 120 - 180 Hz or so seen when the plots are averaged by eye. The envelope of the five plots is about +/- 9 dB between 30Hz and 650Hz.
Obviously this response is a candidate for equalization. Upon first approximation I
dialed in a +6dB boost at 140 Hz over a 1.2 octave bandwidth. Subsequently, based upon
subjectivity, I reduced the boost to +3 dB and the bandwidth to 0.6 octave. The crossover to
the RD-75 is at about 480 Hz 4th order L-R active filter, again based upon experience and
subjective criteria. These parameters will no doubt change as I acquire more information.
Unsmoothed semi-anechoic response at listening postion.
To the left is the unsmoothed semi-anechoic response above ~200 Hz of 4.61 mS of
reflectionless data from the listening position taken at 61.2KHz sampling rate. The plot
range is 200 - 10,000 Hz. Since the intended crossover point is in the 400 - 600 Hz
range, the relative linearity below 1600Hz is heartening.
My previous 'woofer' had been the venerable Hsu W-10, with a crossover to the RD-75s at 135Hz. I certainly don't play music as loud as you do, ;o), and the line arrays do lack the last few Hz that the Hsus can provide, but they are much more satisfying even in prototype stage as woofers, with the baffle uncompleted. Imaging is much improved, and the RD-75s certainly do like to be 'relieved' of duty in their last few lower octaves.
With an old favorite warhorse of the organ repetoire, Guillou's "Pictures at an Exhibition"
I don't get that last bit of pants leg flapping bass with the line arrays unequalized (beyond
the +3dB at 140 Hz); but, the columns of sound which emit from the arrays are wholely
satisfying. How loud will they play? I'll probably never know. To date, as loud as I have
attempted, the cones of the arrays don't seem to have shown much beyond 2mm or so of Xmax
excursion, but that's a very subjective observation. There is a Behringer Feedback
Destroyer (manual parametric EQ mode) in their future, after some dipole research with the
semi-anechoic measurements outside.
The polar response turntable.
My DIY polar response table for the coming line array measurements, and hopefully also for future projects,
is shown. The new computer is onhand, so all is that is needed is a stiff dose of
learning the new measurement system. With "Praxis" there will be a fond goodbye to
the MLS 'noise' and a hello to the 'chirp.' All that is needed now is the will to get
everything into the backyard for a major 'time of tedium' - and a few windless days, which
are really hard to come by this close to the Pacific.' Look for the that research in "The
Fullness of Time."
The New Line Array for 2003 - Page 1.
The New Line Array for 2003 - Page 2.
The New Line Array for 2003 - Page 3.
Acoustic Line Source Research - Table of Contents.
