Copyright 2004-14 © Troels Gravesen
|Why another 18W/8531G00
sliced paper construction? Didn't the SP95 and
the SP98 do well? Why possibly a reduced cabinet volume?
And why does USXX predict an optimum 22 litre cab for the
18W driver? Who is USXX and what has LspCAD to do with
Well, first of all, this construction dates back to the SP95 where I tried the Scan-Speak D3806/8200 mid-dome from 1600 Hz and the HIQUPHON OWI on top from 13 kHz. I only made a single cabinet from this set-up and never tried in full stereo. The measurements looked good and the sound from the single speaker was promising, so here we go with the SP38/13, a potent 33 litre floorstander providing deep solid bass from the 18W/8531-G00 bass driver. Overall system sensitivity is around 85-86 dB/2.8V and you need a min. 50 wpc amplifier to drive the speakers. The large 3806 dome goes low and releaves the bass driver of handling upper mid/lower treble area, an ideal situation for the bass driver. The cabinets are tilted 5 deg. to provide an ideal listening window and keep driver phase tracking at optimum. Due to crossover topology an even vertical dispersion is allowed.
hardly needs another introduction and for this
application I have compared the damar-coated to the
uncoated drivers and I still prefer the damar-coated.
However, you may use the un-coated here without any
changes to the crossover.
The D2010/8513 does a good job from 13 kHz. This may not be my favourite tweeter when we are talking two-way systems with a point of crossover around 2.5-3.5 kHz, but as a super-tweeter I have no objections. And it's cheaper than the HIQUPHON OWI.
The D3806/8200 driver: Well, this is the driver that makes this construction special. To be honest, few 6-7 drivers manage to perform well in the 2-4 kHz range. Most drivers will start rolling off around 3 kHz or they start beaming when the wavelength equals the membrane diameter and would be better off with a smaller driver taking over from around 1-2 kHz, but in commercial designs, cost is an object and the crossover may be more complicated and final production control more time consuming. Not a problem for the diy people. We have all the time in the world for our hobby. Well, cost may be an object, but the 3806/8200 is no more expensive than the 9500 tweeter and a cheaper super-tweeter could be used, like the Vifa D20TD-05-06 or a ¾ from SEAS. We only need to have a linear response between 10 and 20 kHz and this may be easier to find around 19-20 mm domes.
As mentioned in the Acapella files, its hard to ignore the history of Spendor. Spendor will gladly use a 1½" mid dome (actually the 3806/8200) from only 3 kHz in the SP1/2 construction. A 19 mm dome will not go lower than 4-4.5 kHz, and so on. The lesson to be learned from this practise is this: Always use drivers capable of reaching preferably 1-2 octaves below the intended operating range (for good reasons this doesnt apply for the bass driver!).
frequency response of the D3806 driver (blue) is not that
flat, but it is easily equalised to gain a flat response
up to 15 kHz. This driver can almost be used as a
stand-alone tweeter. I guess some damping material on the
pole piece might do well.
Left; Green = 33 litre volume, vent = 72 x
200 mm. Red = 23 litre volume, vent 46 x 200 mm.
More on sound
Making A-B tests from a variety of music
on the 23 litre and 33 litre cabinets didn't give any
conclusive results, but I'm in favour of the sound coming
from the 33 litre cab being more round and deep. The
upper bass and lower midrange appear to have more weight
and remember, we're still dealing with a relatively small
speaker here.150 square centimetres membrane area is the
determining factor for the scale of sound we can expect
from a basically two-way floorstander.
After having decided on cabinet construction the following drawings were made and the tests cabs were modified to off-set the 3806 and 8513 tweeters as depicted above. Quite some task to modify the test cabs but worthwhile before handing over the drawings to the cabinetmaker. This time I wont make the final cabinets myself.
Left: SPL from individual drivers again. Black = 8531. This is a damar coated 8531 showing a dip at 3 kHz and a peak at 3.7 kHz and the lesson to be learned from this is to be prepared to all sorts of surprises when you apply coatings. No problem in this construction due to low point of crossover between 8531 and 3806, but this wouldn't work in a 2-way system with higher point of crossover. Blue = 3806 and red = 8513.
Right: Impedance of drivers, already commented above.
Left: Red = overall system response. Blue = same without supertweeter. Green = inverted 3806 polarity. Right: Wavelengths at 13 kHz are short, very short and moving the microphone just a little up and down easily creates dips at point of crossover between 3806 and 8513. Fortunately this is very hard to hear.
Comments to measurements:
SP38/13 construction pictures
Here are a number of pictures taking during mounting the vent, adding damping material and mounting drivers and crossovers. A vent tuning discussion will appear at: http://www.troelsgravesen.dk/vent_tuning.htm, as I here have tried various flanged and flared vents to measure the impact on performance.
SP38/13 Superior Z-caps