|The system |
This project is a 4th order bandpass isobaric systembased on two Pyramid W61 6.5" drivers. "Isobaric"basically means here that the drivers are mounted face to face with theconnections to one driver reversed, the idea being to reduce by half thevolumes required for the sealed and ported sections of the system.
The W61, like most car audio drivers, is pretty solidlybuilt, but the cone is made of paper and is basically too soft to do anyserious thumping. Pyramid didn't provide any Xmax specifications for thedriver, but the cone does have about 0.5" of excursion before thesuspension brings it to a dead halt. For my calculations, I guestimatedan Xmax figure of 0.1 in. Pyramid DID provide figures for Vas (19 litres),Qts (0.47) and Fs (64.5 Hz), but a quick check of the drivers showed thatthe published specs were a bit off. Average Fs for the two drivers checkedin at 52Hz, quite a bit below the published spec, which is nice as it meansthat I should get a bit more bass out of it.
The average Qts for the pair of drivers checked in at0.50, and the average Vas at 'round 28 litres or so. No probs - the lowerFs and higher Qts suggested to me that the W61 could give decent resultsin a sealed or 4th order bandpass system. I'd expected the measured specsto be different anyway, as I'd quite recently replaced the original dustcovers for the drivers with some new ones that I'd purchased locally.
The specs for a single W61 driver indicated that, fora sealed system with a Qtc of 0.71, the net box volume would have to beabout 28 litres (almost 1 cu.ft.), pretty big for a 6.5 incher. The F3would also be around 73Hz, which isn't so hot either. An isobaric systemusing two drivers would reduce the net volume to 15 litres, but the cutofffrequency would still be a bit too high for my tastes.
In steps the 4th order bandpass system!
My calculations showed that a single W61 used in a bandpasssystem could give a pretty decent response, but the resulting box wouldbe pretty big. My intuition also told me that the power handling wouldn'tbe so hot because of the driver's limited Xmax (rule #1 - bigger isNOT always better!).
An example of a 4th order bandpass alignment using oneW61 driver would be as follows:
Vf = 13.9 litres Vr = 27.2 litres Fb = 74.3Hz Fl = 45.7Hz Fh = 120.7Hz Gain = 0.00dB
Here's where the flexibility of the 4th order bandpassdesign steps in!
Say we use an isobaric system to decrease the volume requirements,then reducing the size of the rear volume even further by settling fora higher cutoff frequency? With this in mind, I fudged around with thecalculations again and came up with the following alignment:
Vf = 7.0 litres Vr = 9.9 litres Fb = 81.2Hz Fl = 52.0Hz Fh = 126.9Hz Gain = 1.56dB
Designing the box
Designing the box - the vented section
There is one potential problem here - the damping materialmay increase the effective size of the vented section and lower Fb, theresonance frequency. I didn't foresee this being much of a problem, however,as I could reduce the effective volume by adding additional braces to bringthe resonance frequency back up to specification. I decided to opt fora vented volume that was slightly more than that required for the alignment.If necessary I could use bracing and/or damping material to make anyfurther adjustments.
Designing the box - the sealed section
Designing the box - response variations
Designing the box - the final plan
Determine Lv, the length of port of diameter Dv, requiredto tune Vf, the vented section of the enclosure, to Fb.
Build the enclosure such that Vf', the gross volume ofthe vented section, is slightly more than Vf, the net volume predictedby the calculations. Also, Vr' the gross volume of the sealed section,will be equal to, if not slightly less than, Vr, the net volume predictedby the calculations.
Add a port of length Lv and diameter Dv to the ventedsection. As Vf' is more than Vf, the tuned frequency of the vented sectionshould be lower than that called for by the calculations.
Add bracing/damping to the vented section until the tuningfrequency is equal to Fb, the tuning frequency predicted by the calculations.
Add bracing/damping to the sealed section until a flatbandpass characteristic is obtained.
At this point, the 4th order bandpass system should havea frequency response that is as close to ideal as I can get.
Building the box
Basically, a cross-section of my final design looked somethinglike the following:
+---------------------+| Vf +-------+| /----\ | Port | | / \ +-------++---------------------+| \ / Drivers || \----/ || Vr |+---------------------+ removable panel
And this is what the finished product looked like, frombelow (with the bottom removed):
The drivers were mounted from below, in the sealed sectionof the enclosure. The bottom of the sealed section was made removeable,so that the drivers could be accessed at any time. The bottom of the enclosurewould be facing the floor when the system is in its normal position, sono screws are visible. Vf is 7.5 litres, and Vr about 10 litres. 3/4 marineply, butt joints and aliphatic resin (wood glue) was used throughout. Drywallscrews were used to hold the sections together while the aliphatic resindried, then they were removed and the holes filled with wood filler. Theenclosure was then sanded and painted with a white laquer (basically tostop the ply from splintering along the edges!). Tuning was done as outlinedin the previous sections.
This is what it looks like from the front:
Frequency Response - 22nd June 1997