This project is a 4th order bandpass isobaric system based on two Pyramid W61 6.5" drivers. "Isobaric" basically means here that the drivers are mounted face to face with the connections to one driver reversed, the idea being to reduce by half the volumes required for the sealed and ported sections of the system.
The W61, like most car audio drivers, is pretty solidly built, but the cone is made of paper and is basically too soft to do any serious thumping. Pyramid didn't provide any Xmax specifications for the driver, but the cone does have about 0.5" of excursion before the suspension brings it to a dead halt. For my calculations, I guestimated an 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 that the published specs were a bit off. Average Fs for the two drivers checked in at 52Hz, quite a bit below the published spec, which is nice as it means that I should get a bit more bass out of it.
The average Qts for the pair of drivers checked in at 0.50, and the average Vas at 'round 28 litres or so. No probs - the lower Fs and higher Qts suggested to me that the W61 could give decent results in a sealed or 4th order bandpass system. I'd expected the measured specs to be different anyway, as I'd quite recently replaced the original dust covers for the drivers with some new ones that I'd purchased locally.
The specs for a single W61 driver indicated that, for a sealed system with a Qtc of 0.71, the net box volume would have to be about 28 litres (almost 1 cu.ft.), pretty big for a 6.5 incher. The F3 would also be around 73Hz, which isn't so hot either. An isobaric system using two drivers would reduce the net volume to 15 litres, but the cutoff frequency 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 bandpass system could give a pretty decent response, but the resulting box would be pretty big. My intuition also told me that the power handling wouldn't be so hot because of the driver's limited Xmax (rule #1 - bigger is NOT always better!).
An example of a 4th order bandpass alignment using one W61 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 bandpass design 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 for a higher cutoff frequency? With this in mind, I fudged around with the calculations 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 material may increase the effective size of the vented section and lower Fb, the resonance frequency. I didn't foresee this being much of a problem, however, as I could reduce the effective volume by adding additional braces to bring the resonance frequency back up to specification. I decided to opt for a vented volume that was slightly more than that required for the alignment. If necessary I could use bracing and/or damping material to make any further 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, required to tune Vf, the vented section of the enclosure, to Fb.
Build the enclosure such that Vf', the gross volume of the vented section, is slightly more than Vf, the net volume predicted by the calculations. Also, Vr' the gross volume of the sealed section, will be equal to, if not slightly less than, Vr, the net volume predicted by the calculations.
Add a port of length Lv and diameter Dv to the vented section. As Vf' is more than Vf, the tuned frequency of the vented section should be lower than that called for by the calculations.
Add bracing/damping to the vented section until the tuning frequency is equal to Fb, the tuning frequency predicted by the calculations.
Add bracing/damping to the sealed section until a flat bandpass characteristic is obtained.
At this point, the 4th order bandpass system should have a frequency response that is as close to ideal as I can get.
Building the box
Basically, a cross-section of my final design looked something like the following:
+---------------------+| Vf +-------+| /----\ | Port | | / \ +-------++---------------------+| \ / Drivers || \----/ || Vr |+---------------------+ removable panel
And this is what the finished product looked like, from below (with the bottom removed):
The drivers were mounted from below, in the sealed section of 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 enclosure would be facing the floor when the system is in its normal position, so no screws are visible. Vf is 7.5 litres, and Vr about 10 litres. 3/4 marine ply, butt joints and aliphatic resin (wood glue) was used throughout. Drywall screws were used to hold the sections together while the aliphatic resin dried, then they were removed and the holes filled with wood filler. The enclosure was then sanded and painted with a white laquer (basically to stop the ply from splintering along the edges!). Tuning was done as outlined in the previous sections.
This is what it looks like from the front:
Frequency Response - 22nd June 1997