Most PVC pipe used for ports has a wall thickness of only 2 - 3 mm, meaning a very tight turn for the air. This can cause whistling, turbulence and "head loss"

The normal approach is to flare the port intake with a flare that is a similar size to one fitted the port exit.

Where this is not possible, a "donut style" ring is worth fitting.

If there isn't enough clearance for a "donut style" ring, a smaller "smoothing ring" can be used

Without a correctly sized flare, you will still have some turbulence as air flows back into the box, but this can be masked to some extent by the inrush of air.

The inspiration comes from ram tubes used in race cars. You can click the picture to see a larger picture of a full V8 setup worth US$3000+

The easiest way to make them would probably be with a lathe, but most DIY speaker builders don't have one. Most of us however do have a router. Here are walk-thru's for both the "donut style" rings and the "smoothing rings" - You will need the following bits:

Router bits used - Small Groove, 45 degree Chamfer, Rollover, Flush-trim

Firstly, the "donut style" ring

Step 1 - Cut the starting rings

Fit a small groove bit to the router and fit circle cutter

Make a ring that has an internal diameter equal to the OD of the port tubing. The outside diameter of this ring is the ID of the port plus the radius of the rollover bit times four. This ring is the bottom ring in the following drawing. If you are only making treatment for one port, you will need to make another identical ring to use as a spacer in the next step.

Make another ring that has an ID equal to the port ID minus 10mm, and an OD that is 10mm larger than the first ring.

Step 2 - Cut the bottom rollover

Make up a mounting board with a stub of pipe that is 3 thicknesses long. Slide a pair of the small rings onto the stub as shown. Remove the circle cutting jig from the router and fit the rollover bit. Cut around the ring as shown here.

Step 3 - Glue together

Remove the rings from the stub and glue the freshly cut ring to the larger ring made in step one.

Step 4 - Flush trim

Replace the stub of pipe with one that is only two thicknesses long. Push the glued rings on and use a flush-trim bit to open up the ID of the donut to match the ID of the port. Also trim around the outside so that the two halves match

Step 5 - Cut the top rollovers

Fit the rollover bit and cut the inside and outside rollovers to complete the donut shape

The completed donut ring

Next, the smaller "smoothing" ring

Cross-section of small smoothing ring showing cutting steps

Step 1 - Outside face

Fit small groove bit

Fit circle cutter - adjust radius to suit required OD of finished smoothing ring

Secure centre of ring with 2 screws

Cut all the way through

Step 2 - Recess to accept port

Adjust circle cutter to cut inside the pipes OD

Use a test peice to get a snug fit for the pipe

Set depth limiter to half way down

Cut groove to accept pipe

Reduce radius on circle cutter and widen groove in preparation for cut-through in step 5

Step 3 - Chamfer on rear face of ring

Fit 45 degree chamfer bit

Remove surrounding board to allow room for bit

Cut chamfer around outside of ring

Step 4 - Outside rollover

Turn ring over

Fit rollover bit

Cut rollover on outside edge

Step 5 - Inside face - initial cut through

Replace surrounding board

Refit small groove bit and circle cutter

Cut groove that is 5mm smaller in radius than pipe ID

Cut to about 90% depth all the way round

Cut right through for about 90% of the circle

Unscrew ring and cut the last bit with small hand saw

Step 6 - Inside face - clean-up

Mount ring onto a stub of pipe that is secured onto some scrap sheet

Remove circle cutter and fit flush-trim bit

Flush-trim centre of ring using stub of pipe as a guide

Step 7 - Inside rollover

Fit rollover bit

Cut rollover on inner edge of ring using stub of pipe as a guide

Remove ring from stub and hand sand any sharp edges.

It is possible to arrive at this page from several locations

Please use your browser's Back button to return to the previous section