Donuts and Smoothing Rings

When you are unable to flange mount a port intake, don't just leave it hangin' - fit a Donut ring or a Smoothing ring

Photo - Donut and smoothing ring
Donut ring vs Smoothing ring, as used in the buddy subs upgrade

Most PVC pipe used for ports has a wall thickness of only 2 - 3 mm, meaning a very tight turn for the air, causing whistling and turbulence. 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 this, a smaller smoothing ring will always fit.

The inspiration comes from ram tubes used in race cars.

Chevy V8 injection system

Race tuned engines use Ram Tubes to increase airflow. The tube is of a tuned length and has a flared "bell" to control turbulence. The cheaper models have a partial roll on the flare, but the best ones have a "full radius" bell. This picture of a Chevy V8 injection system comes courtesy of Top End Performance in the states. They have more pictures....


Anyway, back to the walk-thru...


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 styles of rings - You will need the following bits:

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


Quick tip

If you are making more than one, it is easiest to do them all at once. Screwing your sheet to a sacrificial board underneath allows the use of screws to hold the centres of any circles cut out, preventing the circle jig from moving when the last little bit is cut.


Firstly, the Donut Ring

Here's a quick overview, and then on to the details ..

Infographic of steps to make a donut flare for a speaker port

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.

Drawing - initial rings


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.

Drawing - cutting the bottom rollover


Quick tip

Use some scrap to support the outside edge of the router to keep it level.
This photo shows the mounting stub surrounded by a large ring at the correct height to support the router.
Five completed "donuts" can be seen in the background


Photo - Jig to keep router level



Step 3 - Glue together

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

Drawing - halves glued together


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

Drawing - flush-trimming


Step 5 - Cut the top rollovers

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

Drawing - cutting the top rollovers


Drawing - the compled donut
The completed donut ring


Next, the Smoothing Ring

Drawing - cross section of 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.

/ring.htm last modified: 28 January 2024
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