Acoustic wave speakers combine quarter-wavelength transmission lines and Helmholtz resonators. A bass driver is positioned at the end of a tube the same diameter as the cone and a quarter of the wavelength of the system's desired low-frequency limit. On the other side of the cone is a tube one-third the length of the first tube. The design theory posits greater efficiency, by using the short Helmholtz resonator tube in conjunction with a resonant column tuned to 0.714 of the driver free air resonance frequency to produce a standing wave. Neither tube has to be straight.
- Skill level:
- Moderately Challenging
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Things you need
- Woofer with published Thiele-Small parameters and high Qts
- PVC or Sonotube
- Medium density fiberboard
- Sabre saw
- Wood rasp
- Gray speaker putty
- Electric drill
- 1/8-inch drill bit
- Phillips bit
- Number 10, 0.75-inch pan-head wood screws
- 3/32, 2.5-inch machine bolts with locknuts.
- Phillip screwdriver
- Speaker wire
- Crimp-on spade connectors
- Sine wave generator and amplifier
Multiply the free air resonance (Fs) of your woofer times 0.714. Divide 1128 by the result of your first calculation. Divide that result by 4, and you have the length of the long tube. Divide that result by 3 for the length of the short tube.
Use Sonotube for an 8-inch or larger speaker. Use PVC pipe for a 6-inch or smaller speaker. If you are convoluting your PVC pipe with elbows and U-bends, measure a centerline through the bends to maintain proper length.
Cut tubing perfectly perpendicular to the tube length. Use a sabre saw for Sonotube or a hacksaw for PVC. Smooth burrs with a utility knife.
Sabre-saw two MDF circles to fit tightly inside the ends of your tubes. Cut them slightly too large and rasp them to fit. Cut out circles inside your circles to fit the inside diameter of your woofer flange. Use the woofer to mark bolt holes on both circles. Drill 1/8-inch holes.
Cut speaker wire 2 feet longer than the long tube; split and strip half-inch insulation off both ends. Crimp spade terminals to wire ends on one end to fit speaker spade lugs and connect the speaker.
On one side of each circle, apply a bead of speaker putty around the inner rim. Sandwich the woofer flange between the puttied sides of the MDF rings, aligning all bolt holes. Drop all bolts through and tighten nuts over bolt ends.
Drop the speaker wire through the long tube and slide the tube over the MDF ring attached to the back of the driver. Drill 3/32-inch holes through the end of the tube and into the MDF, taking care to miss the speaker bolts. Insert drywall screws and seal edges with putty. Repeat with the short tube on the other side. Experiment with loose Dacron stuffing in both tubes for damping. Use a sine wave generator to test your speaker.
Tips and warnings
- If convoluting pipe into a labyrinth, treat all joints inside and out first with PVC primer, then glue. When putting PVC joints together, twist 90 degrees as you insert them for a leakproof fit.
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