• The port had to flow enough air to generate turbulence with the largest flare fitted.
  
  
• The tuning frequency for the port should be in the normal operating range of a subwoofer - 30hz was chosen.
  
  
• The port shouldn't be too short, which might lead to an "all flare and no port" situation.To have a reasonable length straight section when testing large flares, a small box is desirable.
  
  
• To have a high enough airspeed to test larger flares, a large box is desirable

Round one tests used a 50litre box and had a pair of 10inch drivers driven by the Jaycar 350w amplifier. Into the 7ohm load presented by the two drivers in series, it delivers up to 240w.

The test port is mounted externally and is forward firing, whilst the drivers are located on each side.

Foam gaskets were used under the drivers and the port. The port was mounted with eight roofing screws, which, along with a cordless drill, made changeover time much easier.

Round one test box sitting on top of "Blast Furnace" sub, utilising its amplifier

Round two tests used a 100litre box. and started life as the very first subwoofer I built. Four 10 inch drivers mount to the sides and a 255mm opening for the port is at the front. An identical opening at the top allows fitting of a plain plate, a 50 litre add-on box, or a 30 litre reducing cylinder. Both openings have foam gaskets and eight studs - this proved much more durable than the roofing screws used with the round one tests.

100 litre test box with 30 litre cylinder fitted, reducing volume to 70 litres

Test box with add-on box to give 160 litres