I built one once with a 2-1/2" (6.4 cm) diameter PVC tube. Unfortunately I don't remember the exact length, but it was about 7" (18 cm) long. I simply glued a 2-1/2" speaker to one end. It resonated at about 600 Hz.
At one point I placed a tightly fitting lid from a spray can over the back of the tube and the speaker. It became significantly louder.
My experimental 2 cents. 73, Vic, 4X6GP Rehovot, Israel Formerly K2VCO http://www.qsl.net/k2vco/ On 19 Feb 2017 01:32, David Anderson via Elecraft wrote:
Stan, Thanks for the video, most impressive. I have one comment regarding the need for an amplifier with the KX1 /KX2 or KX3 radios, there is another YouTube video from a couple of years ago that appears to be almost what the more recent QST article is based upon and in it the constructor has it connected to a KX1 and it seems loud enough. https://youtu.be/4iNELueaU6M Then there is another video where another constructor dissects the original commercial design and makes one, this really shows how efficient the design is when at the end he compares the bare speaker on its own and how weak it sounds compared to when it resonates. https://youtu.be/MF8bk6958Hg The key seems to be that in both of the above cases the lower part of the resonant speaker is + not + sealed completely, air is allowed to enter and exit, though there is sound deadening material in the cavity. Perhaps that explains the greater sound efficiency? This is something I plan to investigate myself with my KX3, once I get the parts here, of course being in the UK we are now metric and so some slight adjustments are necessary as our pipe is 50mm and so on. A friend has offered the services of his laser cutter and 3D printer which will come in handy too! 73 from David GM4JJJ
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