An acoustic CW Audio Peak Filter.
This is the the details of the construction of an acoustic audio filter for CW operation.
Whilst the normal rig DSP filters are usually pretty good on most of the
modern rigs nowadays but I have noticed they still ring slightly on narrow
bandwidths and they are not as good as an acoustic filter.
I built this audio acoustic filter years ago to use on my old K3 and found it to be a great asset at pulling out those really weak stations close to the noise level, although the filters in the K3 were very good I still found the acoustic filter still had the edge over the internal K3 filters. I still use it on my FTdx10.
The construction is really simple, it works on the principle of an organ pipe where the air blown in at the bottom that resonates a column of air in a tube, in our case it's a loudspeaker.
Reference: https://www.exploratorium.edu/snacks/organ-pipe
The parts of the pipe are made out of 68mm rainwater pipe. I have used a 112.5° ROUND OFFSET BEND fitted over a short piece of 68mm pipe that houses the loudspeaker and another short piece of pipe fitted over the end as the resonator, the pitch and be varied by extending or shortening this on the end of the bend.
As it stands without any extension it resonates at 690Hz which I find too high pitch for me personally, by adding a 59mm length of 68mm pipe to the end of the bend bought the resonant frequency down to 620Hz which I find is my personal sweet-spot. You can easily find yours by varying the CW pitch control on the rig and you'll find there is a certain frequency where the pitch seems to suddenly sound loud. If you find your pitch is a lot lower, some peoples is, just lengthen the extension pipe a bit by pulling off the bend. Once the sweet-spot is found it might be worth taping it place so it doesn't move.
The bandwidth of the filter is about 60Hz, so use the wider filter in the rig and then switch the acoustic filter on if needed, otherwise you can easily miss stations calling off frequency, I have had stations call me off frequency by 1kHz if it wasn't for the spectrum display I would have missed them anyway.
This is a link to the demo video of the filter in use: https://www.youtube.com/watch?v=776dxa1MGU4
Finished filter
Component parts
Bottom tube top
Note the loudspeaker mounted behind the perspex plate, make sure this is airtight, this is important.
The hole in the perspex sheet over the loudspeaker is 3mm.
Bottom tube underneath
Bottom tube underneath
Filled with cotton wool as sound dampening.
