About a week ago, my CW technician, Paul, KW1L, brought over one of his latest toys—the Alaskit Resonant CW Speaker. Resonant speakers aren’t a new idea. Several companies have sold them in the past, and if you Google “resonant CW speakers,” you’ll find many references to homebrew projects. I believe, however, that this is the only currently-available commercially product. It sells for $35.
I’ve always been intrigued by these devices. I can remember even as a kid contemplating building one of these things. Like many projects in my hopper, though, I never got around to it.
What I told Paul that I would do is to do a frequency response test. My first thought was to hook it up to my fancy-schmancy Rigol signal generator, sweep through the bandpass of the speaker, and measure the output on my scope. For an audio transducer, I planned to use an old computer microphone that I had.
That didn’t quite work out. For one thing, the microphone seems to have gone kaput. For another, I don’t think that the scope would have given me the display I was looking for anyway.
What I ended up doing is to use a small speaker that I had laying around as the audio transducer. This is perhaps not the best way to do it, but that’s all I had on hand. I set it up near the speaker and manually stepped through a series of frequencies, recording the output of the speaker at those frequencies. To minimize the effects of noise, I set the scope to averaging.
Here’s the result:
As you can see, the filtering effect is quite sharp, except for the weird resonance at 680 Hz (or the notch at 660 Hz, if you look at it that way). Perhaps someone with more audio experience than me can explain that.
Another thing that I’m thinking about trying is to set up the signal generator to output white noise up to say 1,000 Hz and then measure that signal with the scope. The scope can supposedly FFTs and should be able to give some kind of frequency response display. I could probably find some kind of audio analyzer app to do this on my PC, too. Any thoughts you might have on that would be appreciated.
Of course the proof of the pudding is in the eating, as they say. The next step would be to plug this into my radio and actually make some contacts. I’ll let you know how that goes when I get around to it.