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.
Walter Underwood says
White noise would be a good test, because it is similar to how it will be used.
Don Keith says
Dan, if you have a speaker that is resonant at 680 KHZ, you may have other problems! :-) Especially since we have a 50KW broadcast station locally at 690 khz.
Seriously, we’ll be interested in how you like this device. We all need help as we get older. And especially those of us with “disk jockey” ears…over 20 years of using headphones while on the air with the gain cranked up to “11.”
73,
Don N4KC
Dan KB6NU says
Geez. I’m so used to typing kHz instead of Hz. Thanks for catching that.
Don Keith says
Believe me, as a writer I know how easily those things happen! But with a call sign that ends in “KC” you know I am sensitive to such stuff!
Always enjoy your blog posts. Keep ’em coming.
73,
Don N4KC
Steve C - KE8HXM says
In the past I used military headphones to listen to CW and after a few hours, even placing them just forward of the ear those things were very uncomfortable. Who would have thought a blocked and insulated speaker in a frequency tuned length of tube would be the solution? Resonant speakers may be a wave of the past, but then we are talking CW and it does not get much more retro than that, does it?
Nolan Kienitz says
And then there are some helpful pages providing help in building your own resonant CW speakers. I’ve been working on one and just need to get around to finishing the project.
http://www.wcarc.com/images/elmer-files/CW%20Resonant%20Speaker.pdf
http://www.qsl.net/n5iw/RESONANT.htm
I’ve been to the Alaskit website, but information is very limited and one can’t click on CW Speaker label to see more data.
Dan KB6NU says
About a week ago, I purchased the parts to build an N5IW speaker. That will be one of my next projects.
Dave K8WPE says
What is the final verdict in actual use of the resonant speaker?
Dan KB6NU says
Like many of my projects, I got sidetracked and never completed the speaker. It’s still on my list of things to do, though.