want to build a VHF sounder device

First, 18KHz sound is not considered "VHF". VHF is a radio term.

Are you after a noisy dog? If so, I have a lot of sympathy. A neighbor had a pit bull that would not STFU! So, took a cheap piezo tweeter, an eighty watt amplifier, and a signal generator. As soon as the pooch would start up I would blast it. It worked every time. I set the frequency to just above what I could hear. It was about 18KHz, as I recall. The trick is to only leave it on long enough to stop the barking, otherwise it'll get used to it.

The easiest thing, for you, would be to build the signal generator (a 555 would work nicely) but purchase the amplifier and speaker. A cheap car amp and a cheap 12V supply (like from a computer) will work well.

Bob

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You should set it up with a microphone, so that when the dog barks, it gets pinged.

I concur here, but OP will probably need to invest in a book or two (or at least invest some time studying on-line).

Cheers! Rich

18 kHz(+/-3%) is not VHF, not even technically ultrasonic.

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If you'd like to test the feasibility of your project before you get into circuit construction, you might want to try generating the tone with your computer's sound card. You can use the signal generator in Daqarta to do this. (It's free.) This will allow you to see what sort of frequency and waveform work best.

One thing to be aware of is that tweeters (especially piezo tweeters) tend to have *very* irregular frequency responses, with serious dips and peaks. Changing the frequency by a few kHz can mean a huge change in output level... easily

12 dB or more. That's a factor of 4 difference in the required amp drive voltage. So it would make sense to tune the system to find an optimum frequency for the tweeter you are going to use, before building anything.

And I mean literally the same physical speaker, not just one of the same model number... the device variability is huge in this frequency range.

Another issue is whether you can hear in this range, in order to tune for best response. Most adults have poor response here, but you might be able to make a relative determination by getting your ear next to the speaker. Better yet would be to get a kid who can still hear these frequencies to help you.

You can also use Daqarta to measure the response of the speaker, though that part is only free if you can do it in 30 sessions or 30 days, otherwise US$29 for a license. The problem for a one-off design (like I presume your project is) is that you need a microphone with a known frequency response, but calibrated mics are expensive. If you wanted to get involved, you could build a pretty good one yourself from an inexpensive Panasonic WM-61A mic capsule (couple of bucks, from Digi-Key). You wouldn't need to calibrate to get absolute sensitivity, and the response is fairly smooth in the frequency region you need, so you could make relative measurements to find the best frequency. But for starters I recommend just doing it "by ear".

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!