Sound Power

[Robert Reed]

I need a high power , high frequency sound source. 15 to 20 Khz & 120 db,150 db? My original attempt was a 50 Watt amp (@8 0hm) Driving a pair of Piezo horn tweeters. Coulnb't get the sound power needed as I later discovered these have an impedance of approx. 120 ohms.Now I am thinking of switching hi-voltage DC thru them via a high power FET (at 15-20 Khz).Fidelity is not an issue here, only power. Could even be generated by some other means (mechanical ?) Have a real barking dog problem at 150-200 ft. Has anybody had experiience on this subject?

[Chris Smith]

FET amps are the best there. <p>About 20 years back I used my 120 watt amp into a pair of Piezo’s and cranked it up. <p>Not one cat could be found in my neighborhood for several days, including all eight of my own.<p> Actually your sound choice is a little low in the spectrum, I used between 25 k and 40 k and used one of my cats at low power test, to Tell me "when it was hot", and then my neighbors dog for the "other" sound frequency.

[Joseph]

One problem you may encounter is that piezo tweeters generally cannot withstand more than about 25VRMS at around 18khz. Square wave drive can be problematic since the impedance drops at higher frequencies, including harmonics.

[Robert Reed]

Thanks all for replys. As to the 120 Watt Amp and two piezos , what was the rms voltage output? My problem has been a high enough driving voltage to get max power into these tweeters (example--75 watt tweeter & 120 ohm Z, would require 95 volts rms according to ohms law E sq./120)
Or are the manufactures lying when stating rated power of these devices? Bullhorn?? Don't know if it would be efficient at supersonic frequencys. Also .from what I've been able to gather , I'm not sure if piezos get into the 25 to 40 Khz range,but I am also not that familiar with them.

[jollyrgr]

Chris,<p>What frequency upset the cat? Any experiments with dogs? My problem is not cats but dogs around my parents house. A neighbor lets their dog run free at night and it "uses" my parents yard. And since I'm the one that mows their lawn it becomes MY problem. <p>Many years ago I tried an experiment based on my high school electronics teacher's suggestion. He said use a 555 timer and a tweeter. I took a 555 timer and drove a normal speaker with audible waves. Sounded loud. I then drove a tweeter at frequencies above 25kHz, or there about. (I don't recall exactly I just know I could not hear anything but could see a distorted square wave using a scope.) Neither my cat or neighbors' dogs responded. The tweeter used was one of the things Radio Shack used to sell. It sat on top of your normal speakers and was a add on tweeter. I think it worked up into the 60kHz range if I recall. Dogs didn't even flinch. I have three ultrasonic pest repellers in my garage. Still I find mice from time to time. So I ask, do these ultrasonic ides work at all?

[Joseph]

That rating most likely should be taken as system power (for everything). The crystal inside the piezo tweeter can handle only a small small amount of wasted power before it overheats and cracks. But it converts input energy to sound more efficiently than a voice coil driven speaker, save maybe the horn type. I'd say 120w and 8ohms implies an output voltage of around 30v, just a bit too much.<p>I do not like intruding varmits including mice, cats, dogs, raccoons, and opossums, but I welcome garter snakes. I need to check the piezo output at the front of the house since a young human critter was loitering on the porch recently. It fits that it is not operating properly, too, given that a cat was lying there earlier.
The piezos are driven by an isolated soft-edged square wave put out by a SMPS I built and set to run at around 17khz. Isolation is very critical for safety. I placed a resistor in series with each tweeter to damp resonance between the filtered transformer output and the capacitive piezo crystal as well. I set it to run at a rather low frequency figuring that it should annoy a broader range of animals, but hopefully not the snakes.<p>[ May 13, 2005: Message edited by: Joseph ]</p>

[Chris Smith]

In my case I may not have had full power, but I had a pair of balanced 8 ohm Piezo speakers designed for the amp, so there was no mis match and I ran it at about 80% of the knob full power setting, and I used a 555 as one input as a trial at lower frequencies [just above human audio response 28k] which drove my X out of the house, and a signal generator input on the other for the higher frequencies. <p>From memory I did a scope on it to double check the output was working, and the frequencies Im not 100% on?<p> I think the cats went berserk at around 32k and the dogs were up there even higher, either 37k, or maybe 42k? I used to test it on guests also, silently, and watch them squirm and get uneasy, and then go to the bathroom shortly there after.<p> You could FEEL a pressure wave if you stood in front of it, but it was hard to describe what you felt. <p>Most Piezos can handle ultrasonics as they are basically just a crystal that vibrates, but the sound performance probably drops off at higher frequencies. <p>The neighbors dog just became uncomfortable and went and hid, while the cats were not to be found for several days. <p>I did all this in 1984, so I cant really remember the settings. I don’t think the volume is as important as the sound frequency it self. <p>I had mine cranked up to “disturb” a hound dog three doors down, and it worked,.... Sort of?<p> Hounds like to howl just because their being disturbed also?

[rstofer]

One thing to consider is that even though you can't hear ultrasonic, it does impact your hearing. dB limits and exposure times are still relevant.<p>In the beginning, there were ultrasonic motion detectors to control office lighting. Guess what? You couldn't use them in an office environment because the sound level was over 80 dB and would exceed the legally mandated exposure limit.<p>FWIW, the manufacturers didn't know this or even bother to think about it. Cal OSHA eventually came out of their coma and took the position that the devices couldn't be used if they exceeded the exposure limitations. So, the industry moved to passive infrared.<p>[ May 15, 2005: Message edited by: rstofer ]</p>

[jwax]

I've irritated a few critters with the "Sonic Pistol" sold by: www.amazing1.com.
They have higher power versions also. The little "pest repellers" are worthless- not enough power. The higher the power, the more the effect!
Like Chris, I've tested the pistol on folks at a distance (100 ft) and watched them wonder what that "noise" was. They dispersed soon afterward.
Understand they are used in front of convenience stores to dispel crowds from gathering.
The pistol is perhaps 30 watts of DC input, and the output is rated at 130 db. Sweep the frequency from audible to not audible, and you'll annoy every living thing in its path.
BTW, I'm currently building their 100 watt version. Just for science, you know.
They have a forum full of dangerous children too, kind of like this one!

[Robert Reed]

Thank you Jwax for your web site-will check it out. Have been trying to get detailed info on tweeter power ratings and I guess I'll try the manufacturers. Hopefully end up with their engineering Dept. I calculate that at 150' distance I would need 125 DB at the source to end up with 90 DB at stated distance. I think that would be loud enough at the targeted area.

[jwax]

http://www.lsu.edu/deafness/HearingRange.html
Another reference to animal hearing sensitivities.

[ezpcb]

interesting project. I'm also boring by the cats and dog barking. I'll try LM12 from national semiconductor, it's a high power high voltage amplifier

[Robert Reed]

jwax: Thanks again for another informative web site.
ezpcb: There are a number of ways to driive the end device (tweeter?),which are not to difficult to construct. However my big hangup so far has been the end device. Getting a lot of mixed signals on piezo information.The most credible device (suggested by an expert audiophile) so far has been a compression horn tweeter of the type used in rock concert bands. These have an SPL of 110 db, 8 ohm "Z", and the ability to truly drive them at their 75 watt capacity.The down side is they cost upwards of $200. Have located similar devices at 106db for around $50. I juist wish I could find something specifically for this application without adding the extra cost for midrange and fidelity.

[jwax]

True. Nothing special about the amp- simply has to handle high power at u/s freqs. Bipolar, monolithic, or FET's will work.
If high SPL is desired, only way is to use several "emitters". Unless you're military, I don't know of any one tweeter that will go really high power. They're working on destructive ultrasonic weaponry. Go figure.
If I remember right, two 106 db sources will produce a net power of 109 db, correct? A third 106 db source nets 112 db. All in phase of course. Nine will get you 130 db!

[Robert Reed]

jwax: Correct - every doubling of power will get you 3DB's.
I had considered a flock of cheap piezos, but was warned of possible acoustic cancellations, Not familiar enough with this subject to make any intelligent decisions. It's like Rossana Anna Danna says-Its always something. Have checked out rock concert tweeters and they appeared to be ideal,exept that most of them take a nosedive at about 19K -Right at the point where I want to pick up. As to the subject of cheap piezos ($3) , most are rated at 93 Db spl.This would require 8,192 tweeters if my math is correct.A bit bulky for this application. Meanwhile the dogs continue to bark, coming through the window even as I type this,
mmmmm---maybe a .357 Magnum would be a cheaper solution.