How to connect piezo speakers to an amplifier

[Audioguru]

A piezo beeper is a piezo transducer with a transistor oscillator inside it. Some of then are louder when powered from 30V and some have their loudness measured at a distance of 30cm. They are usually used at about 4kHz which is their resonant frequency (the loudest frequency).
Digikey lists 3800 piezo buzzers but some have spec's that have many translation errors.

What will your piezo noise maker be used for?

 

[miker123]

It's needed to discourage a group of unruly juveniles. I'm not trying to backward-engineer the https://www.compoundsecurity.co.uk/wp-content/uploads/2019/04/mosquito-setup-instructions.pdf but would like to achieve something of similar impact https://www.compoundsecurity.co.uk/faq/what-is-the-specification-of-the-mosquito/

I see https://www.americanpiezo.com/standard-products/ultrasonic-power-transducers.html markets piezo transducers with high frequencies.

 

[Audioguru]

Ultrasonic piezos cannot be heard by humans, not even juveniles with good hearing.
Young people and most people used to be able to hear frequencies to 20kHz but the people who deafened themselves with loud rock music cannot. Maybe the juveniles you need to discourage.

The unit that you want to build does not use the little 4kHz piezo buzzers that you can buy.

 

[miker123]

I know that about over 20kHz. I shouldn't have risked another detour by posting that link. (I don't want to risk another detour, but I used to always blow woofers and tweeters when younger, and had more than my share of loud music).

If I could ask, very specifically, what parts would you use to create a 17,500 Hz tone at about 95 - 100 dB three feet away. Maybe an unweighted (opposed to A-weighted) sound pressure level 10 feet away of about 84 dB.

Please bear in mind when I ask what parts would you use, that the terminology and subsets of broader classes (such as piezo transducers coming with or without their own driving electronics) are very confusing, so please be specific.

 

[Audioguru]

There is a piezo tweeter that has been sold for many years and it produces awful music but it is loud and goes up to your required 17kHz. I think its high frequency sounds are VERY directional and are loud only where it is pointing towards.
2.83V RMS produces about 94dB at a distance of 1 meter. 5.66V produces 100dB. 11.32V produces 106dB.
11.32V at a distance of 4 meters would be 100dB. 11.32V is an audio power of 16W into 8 ohms which needs a power supply that is
about 36V for a single ended amplifier or needs to be 17V for a bridged amplifier.

The deterrent might be better if the frequency is warbled a little.

 

[miker123]

As you just pointed out, you cannot buy those buzzers that would be needed (unless you were having them made specially for mass production), so the KSN1001A makes sense here. That is even if they do occassionally burn out. Hopefully the useful life will be long enough that it doesn't get very expensive, but it seems to be a simple means to an end.

As far as your 11.32V equation, I would be using a HiFiBerry Amp2 which specs https://www.hifiberry.com/docs/data-sheets/datasheet-amp2/ for an 18 volt power supply at 8 ohms shows typical power output per channel of 17 watts, and maximum power output per channel of 20 watts. The highest volts power supply the Amp2 can take is 24 volts. But if I used its standard 18 volt power supply, that would be HALF of your suggested 36 volts. Wouldn't that only mean a neglibible loss of 3 decibels?

Also, though I may not need a capacitor for this I am concerned about possible accidents as well as pops when turning the unit on. Would it hurt anything to use a crossover to protect from accidents and on/off pops? I know sometimes folks ask about capacitors on tweeters instead of a crossover, but a crossover has a capacitor in it. I have a 5kHz 8 ohm 12dB/octave high pass filter here https://www.parts-express.com/5-khz-high-pass-8-ohm-crossover--266-474 I'm thinking of using for this purpose. Wouldn't that steep 12dB/octave slope give me the sought protection?

Another concern is the KSN1001A datasheet warning [pg 2] https://www.gad.net/Blog/wp-content/uploads/3017/04/KSN-1001A-1005A-Datasheet.pdf saying that

"Warning: A 30 ohm series resistor is
recommended to assure stability of
extended range amplfiers and preclude
hazard of burnout."

What 30 ohm resister should I put in series with this tweeter? If I do really need one, how many watts? I'm not sure how this would impact the rest of the circuit.

Finally, is "warbling" needed if I'm only using a single MONO channel for this application? ie; no concerns about 'phasing'. Is a good example of "warbling" alternating between 17 kHz and 18 Khz a few times per second?

 

[Audioguru]

As you just pointed out, you cannot buy those buzzers that would be needed (unless you were having them made specially for mass production), so the KSN1001A makes sense here. That is even if they do occassionally burn out. Hopefully the useful life will be long enough that it doesn't get very expensive, but it seems to be a simple means to an end.

I looked and found that the kSN1001A piezo tweeter is obsolete but an unknown Chinese company makes copies.

As far as your 11.32V equation, I would be using a HiFiBerry Amp2 which specs https://www.hifiberry.com/docs/data-sheets/datasheet-amp2/ for an 18 volt power supply at 8 ohms shows typical power output per channel of 17 watts, and maximum power output per channel of 20 watts. The highest volts power supply the Amp2 can take is 24 volts. But if I used its standard 18 volt power supply, that would be HALF of your suggested 36 volts. Wouldn't that only mean a neglibible loss of 3 decibels?

17W into 8 ohms is 33V peak-to-peak and the Hifiberry produces it with a 18V supply because its amplifier IC is bridged (a separate amplifier for each speaker wire) to double the signal voltage which also doubles the signal current for about 3.5 times the power.

Also, though I may not need a capacitor for this I am concerned about possible accidents as well as pops when turning the unit on. Would it hurt anything to use a crossover to protect from accidents and on/off pops? I know sometimes folks ask about capacitors on tweeters instead of a crossover, but a crossover has a capacitor in it. I have a 5kHz 8 ohm 12dB/octave high pass filter here https://www.parts-express.com/5-khz-high-pass-8-ohm-crossover--266-474 I'm thinking of using for this purpose. Wouldn't that steep 12dB/octave slope give me the sought protection?

A piezo tweeter is a capacitor and does not produce low frequencies so it does not need another capacitor and does not need a crossover.

 

[Audioguru]

A concern is the KSN1001A datasheet warning [pg 2] https://www.gad.net/Blog/wp-content/uploads/3017/04/KSN-1001A-1005A-Datasheet.pdf saying that

"Warning: A 30 ohm series resistor is
recommended to assure stability of
extended range amplfiers and preclude
hazard of burnout."

What 30 ohm resister should I put in series with this tweeter? If I do really need one, how many watts? I'm not sure how this would impact the rest of the circuit.

Since the piezo tweeter represents a 0.13uF capacitor then its impedance drops at high frequencies and some amplifiers become unstable and overloaded when driving a capacitance. The 30 ohm resistor limits the impedance drop and reduces the 17kHz output level by about -3dB. Using the 17W Hifiberry amplifier then the 30 ohm resistor will heat with 3.5W so use a 10W resistor.

Finally, is "warbling" needed if I'm only using a single MONO channel for this application? ie; no concerns about 'phasing'. Is a good example of "warbling" alternating between 17 kHz and 18 Khz a few times per second?

A single frequency tone sounds ordinary. A tone that warbles its frequency maybe 5 to 10 times per second a little is obnoxious and will be better to drive away loitering teens.

 

[miker123]

I think you "nipped it by the bud" and I thank you very much as it would have taken me forever to figure this stuff out.

Correct me if I'm wrong, 11.32V ** produces 106dB at one meter, so I lose 3 dB from the resistor then I lose maybe a few more audible decibels from a 100+ foot speaker cable run (12 AWG wire), so that would bring me to roughly about 100 dB at 1 meter, just where I want to be; not overkill.

Are these https://www.parts-express.com/dayto...watt-precision-1-audio-grade-resistor--006-30 acceptable connected inline along the one positive wire between the amp and the tweeter?

** Or something about that amount, as the Amp2 would be 33V peak-to-peak.

 

[Audioguru]

2.83 RMS produces 1W and the tweeter produces 94dB at 1 meter.
11.32V RMS produces 16W into 8 ohms but at +3dB per doubling of power then the tweeter will produce 106dB and the 30 ohms resistor will reduce it to about 103dB at 1 meter.

The resistor is mounted at the amplifier then the tweeter that has a much higher impedance than 8 ohms will not load a cable much. Use ordinary 16AWG or 18AWG lamp cord.

 

[miker123]

I can put the https://www.parts-express.com/dayto...watt-precision-1-audio-grade-resistor--006-30 resistor inline along the positive tweeter wire, mounted right over near the amplifier, but I cannot use 16 - 18 AWG wire because see the table at http://www.roger-russell.com/wire/wire.htm#wiretable
The problem is that it is a GIVEN that I have a 100 - 120 foot speaker wire to run, necessitating 12 AWG wire for 8 ohm, going by that table... It has to be that long of a speaker wire.

 

[bertus]

Hello,

For the cables, have a look at this article:
https://sound-au.com/cables.htm
There are 6 pages with info.
Use the next button on the bottom or the links in the menu.

Bertus

 

[WHONOES]

I think you are overthinking this. Piezo tweeters are capacitive. The datasheet for the one you have linked shows it as 0.13μf. Therefore its impedance at 17Khz is 72 ohms. Its maximum voltage is 15V rms which equates to 42,6V pk to pk. You don't need an amplifier with vanishingly small distortion figures. In fact, you would likely be better of driving it with a square wave as it would be much more annoying.

 

[Audioguru]

I
You don't need an amplifier with vanishingly small distortion figures. In fact, you would likely be better of driving it with a square wave as it would be much more annoying.

Hee, hee. The kids are bats and can hear the harmonics of the 17kHz fundamental squarewave. The ultrasonic 51kHz and 85kHz harmonics will be annoying.

Oh, maybe you are saying that the harmonics will beat with the amplifier's class-D switching frequency producing lower frequencies?
The Hifiberry amplifier manufacturer does not say the amplifier IC part number used so we cannot see what is its switching frequency.

 

[WHONOES]

Nope. Just saying that HF square waves are particularly annoying. As it happens, I designed a cat scarer a few days ago after chasing one through my garden with a Blackbird in its mouth. It runs at 28Khz switched at 1.6Khz. Should be nice and annoying.
It's dead simple. I will add it to this post when I have the time.

 

[Audioguru]

Since the piezo tweeter is 72 ohms at 17kHz then its current is much less than if an 8 ohm speaker was used so the wire thickness can be small. The RMS voltage from the amplifier is 11.32V but the 30 ohms resistor in series with the tweeter reduces the voltage at the tweeter to about 8V RMS (11.3v peak) and the current in the wire and in the tweeter will be 111mA (157mA peak) RMS.