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I have tried to drastically lower the pitch of a piezoelectric buzzer, but to no avail. All the piezo buzzers I have emit a very high pitched whistle-like tone. I've tried changing all the parameters of my pulse generator, i.e., frequency, duty width, spacing, amplitude, etc. but the pitch of the tone remains the same, very high! Is it even possible to make a base tone from a piezo, or is the high pitch an intrinsic characteristic of all piezo units?

The piezo I have is just a plain transducer, no internal circuitry. That's why I am using a pulse generator. What I am trying to do is move as much air as possible with a vibrating membrane unit that is extremely light and simple, which rules out heavy magnet/cone speakers of any size. I figure that a low bass sound would be the key. If not with piezo buzzers, what else could I try?

Eddie
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    Post is missing a part number, could you please add it so we can get a better idea what we are working with, as well as a link to the datasheet? – James S Feb 24 '21 at 22:57
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    The pitch is an intrinsic characteristic of a buzzer. It doesn't have to be a high pitch, but changing the pitch is done during manufacture, not externally. If you want a buzzer with an intrinsically low pitch, select a different part number. – Ben Voigt Feb 24 '21 at 23:02
  • You could try driving it with too low of a voltage – user253751 Feb 25 '21 at 09:18
  • A piezo is like a bell, pipe, or other simple mechanical oscilator - it has one pitch and can be either ON or OFF. It's meant to be used as a state indicator - a digital output device for a single bit of information. Being a resonant device, they have the benefit of being able to produce a loud, sharp, piercing tone with little power. The typical use case for them is altering a user to a problem in a noisy environment. If that's not *your* use case, then perhaps a piezo is not the correct solution for your problem. – J... Feb 25 '21 at 11:23
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    You could lower its natural resonant frequency by adding weight to he centre of the piezo disk : this will also greatly reduce its volume and efficiency. –  Feb 25 '21 at 12:58
  • @BrianDrummond Not within a terribly useful range - maybe 30%, I'd guess, depending on geometry, but certainly not more than an octave, at absolute most, I wouldn't think. You won't get a piezo to drop from 2kHz to a usable 200Hz, for example. PVD uses piezo sensors at 6MHz for thin film thickness measurements and they're considered "spent" when they drop to about 5MHz, for example. You can tweak, but that's about it. – J... Feb 25 '21 at 14:29
  • Thanks , your info makes me realize that piezo buzzers are not suitable. – Eddie Feb 25 '21 at 18:35
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    @J... "...use case ...is **altering** a user..." Best typo of the week! :-) – Carl Witthoft Feb 26 '21 at 12:07
  • @CarlWitthoft hah, if only a simple component had such power! – J... Feb 26 '21 at 16:44

7 Answers7

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A piezoelectric buzzer has a built-in oscillator that will make a fixed frequency sound when you apply power - you can pulse that sound by turning the power off and on, but you can't change the frequency.

There are also piezoelectric transducers, where you can drive the thing with an audio frequency and it will produce that frequency sound. With these, you can change the frequency to some extent by changing the driving frequency - but i think most transducers will work best over a fairly narrow range of frequencies.

Peter Bennett
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The frequency response of a normal coil, magnet and cone speaker in a proper enclosure is a fairly flat line of sound levels from a low frequency to a high frequency with a low frequency resonance. But the frequency response of a piezo transducer is a resonance at a fairly high 4kHz and not much else. If you add weight to the disc then the resonant frequency will be lower but the output level will be much less.

A piezo beeper has a 3rd connection for its transistor oscillator input to cause oscillation to occur at the loudest resonant frequency and the size of its enclosure also is selected to resonate at that frequency for a loud beep. Here is the frequency response of a common piezo transducer:

piezo transducer

Audioguru
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  • Thank you Audioguru! Your most informative graph and explanation certainly gave me much data to consider. The piezo I have is just a plain transducer, no internal circuitry. That's why I am using a pulse generator. What I am trying to do is move as much air as possible with a vibrating membrane unit that is extremely light and simple, which rules out heavy magnet/cone speaker of any size. I figure that a low bass sound would be the key. Obviously, these piezo buzzers do not fill the bill. Thanks again for your outstanding answer! – Eddie Feb 25 '21 at 18:32
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If you have a bare piezo transducer which you are driving with your own circuitry, I have a way for you to get a loud, arbitrarily low-frequency sound. (I can't help you if it's a piezo "buzzer" that has its own fixed-frequency driver powered by DC.)

Say you have a piezo disc with a natural resonant frequency of 4000 Hz but you want a tone at 125 Hz. Your desired frequency is in the ratio 1:32 of the natural resonant frequency. Here is what you do:

  1. Make a square wave at 4000 Hz to drive the transducer.
  2. One time in every 32 square waves, inhibit the pulse. You now have a "missing" wave 125 times per second in an otherwise continuous train of square waves at 4 KHz.
  3. Listen to the result. You will hear a loud, raspy 125 Hz tone superimposed over a rather faint 4 KHz tone.

I'm not sure why this works, but it works. Call it psychoacoustics. Interestingly, if you present this sound to someone who is not expecting a high, screechy tone, they often say that they only hear the low tone. People experienced in listening analytically, such as musicians, easily hear both the low and high tones.

For a commercial product that I engineered using this effect, I used a microcontroller running a simple program loop to "skip" one square wave in X to make the tone that I wanted. You could just as easily use a binary counter and a little mickey mouse logic to get the same result if your divisor is a power of 2.

MTA
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  • MTA, yours is a very good method to try, I will certainly pursue that line of construction. It makes sense and it would make a very interesting project. Thanks for your most informative information! – Eddie Feb 26 '21 at 16:37
  • If it helps, you can visualize it as the sum of a 4000Hz square wave @ 1/2 duty cycle and an inverted 125Hz square wave @ 1/64 duty cycle. It's the world's worst PWM. 1 bit; 0% or 50% duty cycle. – TLW Mar 13 '21 at 05:04
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Since you are using a pulse generator I’ll assume that you have bare transducer, rather than something with active circuitry. I’ll further guess that it is the common kind, a brass disk with a layer of PZT bonded to it. If this is what you have, the only way to make it respond loudly at a lower frequency is to lower its mechanical resonant frequency. The vibrational mode shape is similar to a drum head, with the largest motion at the center of the disk. If you add additional mass there the resonant frequency will go down. You could attach a steel nut with epoxy.

10ppb
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  • Yes, 10ppb, it's just a bare bones transducer with a brass disk., that's why I'm using a pulse generator. I see now that the resonant frequency is fixed, so I'll try something else. Thanks! – Eddie Feb 25 '21 at 18:24
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The crystals are tuned to a mechanical resonance. Stimulating them off resonance isn't the easiest.

Scott Seidman
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    It's just a standard piezo diaphragm with a drive-circuit. It's the drive-circuit that sets the frequency. It would be foolish not to take advantage of the diaphragm's resonant frequency, but that's not the same thing. Have a look here for more: http://dangerousprototypes.com/blog/2017/06/01/piezoelectric-buzzer-teardown/ – enhzflep Feb 25 '21 at 06:48
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What I am trying to do is move as much air as possible with a vibrating membrane unit that is extremely light and simple

If you really think you need a vibrating membrane, then the membrane has to be tightly coupled to a resonating chamber. Since the piezo transducers are fairly stiff, there is quite an acoustic impedance mismatch between them and any chamber filled with air, and an intermediate low-viscosity fluid could be used to enlarge the coupling area. This is not particularly light but can be simple: think an Altoids can soldered shut, filled with water or alcohol, with one or more flat transducers soldered to holes on one flat side, with the other flat side coupled to an acoustic resonator.

Another solution to that problem is a (possibly folded) organ pipe (3D printed) with a small fan (1.5-2" diameter should be plenty) as air source. Should work with just 1" of air pressure.

It's not small but it can be fairly light for its size, and it doesn't get much simpler. The "pipe" can be a Helmholz resonator formed from the enclosure of the device itself, as long as the enclosure can be otherwise air-tight. The mouth and fan unit (3d printed with fan bolted in) would be mounted inside, with just the pipe mouth exposed to the outside (plastic bits poking through a hole).

But most importantly: you should decide how loud that sound should be - e.g. by tweaking the volume while playing it through a speaker, then actually measuring the sound level at a fixed distance from the speaker using a sound meter. Then you can objectively decide whether it's "loud enough".

Kuba hasn't forgotten Monica
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Theoretically

I think You have 2 put it inside a morphable chamber. you have to find the chamber volume/size +resonant freq(as it is hand in hand)but make sure it's the frequency that's in the middle of the frequencies you want to traverse between..eg Pitch freq for note C is 440hertz, Note G is 391.995hertz so find the resonant frequency/chamber for freq415.9975(freq1 + freq2 / 2), the chamber must be morphable as freq to chamber volume ratio must be the same, so Im assuming if you you change the pitch/frequency you lose strength/power on the piezo and therefore lose sound volume and so I think we must rely on the feedback of the resonance to improve itself

Voltage Spike
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Kaaneii Williams
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