Piezo VS voice coil tweeters
Voice coils, with their endless crossover problems, must be superior to piezo-driven tweeters, since most, if not all of you use them. Is their non-linearity and "scratchy high frequencies" their only downfalls?
Apart from not needing a xover, are there any advantages to using piezo tweeters? How is development of piezo's going? Are they getting better? Will they ever reach close to a par with voice coils in tweeter applications?
I saw a "MIDRANGE" piezo horn in a Wes Components catalogue recently. I don't have the catalogue with me now, but as i remember, it had a frequency response of about 500Hz to 2 or 5 kHz. Is that abnormal for a piezo? What would you expect in terms of performance from such a horn?
Re: Piezo VS voice coil tweeters
A piezo absolutly NEEDs an XO. Jon Risch (somewhere on AA or at his site -- URL not at hand) outlines how to best approach the XO. If you are using the cheap horns, they need external damping (with something like ductseal), the cones benefit from a little treatment (damar, puzzlecoat, or a layer of aluminum art foil), and the internal back chamber needs a little damping. The paper cone can be removed from the horn entirely and used with some loss of effiency (and the horns colorations) -- what PHY-HP uses on their VERY expensive 12" is, from the outside, indistinguishable from the paper cone i have pulled out of Motorola piezo horns. And then there is Audax's big-buck ovoid shaped piezo dome that by all accounts is stunning.
I have used piezos, and Motorola sent me a packet of frequency response charts years ago. They also sent an Application Note with advice about crossovers.
Many of these piezos have a nice response except for one or two nasty peaks that need taming. You can tame peaks on a magnetic speaker with inductor/capacitor parallel networks, but piezos are different.
I have searched, but I am surprised that there isn't some sort of online Piezo Fan Club where more competent electronics hobbyists than myself have worked these networks out, and with other ideas on how to use these nifty drivers.
Piezos seem to have a lot going for them. Pity there isn't more interest in them.
Risch copy & paste
(cut and paste, most of the below information credited to JON RISCH)
The Parts Express catalog suggests putting a 20-Ohm resister inline with any Piezo tweeter to make it a more stable load for an amp.Will this not also attenuate the tweeter? If so, and if I need further attenuation, can I simply add more resistance? Is there a rule of thumb for how much attenuation I will get with further resistance, or a way to compute this number?
The recommended resistor is to help protect the amplifier from oscillating due to the raw capacitance that is a piezo driver. Adding resistance in series with a piezo will actually roll off the highs a bit, adding more will roll off the highs noticably. To attenuate a piezo, add a series cap, which creates a voltage divider with the capacitance that is the piezo drive element. Most piezo elements run in the 0.1 to 0.26
uF range, so a cap of the same value as the piezo element will attenuate it 6 dB.
Piezo's can be crossed over, and to great advantage. I have often thought that some of the bad rap piezo drivers have is due to the "you can use them without a crossover" fallacy. Yes, you _can_ use them without a crossover, but just because you can get away with it, does not mean it is optimal.
Since most piezo's are used in inexpensive systems, the cost of adding in "unecessary" components is often never even considered.
How to crossover a piezo:
Add a resistor in parallel, and the driver can be made to look like a current driven device to any outside components, such as a crossover cap. However, to keep costs and power dissipation down, 8 ohms is way too small of a value. The impedance of most piezo's is still quite high at 20KHz, so use a 22 ohm resistor, this makes any series crossover cap smaller and less expensive, and the resistor dissipates less energy. Use of an 8 ohm parallel resistor will also tend to lose you a little bit of output level.
For most piezos, use of a 22 ohm resistor, and a 4-4.7 uF cap will allow the response to be identical to what it was in stock form, but rolls off the lows at 6 dB/oct below 1 kHz or so. This actually increases the power handling of the piezo, as it is voltage limited. Exceed the voltage used to pole (polarize the piezo element during manufacture) the unit, and it will loose sensitivity, and eventually burn out. Most pro grade piezos will handle 35 volt transients, and 28 volts continuous, which are 150 watts and 100 watts into 8 ohms respectively.
Add in a capacitor and 22 ohm resistor, and the power handling could effectively be quadrupled, as the LF voltages are not imposed upon the unit, just the HF voltages.
Piezo's crossed over in this manner don't sound as harsh and spity, and tend to be quite a bit more reliable. Many of the piezo units have a mild peak just before they roll off in the LF, so making the series cap a little smaller can actualy flatten response, and provide even more protection and smoother sound. For the smaller piezo units that cut off at 4-5 kHz, a series cap of 1.5 uF will do the trick, larger units that go down to 3 kHz can use a 2.2 uF, and the large compression driver units meant to be mounted on a horn need about 5 uF, as they do not peak, and any higher would lose the sloping output even more.
Attenuation, HF roll-off AND the crossing over can all be done at the same time. To attenuate, place a cap in between the piezo and the 22 ohm resistor that is shunting across the unit, then if HF roll-off is desired, use a series resistor in this location too. Then the series crossover cap should be in front of the 22 ohm shunt.
Looking from the amp, first the series crossover cap, say 4 uF, then the 22 ohm shunt from hot to ground, then a series cap of about 0.15 uF for 6 dB attenuation, and then a series resistor of about 30-50 ohms to tame the very top end, then the piezo itself.
Thank you, Dave, that was very helpful. I will do some experimenting with those guidelines soon. Also, thank you for your T-Line site.
I still wonder if you could use networks using an inductor, to tame specific peaks. Maybe by using a parallel resistor? Hmm.
LOL, as good as this information is, I still think that piezos deserve their own Fan Club. :)
Just came across this. Wonder what is down the road for piezoelectrics.
NASA ROLLS OUT AWARD-WINNING "THUNDER"
A NASA technology that could make everything from
audio speakers to heart pumps smaller and more efficient has
been recognized as one of the 100 most significant
technological advancements of the past year.
Dubbed THUNDER, for Thin-Layer Composite-Unimorph
Piezoelectric Driver and Sensor, its potential applications
could be applied in electronics, optics, jitter (irregular
motion) suppression, noise cancellation, pumps, valves and a
variety of other fields.
Researchers at NASA's Langley Research Center,
Hampton, VA, have taken advantage of a well-known phenomenon
exhibited by piezoelectric materials. These materials
generate mechanical movement when subjected to an electric
current, as in a speaker or actuator, and generate electrical
charge in response to mechanical stress, as in certain types
The Langley researchers, a multi-disciplinary
materials integration team, have succeeded in developing and
demonstrating a piezoelectric material that is superior to
commercially available piezoelectric materials in several
significant ways. It is tougher, more durable, allows lower
voltage operation, has greater mechanical load capacity, can
be easily produced at a relatively low cost and lends itself
well to mass production.
The first generation of THUNDER devices are being
fabricated in the lab by building up layers of commercially
available ceramic wafers. The layers are bonded using a
Langley-developed polymer adhesive. The process results in a
prestressed device with significantly improved performance.
In addition, the process is controllable and results in
highly uniform pieces of hardware.
In the ideal fabrication process -- which researchers believe
is technically feasible -- piezoelectric ceramic materials
would be ground to a powder, processed and blended with an
adhesive before being pressed, molded or extruded into wafer
form. The result would be increased ability to tailor
properties, more flexibility in choosing methods of
manufacturing and increased amenability toward mass
THUNDER wafers could be any practical size from areas
of a few square millimeters to several square meters and
thicknesses of fractional millimeters to several millimeters.
Memoranda of agreements have been signed with six
companies to develop THUNDER technologies and related
commercial products. Approximately 15-20 more companies are
THUNDER technology will be honored by Research and
Development magazine at the annual "R&D 100 Awards" banquet
October 14 in Philadelphia, PA. The R&D 100 Award, formerly
IR-100, is presented annually by the magazine to the
innovators of the 100 most technologically significant new
products of the year. Selections are made by a panel of
scientists and engineers after studying new technologies from
around the globe. Langley has competed in the program for 27
years and has earned 31 awards.
LOL, the first letters of "Thin-Layer Composite-Unimorph
Piezoelectric Driver and Sensor" don't spell THUNDER, but who's complaining?
PS: Looks like someone is doing something with it. Apparently this material has 10 times the "wiggle ability" of present products, which I believe is lead zirconate and titanate.
All for the Piezo Fan Club
I'd be in on a piezo fan club! I personally think they're great, which is one of the reasons i want to incorproate one into my first speaker project.
That development by NASA sounds good. I wonder when it will reach the commercial audio market....
Thanks Dave for that information. Sounds good. I think i'll use some of your suggestions. As i am currently doing a course in Electrical Engineering (first year), i may soon be able to design my own low-cost crossover to use, too.
more info on Piezo's
Here's a link for some more info on piezo's in case anyone else is interested in trying them out in a project:
Will our "fan club" grow???
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