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Piezo Tweeter Power Ratings

10/20/2008 11:48 PM

Dear CR4 Denizens,

You often see piezo tweeters shown as having power ratings like 75 W or 400 W or something in this range. Piezo tweeters typically have a max input voltage of around 25 VRMS or so. What makes these power ratings seem a bit optimistic (to say the least) is that piezo tweeters have a very high input impedance. A piezo tweeter looks to the amplifier much like a capacitor in the 0.1 μF to 0.8 μF range, more or less. Look at the capacitive reactance over the tweeter's frequency range of, say, 5 kHz to 27 kHz. It ain't 8 ohms, I can tell you!

The combination of high impedance and a typical max voltage of 25 VRMS forestalls any power dissipation figure much more than several hundred mW at most and certainly not something in the 75-400 W range.

So, with that long preamble in hand, here's my question: Are these power ratings referred to an 8-ohm load? If so, why aren't tweeters rated by input voltage rather than input power? 75 W into an 8-ohm load works out to be about 25 VRMS across the load. The power ratings make sense if an implied 8-ohm load in parallel with the tweeter is the configuration the ratings assume.

(Note: A piezo tweeter dissipating 75-400 watts directly is ludicrous. Even assuming a very efficient tweeter - say, 90% of the input energy is converted to sound - the remaining power is converted into waste heat that has nowhere to go. Think of how hot one of those tiny 7½-watt soldering irons get. Now put all that heat inside a small plastic cavity and see what happens. No piezo tweeter even comes close to 90% efficient; we'd have a meltdown in milliseconds.)

No, those power ratings must be referred to some implied load. It's the only way they make any sense. So, is an 8-ohm load the typical reference impedance when spec'ing tweeter input power?

Comments, suggestions (and money) warmly appreciated!

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#1

Re: Piezo Tweeter Power Ratings

10/21/2008 11:04 PM

http://chadgray.info/projects.cfm

http://www.google.ca/search?hl=en&q=%22tweeter+power+rating%22&btnG=Search&meta=

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#3
In reply to #1

Re: Piezo Tweeter Power Ratings

10/22/2008 12:32 AM

Thanks, Aurizon. Forgive me, but these URLs don't seem to lead anywhere when I paste them into my browser. Perhaps if you made them into links...?

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#2

Re: Piezo Tweeter Power Ratings

10/21/2008 11:22 PM

Hi 'Voices',

You raise some very good questions to which there are few very good answers (unfortunately).

Piezo devices are fundamentally capacitive in nature, so 'watts' makes very little sense. A rating more typical with a capacitor would be much more practical, but in audio, there is little that is such.

There are two very 'fuzzy' issues that come into play, which make the 'ratings' to which you refer totally invalid.

(1) Too often, those implying a 'power rating' to limited bandwidth devices (tweeters, mid-ranges, etc.) rate their devices as to their loosely intended uses in a system. In many cases, when a manufacturer says their tweeter is a (for instance) 40-watt device, they are actually saying it is somewhat 'compatible' with a 40-watt system. In music, generally, if a tweeter is to operate in a bandwidth, let's say, from 3kHz to 20kHz, if the system is capable of 100 watts (disregarding the rated impedance), the tweeter in that system with music won't 'see' more than 10 to 15 watts. Now, don't go getting out your calculator on this, because I am just speaking generally. For power testing, there are several test standards (CEA, IEC, JIS, etc.) who publish a shaped pink noise that simulates the distribution of frequencies in music (sorry, but I couldn't find a graphic reference online to link to you).

(2) You assume in your review that the power is somehow compared to an 8-ohm impedance, but that is a bad assumption ... and, again, makes the publication of 'watts' not very realistic. Generally, home audio is rated at 8-ohms impedance, automotive audio is rated at 4-ohms, multi-media is often rated at 6-ohms, and many commercial and professional products are rated at 16-ohms impedance. You can see, given this situation, rating a piezo using an unreferenced power is, as you have surmised, totally inaccurate.

As a final, but somewhat personal comment, why are you considering piezo devices in the first place? Piezos are notoriously poor in acoustic performance. They are cheap, but sometimes you get what you pay for. If you can say more about your application, maybe I can offer some creative suggestions. (oh, and remember, if you DO use a piezo device, you need to use an resistor instead of a capacitor in the crossover design.)

Kind regards ...

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#4
In reply to #2

Re: Piezo Tweeter Power Ratings

10/22/2008 12:50 AM

Thanks, DCad.

You hit it right on the head: fuzzy. I can't seem to find any hard info on this subject at all. I even wrote CTS/Motorola -- supposedly the premier mfr of this type of tweeter -- asking how piezo-tweeter power ratings are formally determined. No response.

But I have seen a number of references to what I presume to be a loudspeaker test standard called EIA RS-426A or something similar. Would a copy of EIA RS-426A be instrumental in answering my questions? Do you know where I could get a copy of this standard? I'm on a pretty tight budget, so if they charge for it I might to defer for a little while.

I'm not using piezo tweeters in a regular loudspeaker application. I'm most interested in the frequency range 17-25 kHz. Piezos seem to be more abundant in this range (and cheaper) than other types of tweeters.

Kind regards,

TV

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#6
In reply to #4

Re: Piezo Tweeter Power Ratings

10/22/2008 1:45 AM

If you are working in the 17k to 25k range specifically, you can probably narrow it down. I assume your application drives specifically that bandwidth with some kind of signal, so output power for you is pretty easy to calculate.

I know it is a PITA (that's code for pain in the arse), but if you can lay hands on a couple of samples, you can pretty easily determine empercally if those devices will work or not for your application. A larger disk will have more voltage resistance, but it will also have a lower frequency cut-off, so you will need to 'mix and match' performance to your requirements.

Regarding RS-426, the revision level is currently 'B', although the difference between 'A' and 'B' is just a minor adjustment in the Frequency Response. Generally, this spec won't help you much unless you are wanting a repeatable signal that simulates music. From what I can tell, your application probably isn't music AND this spec is limited to a bandwidth of 20Hz to 20kHz.

Kind regards ...

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#5
In reply to #2

Re: Piezo Tweeter Power Ratings

10/22/2008 1:14 AM

I did find this article on Web. It speaks of 'blind power' calculations in piezo devices. Curiously, no mention is made anywhere of input power which is dissipated as actual sound! The article speaks of a piezo device as a kind of imperfect capacitor which dissipates energy via a non-trivial ESR, but nowhere is the conversion of electrical energy into sound mentioned. This would constitute a 'loss' or a load on the amplifer as well, wouldn't you think?

I find this whole business of piezo power ratings very mysterious, but I want to get to the bottom of it. Anything you can do to cut through this 'specsmanship' fog will be greatly appreciated!

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#8
In reply to #2

Re: Piezo Tweeter Power Ratings

10/22/2008 9:12 AM

Good answer. I've been building speakers for a while now, and didn't know all of that. I didn't even know why tweeters are rated like they are, I've often wondered how manufactures say crappy little ones are rated for more than big ones. Of course my latest systems have only had ribbon tweeters for quality, but the more you know!

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#9
In reply to #8

Re: Piezo Tweeter Power Ratings

10/22/2008 10:43 AM

Thanks, Nick ... I've been in the industy for nearly 38 years (sometimes it scares me when I think about it ), and there's always something new to learn. If I stop learning, well I've probably stopped breathing

Kind regards ...

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#10
In reply to #9

Re: Piezo Tweeter Power Ratings

10/24/2008 1:03 PM

You've been in this industry for 38 years? Judging from your apparent youth (see avatar), my guess is you started slinging electrons as a toddler! (Not to worry: Carl Frederich Gauss balanced his father's books at age six.)

At any rate, I tested several tweeters using a sine-wave signal. My test setup consisted of a 50-ohm resistor in series with the tweeter, and I measured the voltage across the tweeter and the current through the resistor, noting the phase shift between them. As you know, power computations in AC circuits aren't as straightforward as with DC circuits if the circuit contains reactance. I computed the power consumed by the tweeter as follows:

P = Vrms·Irms·cos θ,

where θ is the phase shift between the voltage and current.

My power amp is rated at 2080 watts into a 4-ohm load and is flat out to 50 kHz. I didn't run the amp at this power level, of course! But I did need a constant-voltage source with a very low output impedance.

Judging from what I measured, the tweeters consumed 619 mW average with 23.0 Vrms at the tweeter terminals. SPL at 1 meter was 118 dB, avg, with a 4.72 kHz input signal at which they produced the loudest sound. Their performance at 27 kHz - at which they are shown as producing an SPL (@ 1W @ 1m) around 100 dB (!!!) - is positively dismal. As my test mic is pretty flat out to 32 kHz, I know the mic wasn't the cause of this apparent lack of performance.

These tweeters are advertised as having a 'sensitivity' of 94 dB at 1 W input at 1 meter. (One watt consumed by what load? Again they don't say.) Nor do they state at what frequency this sensitivity figure is determined. If it's 1 kHz, they're not going to get 94 dB out of these no matter how hard they pray. Piezo tweeters perform best above their self-resonant frequency which, for these, is around 2.65 kHz.

My general impression is that loudspeaker specs seem to consist largely of smoke and mirrors. Is that the general consensus around here as well?

At any rate, thanks to all of you for your help. Especially yours, DCad. Hope to hear from you guys again!

Kindest Regards,

TV

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#11
In reply to #10

Re: Piezo Tweeter Power Ratings

10/24/2008 9:37 PM

Thanks for your comment's but age is age, regardless of appearances (you all are SO LUCKY not to see the just-out-of-bed-in-the-morning-in-front-of-the-bathroom-mirror photos ) ... I'm 56, so yes, I did start a little earlier than some.

Regarding tweeters and specs and such, the manufacturer should have published a frequency response to you, and hopefully it wasn't with too much smoothing, and they should have published the process of measuring beyond the basics of "1W-1M". High frequencies don't 'couple' well with the air, so how it is 'loaded' (phasing plugs, horns, wave-guides, etc.) is extremely important. I can't really tell you much here, but HOW the tweeter diaphragm is mounted is very critical. If you are planning to operate at a discrete frequency, it gets easier because you can specifically load the diaphragm to actually have gain at that point.

Also, it's pretty important to only use a HF device of any kind well above the Fs frequency. At least one octave higher if using a 12dB/octave crossover. At resonance, the mechanical stress is high, and you will experience mechanical failures (and a lot more distortion).

When you can, let me know your application ... sounds like fun .

Kind regards ...

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#13
In reply to #11

Re: Piezo Tweeter Power Ratings

10/25/2008 7:11 PM

"When you can, let me know your application ... sounds like fun."

Hi DCaD:

I'm using the tweeters in a linear phased array. In the pix below, the T-handles on the back of the array are mechanical phase adjustments that allow me to experiment with beam steering and divergence. Except for the PVC pipe, the array is made entirely from scrap-yard materials which I machined. (The cross-piece in each T-handle, for instance, is made from a section of a stainless-steel rung out of a junked commercial refrigerator.) I'm interested in the 17 to 25 kHz range because the shorter wavelengths reduce both the beam divergence and the distance I must move the phase-adjustment screws - a real PITA after awhile. I'd use DIY electronic phase adjustments, but I can't afford the parts at this time.

<I'll upload the pix in a later post. CR4's servers just seem to hang when I try to submit the pix. They don't even acknowledge the request.>

Kind regards,

TV

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#14
In reply to #11

Re: Piezo Tweeter Power Ratings

10/26/2008 12:04 AM

Still unable to upload images to CR4.

The best coupling between the tweeter and air is an exponential horn, and not the sections of pipe you see in the pix. The pipe is useful, however, because the acoustic impedance mismatch causes reflections which allow me to measure the speed of sound via the transit time of a 'click' out to the end of the tube and back to the tweeter, which acts like a microphone.

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#12
In reply to #2

Re: Piezo Tweeter Power Ratings

10/24/2008 10:12 PM

Piezoelectric materials change shape with applied EMF and tweeters are coupled to the air so the change in shape does work = tweets.

so if they are 10 Khz, 8 ohms, 50 watts(say) power = 50 = I x I x R = E x I

or I x I = 50/8 and I = 2.5 amps. and thus the RMS voltage would be I x R = 20 volts

To measure this you would need an oscilloscope and a non inductive resistor of say 1/4 ohm in series with the tweeter and it would develop 1/32 of the 20 VAC if the current, watts etc are as you say.

I am not sure how efficient these tweeters are, they might be 1% efficient = .5 watts of sound in the air, but I think the whole speaker business is full of low efficiencies something like this. The horn shapes couple the free air to the moving parts to try to maximize efficiency.

http://www.google.ca/search?hl=en&q=speaker+%2Befficiency&btnG=Google+Search&meta=

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#7

Re: Piezo Tweeter Power Ratings

10/22/2008 5:44 AM

I believe the correct way to determine driver power handling is to do distortion profiling (typically, a driver will distort before it runs into thermal or mechanical limits*).

I would start with specifying how much distortion is acceptable for the particular system, then test at what input voltages (vs frequency) the driver reaches those distortion levels. A driver with electromagnet motor will typically run into mechanically-related distortion at the lower frequencies first (excursion limited). I don't know the distortion characteristics of piezos.

See here tweetermishmashfor some tests (no piezos, though).

*Perhaps for a peizo driver input voltage would also be a consideration.

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#15
In reply to #7

Re: Piezo Tweeter Power Ratings

10/27/2008 11:53 AM

Dear Shonver:

Tweetermishmash is a fabulous site - piezo or not. How refreshing to have a resource like John "Zaph" Krutke's website! There are so many "mismash" sites parroting each other's "mishinformation" about loudspeaker testing. It's tiresome to wade through them all. John's website is a joy to behold.

Thank you so very much for sharing this link.

Kindest regards,

TV

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#16
In reply to #7

Re: Piezo Tweeter Power Ratings

10/27/2008 9:20 PM

Sir, the work on this website is truly great ... actually more than I have seen from some manufacturers.

Nice to see that someone is keeping everyone "honest".

One other issue (rarely discussed) that becomes important regarding total 'performance' is power compression. Although it's a good (and typical) benchmark to show low-power response and distortion, it's rarely how anyone listens. Doing the same tests at 10X increments can be very revealing.

Thanks for the Good Answer, and the great link.

Kind regards ...

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#17
In reply to #16

Re: Piezo Tweeter Power Ratings

10/28/2008 4:31 AM

No problem getting to your site and seeing your pics. Just buried lately in visitors and little time to do other 'fun' things. (actually, very little is blocked, but much is 'viewed', so sometimes the speed is slow.

I hope to write more later on your device ... feel free to remind me if necessary ... sometimes I 'forget' when my work gets too deep.

If you like, you can also find me at dennis@sonavox.com ...

Talk more later ...

______________Dennis

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