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Piezoelectric Speakers

09/25/2012 12:02 PM

I've been reading a little on the subject of very small, flat speakers. The obvious choice for minimum thickness seems to be piezoelectric (within some limited range of response). Apparently, one of the drawbacks comes from trying to produce low frequency sound (less than 150 Hz).

My question is how would one enhance or compensate for the lower frequency issues in a cost effective manner? Would some sort of ultra sonic signal modulation be useful or does the overall energy drop by more than 10dB?

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#1

Re: Piezoelectric Speakers

09/25/2012 1:33 PM

There are better technologies than piezo for flat speakers such as electrostatic (ESL).

The problem with bass is simply one of insufficient diaphragm area to reproduce it. Piezo and electrostatic have the same physical limitation, that is, the linear excision of the diaphragm is relatively small. Therefore, moving the volume of air required for bass requires a huge area of diaphragm to compensate.

As you do that you run into other problems at the opposite end of the spectrum, such as beaming and diffraction.

Martin Logan fixes this problem by using a conventional sub woofer built into the speaker enclosure as shown below.

But wait, there is one other problem with ESL and flat speakers - they need to be located far away from walls and other objects to play well. Flat speakers use both sides of the diaphragm to radiate sound and the rear wave must have enough linear space before the rear wave bounces forward off of a wall. Placement is critical for these types of speakers and this defeats the perception that they can be hung on a wall.

What is the advantage of ESL? Well, listen to a pair of Martin Logans in a good listening environment and you will understand that the clarity is truly excellent. The same does not hold true for piezo, which has an inferior sound that is harsh.

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#2

Re: Piezoelectric Speakers

09/25/2012 1:37 PM

As I understand, poor low frequency response goes with the territory.

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#3

Re: Piezoelectric Speakers

09/25/2012 1:46 PM

I have patents pending concerning the generation of infrasound and low frequencies by means of ultrasound (of all things). Yeah, who would've guessed (some have, actually: they're the Competition) The method is very efficient - as a method - but what needs to be addressed now is how to make the transducers themselves more efficient. Current designs are terrible at converting electrical energy into acoustic energy. Absolutely Terrible! In PZT-type transducers, for example, your energy losses in the material itself are comparable to the acoustic energy emitted. A big part of the problem is the acoustic-impedance mismatch between the transducer and free air. It's huge. Consequently much of the energy is reflected back into your power amplifier. Techniques exist for more efficiently coupling this energy into free air, such as horns or impedance-matching layers, but the latter are fixed-frequency. This is fine for applications such as medical ultrasound or for non-destructive testing, but for broad-band audio they're lousy. Horns are better, but the horn takes up a lot of space compared to the transducer element; space you may not have. I've got some ideas here as well which, sadly, I am not (yet) free to discuss.

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#4
In reply to #3

Re: Piezoelectric Speakers

09/25/2012 2:05 PM

Is it still sound if we can't hear it?

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#5
In reply to #4

Re: Piezoelectric Speakers

09/25/2012 2:12 PM

Oh but you can - quite! - but it's not the ultrasound you hear (else it wouldn't be ultrasound, right? ). Wot's even better is that you're not stuck with having to have a comparably-sized transducer (relative to target wavelength). That's the beauty of it.

Mine go well below 20 Hz, weigh under a pound each and measure roughly....lemme see....6.5" x 8".

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#6
In reply to #5

Re: Piezoelectric Speakers

09/25/2012 2:35 PM

Interesting.

Where were you when I was trying to build electronic tank mufflers. I had to use hugh, heavy $10,000.00USD transducers.

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#7
In reply to #6

Re: Piezoelectric Speakers

09/25/2012 2:53 PM

Right here and not all there, probably!

Besides, you couldn't have done it The Army Way otherwise, right? I mean, what would they have said if you'd showed up with, say, little dinky thingies that fit in a briefcase (including amplifier - okay, for tanks, something a little larger). If it's not 300 pounds and olive-drab or tan camo, they're not interested. You gotta think like the Army, dood!

TEN HUT!!

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#8

Re: Piezoelectric Speakers

09/25/2012 10:44 PM

Ya wanna get high sound pressure level at low audio frequencies?

Try THIS if you can compromise on "thin", or maybe use a belt drive to get the motor out from behind:

http://www.rotarywoofer.com/

Mark Bingham
Relativity PL

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#9
In reply to #8

Re: Piezoelectric Speakers

09/26/2012 12:28 AM

Interesting.

The 'voicecoil' (linear actuator) behind the fan's hub looks to be coupled to a mechanical linear-to-rotary converter mechanism in the hub which varies the blade pitch angle around zero. The pitch angle, in turn, modulates the rate and direction of air-flow through the port. The motor speed itself appears to be constant, harder to say how well it is regulated (if at all) without more info. If the motor speed is not regulated it will vary somewhat with blade pitch angle and introduce distortion. Ideally the assembly should be placed in a nacelle to reduce audible turbulence. Of greater concern is the turbulence behind those blades, especially when the flow reverses direction.

Electromechanical sirens - "air raid" sirens - modulate air flow by means of a 'chopper'. the largest U.S. made siren sported a 180 HP Hemi, 3-stage compressor and produced SPLs of around 136 dB @ 100 ft @ 30 kW (40 HP).

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#10

Re: Piezoelectric Speakers

09/26/2012 1:44 PM

If you want thin with good bass, consider something like patent #4384173 - Planar Electromagnetic. These deliver everything you want except efficiency. (you knew there had to be a catch) However, you can heat your house with the power amplifier.

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#11

Re: Piezoelectric Speakers

10/11/2012 9:47 AM

I've been following a start-up called Edge Motion Technologies (aka Emo Labs) that is developing products based on similar technology. They explain that their technology works better across a wide range of frequencies than traditional speakers at producing a wider range of frequencies from a single radiating membrane. However, they also acknowledge that a sub-woofer may be required for the lowest bass frequencies. Perhaps taking a look at their website might give you some ideas.

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