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Old 18th November 2004, 12:00 AM   #1
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Default Piezo compression

Are the terms: "piezo" and "compression" mutually exclusive when reffering to HF drivers?
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Old 18th November 2004, 01:05 PM   #2
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No. Compression refers to when the diaphraghm rear is contained within a small sealed chamber, the intention being to raise the resonance of the system close to fc for maximum efficiency and to limit displacement. Piezoelectric is an alternative way of exciting a diaphraghm versus a voice coil. A driver therefore may be both piezo and compression. Or not.
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Old 18th November 2004, 02:53 PM   #3
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Which frequency is fc?
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Old 18th November 2004, 03:34 PM   #4
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Image Dynamics make a Piezo compression driver for use in the car, it doesn't like being drivern too hard but doesn't sound too bad.
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Old 18th November 2004, 06:01 PM   #5
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The term compression in speakers is actually used two ways, iirc.

One refers to the compression driver, the other to dynamic compression - which is the loss of linearity at high outputs.

The piezo tweeter is usually made with what is called a "bender" type piezo element. This is an element that moves by deforming its shape, bending. Sort of like imagining a coin that is flexible being held on the edge and pushed in the middle... but as it flexes, the diameter shifts as well. So, the bender element can not be held in a clamp. What they do is to sandwich the bender between a ring in the back and a small cone affixed to the center of the bender piezo element in the front and either use a compliant element to hold the front in place or else the flex in the cone itself to hold the thing in light compression.

The flexing of the piezo element is then transmitted via the cone to "phase plug" arrangement of sorts and then out through a horn...

There are expander piezo elements that *could* be set up to interface with a dome or in a true compression chamber, but since they are expensive and work best at ultrasonic frequencies I don't think anyone has made such a thing for commercial audio applications. Expanders move in one axis, like a regular speaker diaphragm does.

The way that the inexpensive piezo bender tweeter is made & works has many compromises that result in a rather non-linear speaker with relatively high distortions.

A compression driver is a type of driver - a generic term. It is not linked to a particular driver or means of generating the motion that creates the compression. Generally speaking what a compression driver does is to create a very strong but relatively small motion diaphragm excursion that is coupled via a phase plug (keeping the audio paths equal and in phase) to an acoustic impedance converter (a horn) that takes the very strong small motion and turns it into a large and much weaker motion (the output of the horn) and equalizes it for response over a wide bandwidth (as compared to the direct output of the compression driver it self in free air)



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