December 20, 2010

Introducing B&C DE250

B&C DE250 is a great value 1" compression driver.

I've now had a chance to measure this driver and now show a section along with some photos of the parts.

Photos, drawing and description >

I've just ordered a compression driver for my next series of speaker experiments. I have different ideas, but what they all have in common is this well regarded compression driver - B&C DE250.

So what is it? It's a compression driver, which is pro audio speak for a tweeter. In many ways it is like a dome tweeter, but it is designed to be used only with a horn or waveguide.

It has an inverted dome. The replacement diaphragm is shown above and this view is from the rear, with input terminals shown. Most of the unit is it's motor. This one has a 1.7" dome - being much bigger than a hifi dome it has greater power handling. It has a phase plug which directs it's output to a 1" exit tube. This is where the compression part comes in since the diaphragm has 2.8 times the area of the exit. That means it needs less excursion than a direct radiator, and that is before adding a waveguide. The dome has 2.8 times the area of a dome tweeter. To match the efficiency of this driver, a conventional dome tweeter would need to require 64 times the piston area of a 1" dome yet have the same low mass of the small dome and avoid any resonances or breakup that would plague such a large yet light piston. In short, it can't be done with existing methods and materials. 

What about ribbons? They do have a large driver area and high efficiency, but they generally have limited bottom end extension and the dispersion is problemmatic. Horizontally they have the wide dispersion  and vertically they have narrower dispersion but in both cases they vary with frequency. 

A compression driver can achieve constant directivity and this is where the other options can't compete. 

Curious about what is inside?

Looking inside

I have taken 

Now let's look at it's response.
The nice smooth response shown in black was measured at the mouth of an oblate spheroid waveguide (6"). In red you see it's response when used in a strange way - firing up with no waveguide so that it works as an omni source. The ripples are mostly room related and you will see this with any ungated measurement. The blue response shows a more typical measurement at 1m. A little EQ at 4k was used to get it flat and you can see an axial dip around 7 - 12k. This is most likely a dip caused by diffraction at the waveguide mouth, which would explain why it doesn't show up in the nearfield. 

Other compression drivers with a look inside:

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