Ultracurve

B&C DE250 1" compression driver - looking inside

Previously I introduced the B&C DE250 as a great value good sounding compression driver. I have since had more time to use this driver in various projects, and I've also taken it apart and measured its dimensions carefully to understand how to design a waveguide for it as part of my point source horn project.

This section shows what I was able to measure. I didn't have access to everything, so there is some guesswork with the phase plug and pole. 

Most of this is just the standard compression driver design that has mostly remain unchanged for a very long time. The phase plug has concentric slots and a conical exit tube. I estimate a 12 degree included angle on the exit tube. It was difficult to measure accurately and I believe Earl Geddes has stated that it is in fact 6.5 degrees to the normal (13 degree included angle). The phase plug (shown in purple) is a one piece cast plastic unit. There is a gap at the end that is significant and should be filled when mounted. 

This rear cap comes off when the diaphragm needs replacing:

It appears to be cast aluminium and there is a small amount of foam. Tweakers might like to add some more material here, although it's difficult to get much in before crowding the small space. For critical applications I'd like to see a much bigger chamber with a lot more foam.

Diaphragm front and rear view. It's a 44mm dome with a copper voice coil and flat ribbon lead in wires. This is a second hand unit, the usual spring terminals have been replaced with a soldered cable. The previous owner apparently has a tendency to tweak everything. 

 You can see it's seen some action already. You can also see the gap around the exit tube (right). You can see the concentric slots on the phase plug (left). Then you can also see the VC gap.

Overall, this driver could have been made better. The diaphragm lacks a spigot to centre the VC so there is no way to be sure that you have it positioned correctly. This could well mean a bigger than necessary VC gap to allow for this. Far better to simply design it to centre the VC so that you can't get it wrong. The exit tube gap is quite poor as well. Despite these complaints, it's a very good sounding driver and I put that down to the polyimide diaphragm which is well damped and the fact that overall it's well designed in terms of its acoustic performance. The response is smooth and flat and I'm yet to hear a better sounding compression driver.

See also

BMS 4550
Faital pro HF10TX

Eminence Magnum 18LF

Eminence Magnum 18LF. This driver is now discontinued and it is unfortunate because Eminence no longer appear to offer woofers with shorting rings in the motor like this one. It has a phase plug of sorts behind a wire mesh dust cap. It has ventilation holes and appears to serve as a means of cooling the motor.

Raw response measured nearfield:

You can see the impact of the high inductance above 700 Hz where it is causing roll off. The response above 1.5k appears to be influenced by breakup modes. In the range of frequencies of interest for a woofer it is nice and flat. Compared to the Peavey Black Widow that I was using previously, it has a more extended midrange response. The Peavey started rolling off earlier despite having a lower inductance. The sensitivity is slightly lower at 95 dB. 

Integrated into my point source horn prototype:

The woofer on top of the box is for show - it's a very serious looking driver!

I haven't had a chance to integrate everything properly, the woofer is too far away from the horn to work as it should, but the first impression is that these will do very well. 

Recommended REW settings

When REW waterfall and decay plots are posted online, often the settings make the charts difficult to read. Here are some settings I recommend for easy to read results.

2D decay plots:

Please note: if you experience problems with generating 2D plots, click on "apply default settings" and then change the slice interval back to 30 ms. The intervals aren't important except that it gives the option to see the decay at 150 ms, which is a reference point used in my bass integration guide. 

Waterfall plots:

The settings on the left for X, Y & Z give a perspective that works quite well. On the right, the time range and window settings are related to the recommendations in my bass integration guide articles.

The graph limits are also important. 

The left and right set the x axis. Set the left for the low frequency limit, depending on how low your system extends. For the upper (right) limit, 200 Hz is a typical value for looking at the bass range. Top and bottom set the Y axis limits. Set the top to just above the highest level measured. Set the bottom to exclude the noise floor. You can see when this has not been done because at a certain point, decay no longer occurs. 

Point source horn - building my prototype

This is my first attempt at building a point source horn. 

previous construction photos >

Fitting the parts together. This isn't as easy as you think! They don't all sit flat, so one pair of sides had to be chocked up with wedges before I could drill.

Assembled:

You can see the edges aren't quite perfect, but it was close enough. Each of the four parts had three edges at different angles. Only the front edge could be left square. 

The corners required some figuring out. The pencil marks show my first idea about how to do it, until I realised it would be better to do it differently. I cut it off square further back.

At this point, it hasn't been glued together. I took it apart and used a 13mm round over bit on the front edge. Then the pieces were glued together. The trick was to stop the round over early. This is the result.

Fixed in place, I then cut off some of what remains with a hand saw, then smooth things off with a hand file.

The threads that came with the DE250 aren't long enough, so I had to buy some and then cut them to length. The head of the bolt is under the flange and a cut away had to be made so I could screw it on with a spanner. The bolt head would otherwise hit the wall of the horn. 

I cut the port holes with a spade bit prior to fixing the pieces together, so I could do it with a drill press. It isn't obvious at first, but for the ports to come in at the right place, they have to be drilled through the side walls as shown. It was a bit tricky. A more accurate way to do it would be with a drill press, but I would need to make a rig to hold the horn at the right angle. This is one of those things that turns out to be more difficult than expected. 

Side view with drivers installed. A simple base allows it to sit on top of my subs securely. 

So many drivers, so little space:

Drivers are B&C DE250 and Pyle PDMR5.

Point source horn - simulations vs measurements

How accurate is Hornresp for simulating a point source horn. When I mentioned it to David Mcbean, his comment was that it's pushing the envelope. 

Black: simulated

Red: measured near field

As you can see, hornresp did a fair job. The dip around 1.6k is shown higher and above that there are probably some complex behaviours you can't expect hornresp to predict, but overall it's not bad. The overall trend is correct. 

Now here is the midrange response, shown raw (grey with the hump at 320 Hz), after EQ (red) and with filters in place (blue).

Adding plastic to bass traps

It's been said that restive broadband bass traps can improve with the addition of plastic film sandwiched between layers of rigid fibreglass. Here is a test to demonstrate. In this case, a low profile bass trap was used over the front wall, 0.3m out from the wall. Compared is the baseline (no traps - grey), raw traps (blue) and traps with plastic added (red).

These measurements were taken by Mellow Fellow and posted on the StereoNET forum. You can see the biggest improvement comes from adding the traps, but the film also increases the damping effect slightly. 

S1 point source horn now up and running

S1 point source horn is now up and running. About to be set up:

The mouth is around 540 x 540mm.

More pics, measurements and comments to follow.



Nearfield measurements shown above, including one broad notch filter on the mid and some shelving for CD roll off. With the bandpass, only the front port is shown. 

B&C 8PS21 pro high sensitivity 8" midwoofer

The B&C 8PS21 is a high sensitivity pro 8" midwoofer. This driver is used by Earl Geddes in his smallest speaker kit, the Harper and it represents very good value. It was measured by John K (Zaph Audio) and compared to some other 8" midwoofers. The distortion performance is comparable to a Scan Speak woofer at 3.5 times the price.

 For a pro midwoofer, it's not bad looking! I like the inverted dust cap and it has a cast frame and a more attractive mounting plate than most pro woofers.

It has a high fs that limits it's bottom end to about 80 Hz in a vented box - this is the price paid for sensitivity. It is around 94 dB 1W1m and has a decent 5mm xmax.

As you can see, the response is quite smooth. Measured nearfield bafflestep compensation but no high or low pass filters. I would say it could work up to about 2k with a fairly steep filter so it does require a tweeter that can handle a low crossover point. Here the value of a compression driver is seen, allowing flexibility in choosing the crossover point. The crossover can be chosen based on getting a polar match, anywhere from 1.5 - 2k.

My initial listening sessions with this in the system is that I like the sound of this driver. It's very clean and neutral and punches well above its price point when compared to hifi drivers. The clarity is immediately apparent.

Pros

• High sensitivity
• Decent xmax
• Suited to small boxes
• Well designed driver
• Value

Cons

• Fit and finish could be better
• Requires a vent to get down low enough to mate to a subwoofer
• Top end limited to 2k
• Octagonal faceplate is a pest to flush mount

The casting on the frame is a little rough in places. This means that when flush mounting, a few parts stick out and a tight neat fit won't happen. Some of the paint was also damaged in transit and it appears to be one area where corners were cut. I'm tempted to attack it with a metal file. 

Polar measurement rig

I'm putting together a measurement rig to take polar plots.

 

It will feature one of these stands which extends to 2.2m. I'll then build a turntable with set steps in angle.

Waveguide surrounds - drivers installed

Drivers are now installed:

Cutting the driver recess:

Template in place:

S1 point source horn - update

The compression driver needs a mounting flange, so I decided to turn it up and do it neat, using some ply scraps.

The hole flares out and is the start of the horn.

Now all the pieces are cut, firstly with square edges. Most of the time was spent getting these angles right, although the horn is 60 degree included angle, the actual angle of the cuts isn't 30 degrees. It took some time to work out what they actually should be. 

The sides need a precise angle and this was done with the "buzzer," a plane with a side fence that can be angled. The buzzer like all power tools is a very dangerous piece of equipment. My poppa chopped off part of his index finger many years ago. When I heard about his accident as a little boy, it inspired caution with power tools. 

In case you are wondering, that isn't me. That's my technical consultant aka Dad. There is nothing quite like a father-son project. 

Testing out the angles:

All pieces now cut with angles:

Holes have also been cut (ports for the mids). 

You can see the holes are quite tricky, because of where they must be placed to come to the corner on the inside. I had to cut these with a spade bit, and actually modify it because none of them were the right diameter. I filed a 19mm bit back to suit with a metal file. 

Here you can see the hole size relative to the driver.

Next step is checking everything, then glue and screw.