Sunday, March 31, 2013

SEOS Project: Finally done (for real this time)

As it turns out, my "final" crossover that I had decided upon from before was no good. The early bass rolloff that I was attributing to measurement error was actually a real problem, and I had to start again ) to try to rectify it. Below is the speaker without accounting for the rolloff in red, and the finished design in teal.

To do that, I started over with a much larger inductor on the low pass filter, which essentially lowered the cuttoff frequency, but since the response was rising for the woofer anyway I was able to end up with about the same crossover frequency as before.

Unfortunately, this decreases the overall sensitivity of the speaker, but I was willing to make that sacrifice for a more balanced frequency response.

Here is the circuit that I came up with for the crossover:

It is a third order low pass on the woofer, with a first order high pass on the tweeter. The capacitor and resistor in parallel on the high pass make up an RC contour filter which adds a rising response to the upper frequencies. This was added to get a little bit more high frequency extension out of the tweeter which rolls off  a little early (15kHz or so) on its own. Finally, the series RLC circuit in parallel with the tweeter is a simple notch that is made to knock down a bump around 4kHz and flatten out the tweeter's response overall.

Below are measurements taken indoors at about four feet away. First is the horizontal off axis performance:

Next is the vertical off axis performance:

The hump at about 500Hz, and the dip at 225Hz both seem to be room related, since they appear in measurements of other speakers as well. I'll take some measurements outside when I get a chance.

Once the crossover design was finalized I ordered the parts, built up the crossovers, and installed them in the cabinets.

Above is the finished crossover, and below are the before and after pictures of the speakers in their final resting place in my current space. I'll put the third one to use once I move to a larger space.

The sound is phenomenal. These speakers don't even break a sweat at levels that will drive me out of the room, and the higher sensitivity means that my amplifier doesn't have to work as hard to do it. The added sensitivity and lower distortion also make them easier to listen to without discomfort at higher levels. Overall I'm very impressed, and glad to be enjoying these speakers that I've been working of for about eight months.

Many thanks to EricH of DIY Sound Group, as well as tuxedocivic and BWaslo of the AVS forums. Without their parts and assistance, this project wouldn't have happened.

Friday, March 8, 2013

SEOS Project: Crossover Design Finished

After a couple months, over a dozen crossover designs (see above), and many hours designing and measuring, I think I've finally come to the conclusion of this project.

When I left you last time, I was convinced that I had a good stopping point because I thought that the problems I was getting at 750-800Hz off axis were just a problem of the room, or some reflection off the wall I was inching toward as I moved the measurement microphone.

It turns out, that was not a problem of the room at all, and was due to some pretty serious phase cancellation off axis, a conclusion I came to after testing out Bwaslo's crossover design and measuring it. While his design didn't end up working for my particular case, it didn't exhibit the same issues in that area despite being placed in the same location in the room.

The low end doesn't look quite the same as the other measurements I've done, but I'm chalking that up to the weird problems I've been having intermittently with my measurement setup. Unfortunately, this design doesn't suit my driver arrangement (because of the z offset of the drivers), which can be seen in the image below, which plots the measured response (in gray) against the modeled crossover. Due to reflections in the room and differing levels the plots don't match up exactly, but they track each other pretty well. 

One thing I did like about this crossover was that it eeked out a little bit of extra extension in the high frequencies, so I set about trying to duplicate that with one of my crossovers that accounted for the acoustic offset of the drivers.

In the above image you can see the result of those efforts. I managed to get some of that extension, but at the expense of exaggerating the hump centered roughly around 6-7kHz. While this design was a failure at the time it may deserve a revisit, since I hadn't properly integrated a circuit that should have helped tame that bump.

After that failed design, I went ahead and tried to tackle the problem I had before with the null at 700-800Hz. Since I knew it was a phase cancellation problem, I played around with the orders of the high pass and low pass filters, to try to get each driver's phase as in line with each other as possible. Below is the model for the design that I finally settled upon, with the phase of the two drivers overlayed on the plot.

As you can see, the drivers stay within about 10-15 degrees of each other until the crossover at which point the tweeter has already taken over completely. This design ended up measuring very well, and despite the fact that it doesn't include the high frequency extension I was hoping for in the other design, I'm very happy with the outcome.

Below is the measured response along the horizontal axis:

Next is the vertical axis. There is a null as you move below the tweeter axis, but it's something I can live with. Ideally I'll have them set up so that the listening position is always on the tweeter axis vertically.

Finally is a comparison of the modeled response (in black), and the actual measured response (in grey).

I'll still do a little playing around with this design before I finalize it, but really all that's left is to order the crossover parts, and build up the boards before closing up the cabinets for good.

Tuesday, February 26, 2013

SEOS Project: Crossover Design

Once the cabinets were completed, I set to designing the crossover, which I soon discovered was going to be a much more challenging task than I initially thought. My naive assumption that the physical crossover would perfectly match the modeled crossover was soon crushed, and I began the sometimes tedious, but mostly fun and interesting, process of designing and testing different crossover networks.

I quickly learned that the room I had available to me was going to present some problems since it is quite small, and not very well suited to the type of measurements I would be doing. With some help, I managed to get my measurements and clean them up in a way that gives me a reasonable idea of what's going on in the crossover region. Doing this required gating of the impulse response of the measurements, effectively removing the effects of signals delayed in time (mostly reflections that don't represent the direct signal from the speaker) at the cost of greatly reducing the frequency resolution of the data, and rendering the low frequency data (everything below about 250 Hz) useless, and unrepresentative of what's actually going on.

I started off with the design mentioned in the last post about the crossover that didn't take into account the acoustic offset derived from my previous measurements:

Omitting a small network on the tweeter's high pass filter, I was left with a fairly simple six component network that I was able to quickly prototype and implement in the finished cabinet. Below is the actual measured crossover from the above design:

As you can see, the actual crossover point is good deal lower than the modeled version, resulting in a more significant dip in the frequency response around the crossover point. On axis, this design doesn't actually look that terrible, despite the shifted crossover frequency, but with such a low crossover, the directivity of the woofer does not match that of the waveguide, resulting in poor off axis performance.

With my less than scientific documentation, I'm not entirely sure which crossover design the following screen captures represent, but I'm fairly certain they are a design very similar to the one shown above, but with the low pass filter altered to raise its cutoff frequency. It is the same kind of design, with a second order high pass on the tweeter, and a second order lowpass on the woofer, and it had similar behavior to the one pictured above, so I'll be using its measurements to show the off axis performance of the crossover.

The traces in the above picture, show the speaker at about 10 degree increments. Notice the large dips that appear at about 750 Hz and 1200 Hz as the microphone position moves further off axis. The null at 750 Hz has been present in other designs, and is possibly a result of reflections in the (less than ideal) room where I'm measuring, but the problem at 1200 Hz seems to be due to poor directivity and phase matching in the crossover region, which is possibly a result of the acoustic offset that I ignored for this particular design.

Even worse is the off axis performance in the vertical direction. When the microphone was placed between the tweeter and woofer, a huge null appeared, and was only slightly less at the woofer level.

The main point that these measurements seemed to suggest was that the acoustic offset of the two drivers is actually an issue, and not just an anomaly of the measurement setup I used to test the raw drivers. From this realization, I designed a very different crossover that took the offset into account, and ended up with better results.

This design uses a very simple first order high pass on the tweeter, and a third order low pass on the woofer, and manages to keep the crossover region flat in spite of the troubles caused by the acoustic offset of the drivers.

Shown above is the new design measured from varying heights. The response does get a little rocky around the crossover region, but it looks nothing like the deep null from the other design. 

Next is the horizontal off axis response of this design. As you can see, the null at 750 Hz is back with a vengeance, but the next plot shows the off axis performance from a different area of the room, which all but removes the problem, leading me to believe it was just a result of the reflections in the room.

Overall I'm happy with the measured performance, and sound of this design, so it will likely be the final choice. However, I would still like to test in a large space (outside preferably) before ordering any parts. Also I need to implement a simple LCR circuit in the tweeter's high pass to knock down the small bump at about 4 kHz and see how that effects everything.

Until I can do those things, my next step is to build Bwaslo's proven crossover and test that so I have something to compare to. This, above anything else, will help show if the acoustic offset is truly a problem, since it was designed without my particular offset in mind and should be flat if the offset doesn't exist. Additionally if it sounds good and measures well, I'll have an option if I decide to give up on this particular design and go with something already completed.

SEOS Project: Cabinets Complete

Around the beginning of the year, I finally completed the cabinets for the speakers, finishing the paint job, and adding some fiberglass insulation to dampen internal reflections.

The finishing was done with a 3/16" nap foam roller, and Rustoleum flat black enamel paint. Two coats were applied, with sanding in between using fine grit (320) sandpaper on a random orbital sander. The cutouts, and tight spaces, especially in the binding post recess, were painted using cheap foam brushes.

Lining the cabinets was the next step. I used standard R13 fiberglass insulation, cut it to size with a box cutter, and peeled off the backing before gluing it to the sides, top, and back of the cabinets.