Bookshelf Speakers Upgrade Fun & Interesting

I mainly built the new speakers for my office as a woodworking project and didn’t put much effort into designing the crossover or picking the best components. The tweeter, in particular, could have been better but I didn’t actually know that at the time. Rather than go into detail on that here, I made a video showing the problems I discovered when I actually measured the frequency response:

It’s now one year after the original build and I’ve decided that I’d like to improve these. I have three major changes to do:

  • replace the tweeter with a better one
  • redesign the crossover but do it properly this time
  • refinish the cabinets

I rushed the finish to get the build video done and had a problem with the final coat of polyurethane I sprayed on flaking off. And I also don’t like the colour anymore – it’s a bit too red.

Like the ones I originally used, the new tweeters are ones I’ve had for many years. These are better, but they do have a problem that needs to be solved. The recommended crossover frequency is 3000Hz, while the recommended top end on the woofer is around 2600Hz. One interesting way to make a tweeter so that it will cross at a lower frequency is to horn load it. In simple terms, the horn boosts the response at the lower end of the tweeters range allowing the crossover point to be lower.
To test this, I made one quickly from 1/2″ plywood and mounted the new tweeter on the back:

I installed it and measured it and saw it was good, but not quite there. I was pretty sure that it would have to be deeper to get the desired effect. But before going any further with the idea, I did a reality check and mounted it flush and measured that:

Unfortunately I lost all of the measurements I had when I accidentally unplugged the laptop I was using. I hadn’t saved anything and the laptop didn’t have a battery.

Anyway, the results of it being flush mounted were not great. I possibly could have redesigned the crossover to improve it, but by now I had my mind set on making a new horn to test. And that’s what I did:

This one is 3/4″ Baltic birch plywood with a deeper and wider horn that made all the difference.

I stripped out the old crossover, measured both the woofer and tweeter in the box and used that data to design a new crossover. To test it, I just hot melt glued the components in place:

And ran several measurements, both on and off axis:

I tweaked the crossover design four times but finally ended up with results I’m very happy with:

To anyone new to this, that probably doesn’t mean much. But it represents a full day’s work – pulling the drivers out, replacing this or that capacitor, resister or inductor, and the putting it back together again. And testing again. And again…

While that was happening I sanded the finish off of the other box:

I did the same for the first box and sprayed them both with three coats of water based polyurethane. No flaking finish on the speakers this time! And I love the new colour.

Also while all of this was happening I ordered new parts for the new crossovers and assembled those:

If you are going to play around with building speakers, you need a stock of parts on hand to use to design and test the crossover. And then when you have that dialed in, you can order the parts to make the final ones.

Final measurements are taken with the speakers reassembled with the new crossovers. The first is the total impedance:

Matches very closely the projected impedance from the crossover design software Xsim:

Then the frequency response on and off axis:

On axis is red, 20 degrees off horizontal is green and 40 degrees off horizontal is blue. The on axis response drops a few db above 10,000Hz, but evens out very nicely off axis.

I also tested the vertical off axis:

About 10″ up and 10″ below. I’ll be using these in my office to edit video and this response is night and day better than it was before the changes.

As a point of comparison while doing these measurements, I took out a similar size Elac speaker I have and measured that:

Green is on axis, blue is 20 degrees off axis. It’s a very smooth response (the scale is different from the measurement above).

Here’s the crossover design I settled on:

Tweeter has a simple first order single capacitor with an L-pad to bring the output down to match the woofer. The woofer has baffle step compensation, a second order filter and a zobel to counteract the rising impedance.

I made a quick video going through some of this, including making the tweeter horn and refinishing the cabinets:

A closeup of that horn:

And the finished speakers: