I purchased a pair of Unity Horn kits from Lambda Acoustics nearly 10 years ago (hard to believe) that got put aside in favor of planar, dipole designs that I have been using for some time. This summer after reading William Cowan experiences with both Planar systems and Unity systems, I decided it was time to complete my Unity project. I had already purchased a used but just re-coned pair of TAD 1603 woofers to mate to the Unity Horn. Learning from Williams past experiences, I decided to built the system as a corner system. Not as elegant as his built in the wall system, but trying to achieve many of the same benefits of corner placement.
Attached it a picture of the rough (no finish yet) speaker in my living room.
I will use the Unity horn by using a DEQX digital processor for crossover and EQ. As soon as I hooked it up and ran some DEQX curves, I found some nasty things going on that were unexpected. The DEQX kept coming back with correction curves with a huge peak at 4.5K. If I by passed the correction curve portion and used it as a straight crossover, things started to sound promising, but I wanted to find the source of the issues with DEQXs correction curves.
I did some measurements. I combined some of the measurements on the attached graphs. I discovered the source of the 4.5K problem with DEQX. On axis, the Unity from Lambda has a sizable dip at that frequency which the DEQX tries to correct. The problem, is that the dip in only on axis and goes away off axis. I did a little web search and found references to the dip being caused by the mid-range holes in the Unity Horn, and a hint that the implementation of the Unity Horn by Lambda, was not exactly what Thomas Danley did on the original. I could not find any definitive references to what was different, but did see other DIYers efforts to experiment with the same issue with other Unity clones. I have also noticed that the radius of the curvature in the “seams” of the horn seem much greater on the Lambda version compared to sharper radius on other I have seen photos of.
Measurements. Attached graphs high frequency and Mid range portions measured separately. FYI they are all 1/10 octave smoothed (to leave detail), and scaled up and down to allow several on a page. Outputs are not absolute.
On the first graph, (Unity DE25.jpg), the Red Line is the stock finished Unity response from the DE25 (image of stock unit holes attached). The 4.5K dip is clearly visible. (I did do off axis measurements that showed that this largely went away off axis but for this discussion, I have not shown them). Next I covered the mid-range ports with metal foil tape. The gray line shows this change. The 4.5K dip is gone and the whole response gets soother. So, I know the ports are the problem. Next I poked holes into the foil - roughly 1/4 in diameter (image attached) and measure again - this is the Green curve. Most of the improvements are maintained. The last curve was to replace the thin foil with a thicker rubber washer with a 1/4 hole drilled in it. There is also a picture of that and the result is the blue curve - which is nearly identical to the hole in foil curve.
So far so good. Now to see what happens to the mid range drivers. The second graphs (Unity Mid.jpg) shows the curves for the mid range. First the red line is the stock Lambda ports. What the red graphs shows is that the stock Unity has elevated response at 300 Hz of nearly 10 dB above the level at around 650 Hz.
I did not do a gray graph with the ports covered as no sound comes out.
The green graph shows the response with the foil with 1/4 holes. The graph is scaled down, but in absolute output, it is within about a dB in the middle of the pass band, and the 300 Hz peak is suppressed a little and moved lower in frequency.
The blue graphs shows the same thing with the rubber washers and again is nearly identical. While the foil and washers now seem on the right track to work, I find both unacceptable in appearance.
So my next step is to find a more refined and permanent way to do this. So I took a piece of birch plywood, drilled holes in it the diameter of those in the unity horn (3/4”)and then filled one with Bondo and the other with Durham’s Rock Hard Water Putty. Then I drilled a 1/4 hole all the way through them, and finally drilled a 5/8” hole from the back side to within about a 1/8” all the way through. This can be seen in the attached photos. So it appears that either of the two material will work, I think I like the Bondo best.
Questions to anyone with knowledge of Unity or Synergy Horns.
1. Does anyone have any knowledge of the preferred geometry of the holes for the mid-range drivers for Unity?
2. Will the 1/4” holes cause any distortion problems (bearing in mind that this is a home installation - not a PA app)? I have no way to measure distortion.
3. Can anyone explain the tuning of the ports? The mid system appears to have double resonant humps at 300 and 1000 with a valley in between. Any suggestions on tuning these - more cavity in the port or less, taper vs cylindrical, etc. I did try filling the original holes with damping material and the pass band response dropped significantly, so I am not inclined to use damping.