This speaker I made with good success. Its bass extended below 30hz, and had good imaging and soundstage. The wings on the bottom helped it not only stand well, but added to the bottom end of the 18" LDA-18 woofer. I tried 14" wings first and they are what got me on some measurements down to -3dbs at 23hz. Room shaking with only a few watts!!
Crossover:
The LDA-18 18" slot loaded woofer has a 10mH choke. The midband LDA-HA15.1 has a 1.5mH choke on it, but sounded very good without it, as the 10" hole slightly slot loaded it so its upper range sloped off to prevent a 2k peak. The LDA1001 tweeter in addition to an Lpad has a 3.3uF cap.
The above screenshot shows a rough measurement using an iPhone with a measurement app compensating for the microphone. I don't trust the measurements above 10k as they all have a peak there no matter a tweeter or woofer, so it is probably some limitation of the microphone or app.
Room response easily gets below 30hz after the drivers are broken in. It is a good idea to use a decently powerful amplifier with the LDA-18 driver to loosen its suspension to get the lowest frequency, I recommend an amplifier of at least 100w RMS power output. This baffle can get deep bass with 14" wings on the bottom portion. The 70hz peak got bigger with a 9" hole, but not as much when using a 10" on the LDA-HA15.1 driver. This measurement was taken with the above-recommended crossover.
In the above baffles I made a slight variation of rather than two 1.5" struts for the slot loading on the 18" driver I went with six 0.5" struts. Had I to do it all over again I would go with two struts as the six struts made for a lot of work.
Regarding the Spacer, crossover, and slot-loading of the LDA-HA15.1: In the plans above there is shown a spacer between the woofer and the tweeter. We feel this is accomplishing two things. 1) Phase aligning the signal from the woofer and the tweeter at the critical crossover frequencies better. 2) Loading the tweeter a little more smoothing its response. Both are bringing added clarity.
We are not certain exactly(and we are not going to pretend we do, although we have measured/calculated as much as we could think of extensively.) what is happening but 2" here seems to be an ideal amount, less or more and there doesn't seem to be as much benefit. This in conjunction with the removal of the back of the tweeter, making it a dipole tweeter, really helps the clarity of the entire combination.
Using other tweeters like the JBL 2414H-1 and others the effects are not as noticeable. And when slot loading the LDA-HA15.1 driver a little, this seems to accomplish similar effects. So with the 15" woofer slightly slot loaded this also gives a similar effect of clarity in the treble range. But certainly, all of these things are tools to achieve different open baffle sound effects that influence clarity, soundstage, and imaging. So each user may like to experiment with what combination they like best, between the spacer, slot-loading, crossover capacitor, and inductor on the woofer, as they all affect the same goal but differently. What is shown above is striking a nice balance between all the variables.
Update: After lots of experimentation the lower bass of this ring slot driver combination is extended by two things. 1) The fs of the driver used, some drivers can attain a fs of 20hz or lower in the ring slot with a compression disk, and those have a -3dbs point of usually around that on one driver we measured 22hz for the -3dbs down point. 2) The second thing other than the driver is the compression disk for sale on this website, with any driver it helps lower and make stronger the bass below 110hz.
Here is a link to the page.
