Through much trial and error, I have learned to build my own cardboard instruments. I enjoy building this way because it is easy and cheap, so I can experiment with new ideas. Below are some descriptions of things I have tried, along with some pictures of them to the right.
I have enjoyed experimenting with 4-string dulcimers. This is because they allow for more complex chord choices, and in some ways, a larger range. All of the 4-string dulcimers I have used have been chromatic as well. There are two main tunings I have been experimenting with lately.
First, DGAD. This is the common DAD tuning with an extra G string as a second "middle" string. This is the tuning I used in my second round of competition at Winfield in 2019. This tuning allows for more notes within a closer range. I found this tuning to be helpful particularly for my arrangement of Rondo Alla Turca. It makes more complex melodic movements slightly easier, and allows for more complex chords. Another cool thing is that it has the usual DAD and DGD tunings built in, so long as you can pick around the extra string.
Second, DADA. (I also have dulcimers with these same intervals but in different keys). This is the usual DAD with another A string a fifth above the melody string. I really like this tuning and have been doing much with it lately. With this new tuning, the total range between open strings is no longer an octave, but an octave and a fifth. Furthermore, I now have a full two octaves within reach, without having to move my hand position at all. I find that the extra string makes busy melodies easier, without sacrificing the chord potential of the middle A string. Of course, this tuning also allows for more complex chords, such as 7th chords and beyond.
Side note: Trying 4-string tunings yourself
If you want to try a 4-string tuning on your dulcimer, there are a few things you need to check. First, you need to check if your dulcimer has 4 tuner heads. Nearly every dulcimer built these days has four. In addition, check if your nut and bridge are set up for 4 equidistant strings. If you want to try DGAD, this is easy! You just need another string, of a similar gauge to the A string, and put it between the bass and melody string.
DADA is more complicated. On a typical dulcimer, the VSL (Vibrating String Length) is too long to accommodate that high A string. The only way to address this is to change keys. You can try AEAE, or BF#BF#. These still have the same intervals between strings, just in a different key. There are two important considerations here. First, you need to find the right gauge so that you have the right tension. Second, you need to check that this gauge will fit in your nut and bridge. This is very important! Many dulcimers are carefully designed for a certain size string, and if you stray too far from this you can cause serious damage. If you are not confident that your dulcimer can handle this tuning, don't try it! You may also want to contact the builder of your dulcimer.
More Than 4 Strings?
One idea I had for a 5-string dulcimer tuning is DGADA. I've built one of these so far, and aside from some final adjustments, (and a body), I think it works! I built it with a fan fretboard. It's hard to learn how to really use all 5 strings effectively, but if I could, there are a lot of possibilities.
A few years ago now, I wanted to build a super dulcimer; that is a 7-string dulcimer. It's tuning is DADADAD. The lowest string is the same as the lowest on a bass dulcimer, up to the highest string on a soprano dulcimer/dulcimette. That is, the open strings span three octaves. I actually build three attempts at this. By the third attempt, I at least had all seven strings on without breaking, though I can hardly claim that it is functional. Mostly the action needs a lot of help. (If any dulcimer builder out there sees this and thinks it's something worth pursuing, well, contact me;))
The main difficulty with this idea is that because of the range of string tunings, a fan fret board must be used. This has been used before on guitars, but I don't think it has ever been done on a mountain dulcimer. It is kind of awkward, and makes designing the bridge difficult.
Is that... Styrofoam?
Yes, when I build my own dulcimers, I use cardboard for the sides and back, and styrofoam for the top. (I have yet to build a fully wood dulcimer). The reason for this? The main book that got me interested in building is Musical Instrument Design, by Bart Hopkins. (You should check it out). It said in there that styrofoam is a great resonator. I figured most dulcimer builders put the softer wood on top, and styrofoam seems softer than cardboard, so I tried it. The volume is similar to a regular cardboard dulcimer, and I think the tone is a bit mellower and more balanced. That said, the shape and surface area of the styrofoam piece, along with the thickness, affect the tone immensely and there are many possibilities.
In January of 2021, my roommate showed me how to play clawhammer style banjo. I really enjoyed this, and I love the sound of the banjo in general. However, I just couldn't get used to using my thumb to pick. One day, I tried fingerpicking somewhat like I do on the dulcimer; without using my thumb. It turned out to work well, and so I've been developing it more. I also made this custom banjo with 7 strings:
I designed this to fit with how I now play the banjo. The most fundamental change is that I moved the drone string to the other side of the fretboard. In clawhammer style, the player hits the drone string with their thumb; but I usually play it with my pinky finger, occasionally ring finger. I also added a second drone string, usually pitched a step above the first drone string, and an extra string, bringing the total to 7 strings. Because the drone strings are on the other side, I had to anchor the ball end of the string in the neck and put the tuners on the other end of the instrument. Oh and by the way, the drumhead is a pop bottle, stapled on, and shrunk with a heat gun:) So far the two tunings I have tried, from low to high, are ADADF#AB (playing in the key of D) and AEAC#EAB (playing in the key of A). Check out my banjo playlist on YouTube to see more of this.
I think that about covers it for the main things I have build. I have many other ideas for other instruments which probably don't fall in the "dulcimer" category, but have yet to build them. I have built one dulcimer in a 19 TET system, but have yet to do much with it. (That means there is nineteen notes per octave instead of twelve). I hope to always be experimenting!