Hey Colin

The keys will always sound a little 'dead' until you have two things in place (a) a mounting mechanism that will allow it to vibrate freely and (b) something to amplify the sound, i.e. a resonator.

(a) Temporary mounting

Place the bar resting on its nodes on some rope or a couple of part-inflated long balloons or some light sponge, you should hear a clearer note then.

(b) resonators - this is a more complicated element of the build. Basically you require a sealed 'vessel' open at the end directly below the marimba key. This can be anything really as long as it fits under the bar and can be cut to length. Go for the widest vessel that you can physically fit in there as this will maximise volume. Reso length will affect the pitch of the resonator and reso width will affect amplitude (volume) with wider being louder. The reso needs to be really tight to the key 1" - too tight will flatten the resonated note, too far will make the note weak.

There is a formula for calculating the reso length

Ok so if maths isn't your thing, try this... Take a plastic bottle and cut it off on the vertical bit below the shoulder. Place the bottle with the open end beside your ear. Sing a glissando, a sliding note from high note to low note. At certain points you will clearly hear the bottle pick up on your voice and make it significantly louder - that's it resonating the note, each time it does that you have matched either an harmonic or the fundamental. If you place a marimba bar of the same note over the bottle and hit the bar then the magic happens.

To tune the bottle you simply use scissors to trim the plastic away and raise the resonated frequency until it is resonating at the pitch you want. I used this singing technique to build resonators (seen above) at home near Belfast for an instrument that was 500miles away in the Stomp workshop - it worked perfectly.

You will notice on the assembled instrument to the left that the last two resonators are about the same length. In this case, the lower one has had the top closed over slightly. Restricting the size of the opening will also lower the note - I uses this a lot on the bass and baritone marimbas otherwise the resonators would be many feet long. It does have an effect on volume however so you need to work out effective ranges.

Hope this helps

Paul