Impedance Versus Dampening

I just thought I’d share some of my thoughts/musings on how different types of wood and their working properties come into play when making a bowed psaltery.

Dampening: This is a term that I hear thrown around a lot. It is usually used in a bad way: “you don’t want the wood to dampen any of the strings’ vibrations…” and so forth. I would define dampening, in regard to musical instrument woods, as the measure of a wood’s tendency to lose (or conversely, to retain) a musical vibration.

For the sake of example, a material that has a very low amount of dampening would be a tuning fork. You strike it, and it retains the vibrations and gives off a sustained ring. The opposite of this would be a rubber band. You strike it (or fling it!) and the energy is instantly transferred and absorbed by the rubber band.

So, is dampening a bad thing?

Not necessarily.

I would like to mention another term, one that is hardly ever mentioned by luthiers, but I think it is important: impedance. Usually, if a wood has a low dampening factor, it has a high impedance factor. (Stay with me, I’ll explain.) In lutherie, I would define impedance as a measure of the wood’s tendency to impede, or hinder a musical vibration from traveling any further.

Can you see how impedance is almost the exact opposite of dampening? A tuning fork, because it is so good at retaining vibrations, has a high level of impedance. Conversely, a rubber band, because it immediately “consumes” all of its energy, is a perfect example of an item with very low impedance—it does not hinder the energy you put into it, but the rubber band transfers it immediately into action.

In Practice

There are areas of the instrument that would benefit from having low dampening, and there are other areas that would benefit from having a lower amount of impedance.

Basically, softwoods tend to be high dampening and low impedance. (They are lighter, closer to a rubber band than a metal tuning fork, figuratively speaking.) Hardwoods tend to have high impedance and low dampening. (There is a wide variety contained within hardwoods, but overall, I place them closer to the tuning fork, rather than the rubber band.)

To get down to the nitty gritty: the top of a psaltery should have a low amount of impedance. The job of the soundboard is to transfer the vibrations of the strings to the rest of the instrument’s body. Does this softwood dampen some of the vibrations? Yes, to an extent. But overall, softwoods help the vibrations more than they hinder them. The top should be thought of as a pathway – it only passes the vibrations on to the rest of the instrument.

If the entire instrument is built with the intention of totally eliminating any sort of dampening, then it makes it much more difficult for the vibrations of the strings to be transferred to the rest of the instrument. The resulting tone is then very shallow.

The back, however, would benefit most from a wood with a higher impedance (and a lower amount of dampening). That is to say, the back of the instrument is basically the final stop for the strings’ vibrations, so a low impedance is not needed. What is needed (or at least, preferred) is a material that will not immediately muffle out and dampen any vibrations. This is where hardwoods come into play.

Now, what about the sides, and the bridge? This is open to debate. There are a lot of trade-offs between acoustics, aesthetics, and strength. Obviously, a bridge would be a prime example of an area that is only transferring vibrations. However, the bridge is also an area that is probably exposed to more stress than any other part of the whole instrument. Likewise, the sides could be considered as an extension of the top – simply transferring vibrations to the back, or it could be considered as a needed structural element, and/or a part of the back.

Regardless, I hope this helps clear up some of the properties of wood, and how best to optimize the depth and richness of the bowed psaltery’s tone.