Temperature changes are very much the enemy of bar percussion tuning!
The situation is worse than the bars and resonators not being affected evenly - they are affected in the opposite direction to each other! An increase in temperature will make the bars go DOWN in pitch, but the resonators go UP in pitch.... therefore it is quite important to fine tune both as close to the final playing room temperature as possible. For example when fine tuning bars I tune them to perfect correct pitch, then place the bars in a temp controlled room - just my lounge room with the air con on set to 21 degrees - and leave them for at least 10 minutes there to equalize. Sanding the bar will create friction and warm it up, so after ten minutes in the room I take it back to the sander and quickly touch tune it again as it will have gone up in pitch above the zero mark a little. I repeat this a couple of times to make sure it ends up correct. I make sure I tune resonators in the same room.
The whole point of having that adjustable height on resonator banks, is to compensate for temperature variations. Sometimes if we play the marimba at higher temperatures it will start to sound a little off - the resonators and bars get out of tune with each other so the system becomes less efficient and we hear a sound that is both less harmonious and less fat sounding. This is a common enough scenario as stages sometimes get warm because of lights etc and studios often just heat up! Warmer temps mean that the resonators get too sharp in pitch for the bars.... bars go down and tubes go up.... so by raising the tubes up a little closer to the bottom of the bars we can actually make those tubes flatten in pitch to better match the bars "high temp" pitch. This is because of a kind of proximity effect. You may have noticed that the pitch of a tube will go down if you partially close the mouth of the tube..... having the tube mouth a little too close to the bar has this same effect... and in this case we are using this effect to our advantage.
Jim
