August 18, 2006
The Kalimba Box Ate My "A" Note

Having played a number of very old Hugh Tracey kalimbas, I have become familiar with a problem that some of them have: one tine becomes dead.

Looking back at last Friday's Tip of the Day, we saw how a glissando of the upper four notes on a treble kalimba (D, F#, A, C) excites those four notes roughly equally, but that after only a tenth of a second, 99% of the power in the "A" note just disappears. I assert that the kalimba box eats the A note. Here is my evidence, taken from the kalimba suffering from the dead "A" note:

The plot below shows the spectrum of the first 0.05 s (i.e., just the attack) of the middle "C" note on the kalimba (523 Hz, which is in the middle of the right side, and is very close to the resonance of the kalimba box). Reminder: any sound you hear is actually made up of many different frequencies. A power spectrum just shows you which frequencies are present in that sound. And it turns out my software had a bug in the x-axis frequency label, so I just cut it off this week.

As this spectrum is taken of the "C" note's attack, it will have lots of high frequencies in it. I've labeled some of the peaks which correspond to the higher kalimba notes: C is the peak that goes off the chart (i.e., lots of power in that frequency), then the E, G, and A. The peaks to the right of the A are harmonics which are higher than the kalimba's highest note. So, even though we didn't play the "A" note, the kalimba system - the box and the tines - is ringing with some A note at 880 Hz. But look at the situation 0.2 s later:

First thing, there is less power in the C (it is no longer off the chart), E, and G. BUT look at the A! The A just fell into a hole! It is down by 50 dB, ie, there is about 0.001% as much power in the A vibration as there was only 0.2 seconds earlier!

Remember, we did not strike the high A, but the middle C, and the A in the attack was just sort of accidentally excited. And then, the kalimba just ate it up. Gone. Nothing left at that frequency!

But don't be sad for that missing A. I know how to get it back! Tune in next week, same Bat time, same Bat web page, to find out!