26. The skirt
The skirt of the pan has two major acoustic functions: First, its length determines the volume of the resonant cavity of the pan. Second, it keeps ringing after a note is struck, giving the pan a natural reverberant sound. A third possible function may be that it acts as some sort of counter-vibrator for the notes in the outer ring. Some of the notes in the outer ring lose their energy when the side is rested against a firm surface. This implies that there is an exchange of energy between the note and the side and that the side may be important for the sound radiation from the pan. The interaction between the side and the notes is still to be further examined and explained.
The length of the skirt determines how the sound produced by the notes will be coloured on its way out into the air. The sound waves emitted from the pan tend to "creep" around to the backside of the playing surface and cancel the produced air pressure, see fig 26.1. If the surface of a note still is moving upwards when the sound wave reaches around, this will cancel the produced sound pressure.
Fig. 26.1 Acoustical effect of the skirt.
This cancelling effect will be most predominant for the lower notes, as a note is vibrating slower when it is producing a low tone. If the side of the pan is made longer, it will take a longer time for the waves to reach around to the backside. This will move the cancelling effect down to lower frequencies. A physical formula for this tells us that the total length the sound has to travel should be more than the half wave-length of the sound, if no cancelling effect is to occur.
If the side of a tenor is 15 cm, the average length between the front and the backside will be about 80 cm, see fig. 26.1. This means that all notes below 277 Hz (A3) will tend to cancel and therefore be weak in sound. Fortunately, this is below the range of the tenor (lowest note D4).
But what if we were to design a cello pan that goes down to B2 (123 Hz)? The formula tells us that the side would have to be at least 40 cm. The side of a regular cello is about 45 cm, so this is in accordance with the theory. What about a six bass with lowest note B1 (62 Hz)? This gives a side of minimum 100 cm, which is about 10 cm more than the length of the standard steel drum. This indicates that basses may be improved by lengthening the side.
As the resonant effect of the skirt doesn't affect the pitches of the notes, it is possible to make pans of the same model with very different side lengths. The length is chosen according to the desired sound "colour" of the pan, see appendix A. The lower the pitch, the longer the side.
The reverberation effect of the side will be more prevalent the longer the side is. The reverberation will also tend to have a pitch of its own. A long side will have a low pitch and a shorter side will have a higher pitch. But the pitch of the skirt is so low that it will not interact with the notes, just "colour" the sound with a constant low tone, resulting in the typical steel pan timbre.