Click on any of the pictures to get really big versions of each.
I have recorded the sound of my tenor drone with varying lengths of the slide. 1950’s Henderson tenor drone, Selbie drone reed. Limits of this study are the resolution of the FFT analysis produced by Audacity using the Hanning window. Below is a graph of the pitch of the tenor drone from the FFT analysis and Audacity “change pitch” function. Agreement is okay/good.
Below are 3 graphs of the FFT analysis. The numbers on the right are how long the exposed tenor drone slide is measured in millimeters, mm. The note (in the bagpipe convention) is given over the lowest pitched tenor drone (longest exposed tenor slide – a yellow/brown colored line). The 2 longest exposed tenor slide recordings have an ‘h’ next to them indicating that hemp is exposed. Note that at E*, the harmonic spectra overlap, looking at spectra beyond the 3rd graph will give you a headache! All the spectra are normalized in such a way that the fundamental has a value of 1 dB. This is because it is ‘impossible’ to ensure that each recording captures the same volume from the drone for each different slide length. This is the only way to draw any conclusions from the changes in amplitudes of the harmonic spectra that accompany changes in tenor drone slide length (proportional to the length of the tuning chamber).
The left-most ‘A’ on the top graph below is the fundamental of the tenor drone.
Remember, dB is a logarithmic scale!
Amended May 5, 2011:
I also tried this on a Naill tenor drone with a Wygent reed and got the same results (it’s not normalized but the effect is easily visible):