4. Procedure
I find it conveniant to keep track of what I am doing with the help of an Excel spreadsheet. Thus, I first go through all the reeds in a pair of reed plates and write down how much each one deviates from the tuning scheme. Then I select those reeds that show significant deviations (1 Hz or more) and adjust them each in turn until the tuning is satisfactory. Finally, I write down the deviations of those reeds that I have retuned.
One reason for the 1-Hz criterion is that this corresponds to the so called just noticeable frequency difference for human hearing up to about 1 kHz (it becomes larger at higher frequencies) [Hall, 1991, p.97]. As noted in Theory of harmonica tuning, 1 Hz corresponds to 4 cent at A4 = 440 Hz but only half as much for each increase with an octave. (At A3 = 220 Hz, 1 Hz is 8 cent; at A5 = 880 Hz it is 2 cent.) Indeed, it is possible to detect smaller frequency differences by carefully listening to beats when playing intervals. For example, an octave mistuned by 1 Hz results in a beating rate of one per second. However, in practice it is difficult to achieve an accuracy better than 1 Hz with the tuning procedure described below.
4.1 Activating the reeds
In the beginning I found it quite difficult to get consistent readings on the tuner. The needle was jumping back and forth and repeating the measurement several times gave different results each time. First one must learn to blow (draw) in a very relaxed way and with a steady air flow. Second, it is important not to bend the notes. This can be quite difficult for certain notes, presumably because of resonance phenomena in the vocal tract. (Personally I have difficulties with notes around Bb5 which easily bend down.)
Another thing I experienced was that the frequency of a reed tended to decrease the more I blewed on it. This is probably due to two things [Epping, 1999a]: First, as the reed gets warmer its resonance frequency drops a little. Second, and most important, extended blowing will make it moist, due to condensation of water from the breath. And the larger effective mass will lower the frequency of the reed (see next section).
The above problem does not occur with the draw reeds. Therefore, I decided to activate the blow reeds also by drawing on them. This can be done with the following simple device. Since most of my harmonicas are from the Hohner MS series, I use a a spare plastic MS comb on which I have put Scotch tape on the complete blow (upper) side. Then I press the reed plate (blow or draw) with my thumbs against the draw (lower) side of the comb with the reeds facing down. It is then an easy matter to activate both draw and blow reeds by drawing in the appropriate hole. I put my thumbs on the reeds next to, and on each side of, the reed I am working on. In this way I can have my mouth quite wide open (important for relaxed playing) and yet only activate one reed at a time.
Of course, if only draw reeds are going to be tuned, it is unnecessary to detach the reed plates since the draw reeds are easily accessible even whith the plates mounted on the comb.
4.2 Tuning the reeds
To change the resonance frequency of a reed one could either remove mass from it (by scraping or grinding) or add mass to it (for example by adding a small drop of solder). An analogy with an oscillating mass at the end of a spring is useful here: The resonance frequency is proportional to the square-root of (k/m) where k is the spring constant (related to its stiffness) and m is the mass attached to the end. Thus, increasing the mass will lower the frequency while a higher frequency will result if the mass is decreased. A harmonica reed responds in a similar way to changes of the mass at the tip (the free end). The stiffness (k) of the reed can also be affected: If ones removes material from the reed close to its root (towards the end where it is attached to the reed plate with a nail), k will decrease and so will the frequency [Baker, 1991, p.55; Oskar, 1996, p.9].
Some of the tools I use
I find it practical to use an electrical rotating grinding tool. There are several types available, for example those manufactured by Dremel or Black & Decker. Mine is a Co-Tech multipurpose tool which is quite cheap (200 SEK) but works satisfactorily. I use it with a small conical grinding stone at its slowest speed (15000 rpm).
A tool like this is very effective and one has to be extremely careful. A slight touch with the tool on the reed to make a mark 1 mm wide and 2 mm long is sufficient to change the frequency of the order of 10 cent.
The reed needs to be supported when one is working on it with a grinding tool. For this I use the thin brass plate that comes with the Lee Oskar Tool Kit (a razor blade or something similar may also do). I insert the plate under the reed to be tuned and over adjacent reeds to protect them from accidental damage.