A means of blowing the reeds outside the instrument is required. This can be done using an old set of bellows attached to a good quality plywood sounding board with two holes cut in it to replicate the reed block base/foundation plate in the instrument.
When the reed blocks are removed from the instrument and placed on the tuning board at least three parameters are changed:-
In the case of 1) (air flow reversal) the small changes of reed frequency response which can occur when it is blown on the tuning board (as opposed to that when blown inside the instrument) MAY be due to variations in reed shape/reed tip height as well as variations of air pressure.
In the case of 2) (un-restrained reed blocks outside the instrument) simple laws of physics tell us that a vibrating reed will tend to try to resonate with the device to which it is fixed. In the case of the accordion the ‘devices’ are the reed block itself and in turn the fixings of the reed block to the foundation plate/accordion body.
Only one of the ‘devices’ to restrain the reed is retained when it has been removed from the instrument. ie. the reed block itself, so that some compensation may have to be made for the removal of the fixings/restraining element. By how much this should be compensated (if at all) would seem to depend on the individual instrument and design/construction of the tuning board. The influence of restraining the block can easily be seen by simple hand pressure on the block. If hand pressure is increased the reed frequency falls and tends towards resonance with the tuning board (which has a much lower resonant frequency than the reed).
Since the reeds vary in size (vibrating mass) it also follows that the dynamic energy generated varies so that a large reed will behave differently to a smaller one.
In the case of 3) (unrestrained air flow) the change can easily be seen by simply cupping the hand over a reed to restrict the air flow (a fact which can be put to use when tuning the reeds. See ‘tuning’ section)
Reed blocks normally contain two opposing reeds which are quite often tuned at slightly different pitches. When they are played together there is a natural tendency for the reeds to try to resonate and the pitches of either pulled up or down by the other. This is an element which is common to both conditions (ie. inside or outside the instrument) but needs to be taken into consideration.
When a reed is blown its frequency will most likely change along with a change of applied air pressure. Quite often the frequency will fall with an increased pressure though this is not always the case. Why some reeds fall while others rise is not clear but the final answer might lie in the reed quality (its material and close fitting within its slot). If a reed’s performance differs greatly from its neighbours it is suggested that its valves, tongue alignment, shape/tip gap and cleanliness, particularly around the root of the tongue, are checked before its behaviour is written off to design.
In any event, tuning would be simpler if all the reed frequencies, in a note set, varied in the same direction and by the same amount with pressure variation. As this is not always the case it is a condition for which allowance in tuning must be made.
The use of the tuning board requires practice to achieve consistency in trying to replicate the conditions inside the instrument and the author incorporated a simple manometer into the tuning board to observe the pressures being applied. Initially an attempt was made to measure the air pressure in a fully assembled instrument and this showed firstly the very low pressure range dealt with (0 to about 0.1 lbs/in2 or 0.007bar) and secondly, how the pressure tends to increase from the low/large reed end towards the high/small reeds. An average pressure range was adopted, based purely on the author’s playing style and ‘attack’ and this range is applied on the tuning board.
Care with the reed blocks on the tuning board will allow the reeds to be taken to or near the required frequencies. However it is likely some fine tuning will be required when the reed blocks have been re-fitted in the instrument.
Two methods to allow blowing of the reeds when fitted in place seem possible:
1) The treble end of the instrument is laid on its bellows and the instrument is blown to listen to the reeds. Where reed adjusment is required the treble end must be removed and turned over to allow access to the reeds for filing etc.
This method works but has disadvantages:-
Repeated placing/removal of the treble end on its bellows degrades the air seal and blowing the instrument without the bellows pins in place can strain the bellows wooden end frame.
2) The 2nd method uses a further set of bellows attached vertically to a firm, fixed base. The upper, open end of the bellows is fitted with a suitable soft air-tight seal where the treble end of the accordion can laid and the bellows operated and a note blown in conditions very close to the instruments own bellows.
The disadvantages noted with method 1 are eliminated.
Methods of reaching the reeds with file etc. when they are fitted in the instrument are suggested in section ‘Reed Pitch Alteration’.