It is felt worthwhile to repeat here a caution given at a previous page ‘Reed Health’.   Reed adjustments made by filing or scraping are liable to be in vain if some basic maintenance and checks are not carried out. ie. the reed set (tip  height), shape, alignment, security, cleanliness, valve condition and register slides should all be in order.

     Tuning, I feel, falls into two categories:
1) Putting right a single note which has become defective
2) Making a complete change (or restoring) a whole instrument.

      The first is probably within the scope of a careful amateur while the second is  a job only for a very experienced person.
 
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                               Basic Pitch

    Accordion basic pitches are tuned using the ETS (equal temperament scale) with note A (A17) set at 440Hz.
(It is noted here that it seems common to tune the basic pitches a few cents high; perhaps this is seen as giving the accordion an edge over other instruments. In any event the basic tune should be close to that of the bass end).
In the ETS scale the 12 semitones of the octave increase in the ratio 1 : 12th root of 2   or   1 : 1.05946
Eg.
If C = 1
C# = 1 x 1.05946 = 1.05946
D=  1.05946 x 1.05946 = 1.1225
D# = 1.1225 x 1.05946 = 1.1892
      etc.
 to the next C = 2  ie. the frequency doubles every octave.

Eg.  If C = 261Hz       C# = 261 x 1.05946 =  276.5Hz

The ETS scale gives an exactly equal division for every semitone, all of which are slightly imperfectly tuned in all keys.   The ETS is a compromise allowing playing in 12 different keys, all slightly imperfect but equal. The very small disparity is not in any way discordant and the ear is now trained to accept it.

                                  Stretch

    I understand some accordion tuners take 'stretch' into account in their initial setting. This is the effect applied to the basic tuning where low notes are lowered below the straight tuned position and higher notes are raised above it to take account of 'imperfections' in the human ear which, apparently, interprets as false, straight tuned notes nearer the ends of the human audio range.
The effect is probably of more significance to piano tuners where the audio range dealt with is much larger (< 7.1/4 octaves)  while a standard 41 keyed piano accordion is only about 3.1/4 octaves. One Steinway piano tuner is noted to favour lowering the lowest note by 18 cents and raising the highest by 27 cents.
On the above mentioned accordion this (according to the piano tuner) relates to the lowest F note being lowered by 0.5 cents and the highest A note raised by 9.5 cents.
The stretch effect (or lack of it) can be heard by listening to some older electronic pianos where all notes are 'straight' tuned and derived from a single set of 12 frequencies.

As with most matters on tuning what sounds good to one person does not to another so that the decision to apply ’stretch’ or raise the basic pitch above the standard A440 is entirely personal.
 

                        Straight Tuned Reeds

    After tuning the 8’ basic pitched reeds it is recommended other ’straight’ tuned reeds (if fitted) are tuned. ie. the 16’ or 4’ reeds are set to the same pitch (+/- 1 octave) as the 8’ reed.
   When two reeds are being tuned to the same pitch it is sometimes difficult to hear if the reed being tuned is above or below the pitch of the target reed especially when they are close. When the reeds are played together and are out of tune a beat (or vibrato) will be heard. This beat needs to be eliminated. While blowing both reeds try covering the reed being tuned by cupping the hand over it (or otherwise restrict the flow of air to/from it). Its pitch will fall. If the beat/vibrato rate falls it means the reed needs to be lowered in pitch and if the rate rises the reed needs to be raised in pitch.
 

                         Tuning Styles and Beats

     No two instruments have exactly the same tuning though it may be true that tunings fall into styles such as Tex-Mex, Irish, East-European/Russian, Italian, French musette and Scottish musette for example. Within these styles there are variations of reed configuration and width ('wet' or 'dry').

     A lot of tunings are derived from creating 'beats' which is the vibrato effect obtained when two reeds are played against each other at slightly different frequencies.
   For instance at Note A17 (approx 440Hz) if one reed is set at 440Hz(0 cents) and the other at 445Hz (approx 20 cents) then the rate of beats (beats/second) will be 5      (445 - 440 = 5)

      Accordions normally have up to five reeds per note and this complicates the tuning process.  I understand one of the most common reed configurations is one with two sets of 8’ reeds and one set of 16’ reeds. This allows a useful mixture of ‘straight’ sounds and simple vibrato/musette .  On the instruments master coupler this reed arrangement might be shown as follows:

The two reeds on the central vertical line are ‘straight’ tuned.

This set-up normally allows 5 different tones:

1)  Single straight  8’ reed.  (Sometimes called ‘clarinet or flute’).
2)  Single 16’ reed. (Sometimes called ‘bassoon’)
3)  Straight 8’ and 16’ reeds (Sometimes called ‘bandoneon’)
4)  The two 8’central reeds (Sometimes called ‘musette’ )
5)  All reeds. (Sometimes called ‘master’)

An approach to tuning the two ‘straight’ tuned reeds has already been given.

The right hand central 8’ reed is commonly set slightly higher so that a beat/vibrato is formed between it and the other two reeds.

It is suggested that the central A, note 17, 440Hz is taken as a starting and reference point in deciding the rate of beat or wetness.

The beats of the notes above and below the reference A 440 are decreased towards the low end and made increasingly fast towards the high end. An even, gradual change of beats is desirable.   Also a faster beat gives a louder sound so that quieter, smaller reeds towards the top end of the instrument can be better balanced in volume to the louder, lower reeds.

So that a gradual, smooth, change of beats across the keyboard is achieved it is recommended a tuning chart is drawn on a piece of graph paper:-

Since the width or wetness and rate of change across the keyboard is endlessly variable it is open to the tuner to interpret what, to him/her, is an attractive sound.

     (Monsieur.T.Benetoux of Paris offers some tuning styles in his book ‘The ins and outs of the accordion’.  See Contacts Page).

     Having chosen a setting for the third, fast reed in the above 3 note example instrument then the reeds are tuned.

    
                             Tuning Example 2

       The above example is of a 2 note musette.  Richer, more complicated sounds are achieved with 3 note musette instruments:-

After having an instrument serviced by an expert tuner a few years ago a tuning graph was plotted. Each individual reed frequency (all 328 of them) was measured and plotted on graph paper. The instrument was a 4 voice, 41 keyed piano type with 3 reeds in the middle and a bassoon reed. The following chart is an approximation of what was found:-