Sound

Further information:
     

The musical tones of a piano are produced by felt covered hammers striking steel wires (strings) when a key is pressed.  This is achieved by the conversion of the motion by the pianist through the complicated mechanism (called the Action) to the hammer (which forms part of the Action).              These strings lie over a wooden bridge which is fixed to the soundboard.  They exert a small downward pressure on the bridge, thereby transferring the sound through to the soundboard which amplifies it. The highest note in the treble has a string which is about 2 inches long irrespective of the size of piano and this gives a pitch of about 3,600 hertz (vibrations per second).

There are three ways of affecting the pitch of a string: tension of string, length of string, thickness of string and these are used in a combination to provide all of the different note pitches on the piano.  It is desirable for instrument stability to make all of the strings have approximately the same tension and so in practice the pitch of each note is varied by a combination of string length and thickness. If the pitch was varied with string length alone, then the extreme bass note would have a string 21 foot six inches long.  This would clearly be impractical and so the strings are made as long as the piano will allow and at the same time progressively thicker so that down in the bass section they are covered with copper so that they are thicker without being too stiff.  The combined tension of all of the strings exerts about 18 tons on the frame of the piano.

Tuning

The thickness and lengths of the various strings are determined by the piano manufacturer but the string is fixed at one end to a pin which can be turned.  Tuning is an operation in which these pins are turned to raise or lower the tensions and therefore pitches of each note so that they are all at the correct pitch in relation to each other.
Sometimes it may be that the piano is required to be used with other instruments which cannot be tuned (other than by a small amount) such as a trumpet or clarinet.  This is achieved by tuning the piano such that it conforms to a Standard Pitch such as A440 or concert pitch.
A440 means the first A above middle C on the piano (A49, the forty-ninth note on the piano keyboard from the bass end) is tuned so that it's strings vibrate at 440 hertz and then every other note on the piano is tuned to it's correct pitch relative to this first note.  This means that the whole piano is in tune with itself but is also tuned to a pitch level which will match other instruments, or practice CDs etc.
Pianos go out of tune for a number of reasons including usage, environment and quality of piano but by far the biggest reason is through changes in humidity. Ever since it's invention the piano has been constructed mainly of wood which gives a piano it's unique tonal quality, but wood absorbs moisture when the atmosphere is humid and then dries out again when the surrounding air is dry.  When the wood dries out it shrinks slightly and there is slightly less upward pressure by the bridges onto the strings so the tension and therefore pitch drops slightly.  Unfortunately when the wood takes in moisture again and expands the pitch does not go up when the bridges exert slightly increased pressure on the strings again. Instead the tuning pin unwinds a fraction.  So the piano's overall pitch tends to drop in steps with humidity changes.  Even most musicians will not notice this if the piano is never used with other instruments but over a period of time the pitch will drop further and further and the pressure of strings to bridges will fall.  When you consider that it is this string/bridge pressure that transfers the sound to the soundboard you will realise that the quality of tone that the piano can produce will also deteriorate.  Pianos are designed to give their optimum tone from the string/bridge pressure strength achieved when the piano is tuned to concert pitch or A440.
Rough tuning is carried out when a piano is quite out of tune having been neglected for some time.  This rough tuning puts the piano approximately in tune and goes some way to equalising the stress caused by a considerable change in string tension across the whole instrument  before a second fine tuning is carried out.  The need for two tunings is such that the pressure of one string on the bridge will affect the pressure of it's adjacent strings if it is adjusted by more than a tiny amount and so during rough tuning each subsequent note tuned will do something to de tune the previous ones. The piano has to be fairly close to being in tune before a fine tune will hold up.
Pitch raising, to bring a long neglected piano back up to A440 concert pitch is really a very crude rough tuning and may have to be performed twice if a piano is very down in pitch before a rough and fine tuning can be carried out.
 

How often should a piano be tuned?

Some pianos stay in tune better than others.  They seem to have wood that is less affected by humidity changes than other pianos.  Generally the better quality the piano, the better chance that it will stay in tune for longer. As far as old pianos are concerned it will be down to condition and in particular the tightness of the tuning pins that will determine how long the piano will stay in tune.
A domestic piano which is given light use should be tuned every six months to ensure that it maintains its pitch.  If it is left for a year then it will undoubtedly need at least a rough tune if not a pitch raise before a fine tuning and to be fair to the tuner the fee for this should add fifty percent to the normal tuning fee.  On this basis it can be seen that if a piano is tuned just once per year in an attempt to save money then to make an annual saving of about £40 the piano will spend half of the year out of tune and falling in pitch.
A little attention paid to the environment around your 
piano will pay dividends in it's tuning stability and general condition.  If your piano is sensitive to humidity changes and this causes it to go out of tune within six months or you are worried about changes in humidity caused by central heating or a sunny room ask your tuner for advice.  It may be that you can fit a humidifier to your piano or if the atmosphere is constantly damp a low wattage heater. Comprehensive kits are also available which accurately control and maintain the correct  environment within the piano.