Inharmonicity as it relates to piano tuning.
Compared to other stringed instruments (guitar, violin, cello, harpsichord, etc) the piano has a unique quality in it's tone called inharmonicity. A string in vibration creates not a single tone, but multiple tones called overtones or partials. These upper partials on a piano are sharp of the pitches found on other stringed instruments. This difference is called inharmonicity.
Each piano make and scale design has a different amount of inharmonicity. Therefore, the inharmonicity in a given piano must be taken into account while tuning. When a piano is tuned correctly, you will find that compared to other instruments the treble of the piano will be tuned sharp based on the amount of inharmonicity, while the bass will be flat as the fundamental pitch is driven down because of the sharp upper partials in the treble and tenor sections. The resultant tuning is more like an S curve than a straight line.
If a piano is accompanying another instrument, it is often wise not to play identical melody notes because the adjustment for inharmonicity in the piano will generate a very noticable "out of tune" quality with the instrument it accompanies even though the piano is tuned correctly for itself.
String Pitch - Every piano has a scale design and is manufactured to sound its best at a certain string tension. Changing string tension by turning the tuning pins will raise or lower the resulting sound or pitch of the string.
So that instruments can play together in tune, we have a pitch standard A=440 cps. This means that the fundamental pitch of the note A above "middle C" on the piano will vibrate at 440 cycles per second. This pitch standard was not established in the United States until around 1925.
It was common for pianos prior to this period to be tuned to a pitch somewhat lower at A=435 cps a very noticeable difference from the current standard.
It has also been a common practice of piano technicians to tune older upright grands/cabinet grands anywhere from a 1/2 tone flat to a whole tone flat to substantially reduce the tension on the strings and frame of pianos not in good structural condition. If your piano falls in this category, the piano must be carefully evaluated before attempting to raise the pitch.
Pianos can be tuned at any pitch that the strings will withstand, but if you want your piano to perform its best or continue to perform at all, you may have to put some careful thought into how you plan to use the piano.
Carefully listening to pitch, and learning pitch interval relationships is a part of learning to play the piano. Playing on a piano that is not tuned to modern pitch can be disconcerting to some students. So if you have a student taking lessons, it is best to make sure the piano they play on will tune to modern pitch.
If your piano is going to be played with other instruments, again it can be very difficult if the piano is not tuned to the modern standard. Add the problems of inharmonicity and the resultant sound can be quite unpleasing to unusable.
For the casual user where the piano is played by itself only occasionally, a piano that is tuned to an old standard or even flatter can still sound good unto itself and provide years of continued enjoyment.
Pitch - Raising - If a piano has not been tuned for a long time (usually 3 years or more), the strings may not be playing the correct pitch for the note played. Depending on how far the pitch has fallen will determine if the piano needs more than one tuning to restore the correct pitch and string relationships. Pianos have approx. 210 strings or more and the tension these strings exert on the framework of the piano and the plate can be as high as 20 tons (40,000 lbs).
By altering the overall pitch, thousands and thousands of lbs of pressure can be added to the frame and plate causing the strings to continue to stretch and the piano frame to compress. Under these circumstances it is best to alter the pitch in one appointment and make an additional appointment 5-7 days later for the actual fine tuning to occur.
If the piano was severally off pitch, it is also advisable to completely retune the piano within 6 months.
Tuning Stability- In order for a piano tuning to remain stable for any length of time, the environment around the piano must also be stable. Pianos are made of a great deal of wood and they react to the surrounding atmosphere. If the room is constantly fluxuating in temperature and the amount of humidity in the air, the piano tuning will not remain stable.
New pianos are more sensitive to these changes because the parts are new and the tolerance of the parts is usually very exacting. Older instruments have more wear and tear on parts and pieces, so it takes more environmental change to cause them to malfunction.
A good rule of thumb is not to let the temperature get below 60 degrees nor above 75 degrees for prolonged periods. The humidity in the room should be between 42%-45%. Again because pianos have so much flexibility, they usually can tolerate measurements outside those I just gave you for short periods of time, but if the piano is exposed to continuing extremes on either end of the scale - high or low - permanent damage can occur and ruin a wonderful instrument.
If you want to know what is going on in the room with temperature and humidity, I suggest purchasing a hygrometer/thermometer. Small inexpensive units can be purchased at many mass merchandising stores, hardware stores, and over the internet for very little cost plus probably a battery. Many will not only tell you what the current temp / humidity is in the room, but they will also track the high and low readings over time which can be a great help in determining the cause if you should find readings that show concern. If you see numbers that differ by more than +/-10 degrees and +/- 10% humidity you should begin the process of discovering what is causing the unstable atmosphere in the room and take what corrective action you can to restore stability.
Correction is usually obvious - too much moisture requires heat to dry the piano down to the desired level. In this case products like Dampp-Chaser systems can be installed right in the piano to reduce the amount of moisture content in the air surrounding the piano parts and case pieces.
Too little moisture is another issue. At least in the Pacific NW our outside climate usually does not present this issue, but with the greater use of wood stoves, fire places, and alternative heating sources that tend to severely dry the air in the room, you may find it more advantageous to use a room humidifier than to attempt to deal with the piano alone if this condition presents itself.
For more information on humidity check out our separate webpage. Humidity