Understanding Piano Tuning Stability

Piano tuning is just one aspect of piano service technology as it relates to string tension and pitch. To best understand piano tuning it is first helpful to understand the properties of piano wire and the construction of the super structure of the instrument.

Tuning is not a finite science. The practice of tuning has many variables. A tuner is faced with the challenge of dealing with these variables daily, which are influenced by climate, quality of the instrument, service history, condition of the instrument, ambient acoustics, and other factors that make success with tuning both an art and a science. Piano tuning requires education and skill.

Piano Wire

Tuning StabilityFirst, let’s take a close look at what a piano string is. Piano strings are made from a classification of cold drawn steel wire commonly called music wire. What distinguishes piano wire form the general classification of music wire is purity, high carbon content, and exacting manufacturing specifications with variations no greater than .0003”. Music wire and piano wire fall under international standards of gauging.

High carbon piano wire has special properties allowing it to be coiled and drawn to tension without changing properties. There are three limits to piano wire:

1) Elasticity – ability to be coiled and drawn to tension without fatiguing

2) Stretch Point – the point at which the wire loses integrity and begins to fatigue

3) Break Point – the point at which the wire fails under tensions and breaks.

As piano wire ages it stiffens. When this happens each of these limits occur at a lower tension.

Understanding this clarifies the point that piano strings do not normally stretch when being tuned or when they go out of tune. This is an important fact to understand in order to understand tuning stability.

Piano wire, installed in the instrument, varies in length and diameter.  Length is determined by the scale design, bridge position, and bearing points. Diameter, or gauge, is also determined by the mathematics of scale design. String length varies from a few inches to as many as seven feet or more, depending on the size of the instrument. Of this, only a portion of the overall string length is responsible for pitch. This section is the majority of length and is referred to as the speaking length.

Piano wire size is measured by standard gauge converted from in both english and metric measurements. Typically pianos utilize wire ranging in size from 12.5 up to 22 gauge. However, each piano is different depending on scale design. When tuned, piano wire is under tension ranging from 165lbs. upwards to more than 300lbs. Average collective tension on a piano, depending on size, is between 36000 – 40000 pounds. It is this collective tension which makes structure the next importance element to understand when grasping concepts of tuning stability.

Piano Super Structure

Super structure is a term that relates to an instruments comprehensive unified construction consisting of the rim (in a grand piano) or back frame (in a vertical piano), iron plate (sometimes referred to as string frame or harp), and soundboard and bridges. These are the components which work together to support and stabilize upwards to 20 tons of tension.

There have been several schools of thought on construction. Some instruments are designed for tension to be supported largely by the plate, while others integrate the rim or back construction into the over-all design. Many of the early American instruments utilize both principles, unifying the plate into the wood structure of the rim and back.

Another area of divergence in concept is whether the plate and rim are part of the resonation of the piano or is most of the energy of tone contained within the soundboard. Makers such as Steinway, Baldwin utilize a hard maple rim to deflect energy into the soundboard while European makers such as Boesendorpher and Blutner use softer wood such as spruce to allow resonation of the plate and rim. The sound is distinctively different. The first is a more dynamic edgy sound while the latter carries more sustain and resonance. In all case quality of construction over concept will determine tuning ability and stability.

The rim and plate work together to support string tension as well as stabilize the soundboard which, under downward pressure from strings, will fail if allowed to push out from its perimeter. This downward pressure is referred to as down bearing. Down bearing is the force exerted by the strings on the bridge. As mentioned, the collective string tension can be upwards to 40000 pounds. This converts to a certain amount of downward force influencing the position and movement of the soundboard.

A formula to calculate down bearing was presented in conference to piano builders in 1919 by Dr.D.R. Hodgdon. I will repeat this formula for fun. The average person does not need to know this, yet however, it will help to illustrate the principle I am attempting to arrive at.

Dr. Hodgdon’s formula:

P=TC (A+B) over (AB)

In the case where the string tension is 160 pounds, string length is 11 inches overall, and bridge height is 1/16”:

=160 x 1/16x (11 divided by 24)

=160 x 1/16 x 11/24

= 110/24

=4 7/12

Down bearing in this case will be 4 7/12 pounds, roughly 1/32 of string tension. Therefore, if over-all tension is 40000 pounds than down pressure will be 1250. This is like having a fattened steer sitting on the inside of your piano, for the life of the instrument.

Piano Tuning Stability

Now that we have an understanding of piano wire and structure let’s look at what influences piano tuning stability.

We now know that piano strings don’t stretch, until they begin to fail. At this point replacement should be considered. We also know that not only is the wire susceptible to instability, but also the super structure of the instrument.

The number one influence on piano tuning stability is climate. Not temperature, but humidity fluctuations. Normal amounts of playing will actually add in tuning stability. The reason for this is, as strings are struck the tension is equalized along all bearing points. If an instrument sits idle the wire segments between bridges, agraffes, hitch points, etc., rendering unequal tension and therefore pitch instability. Moderate playing enhances stability. However, excessive playing will cause strings to go out of tune.

Tuning Instability from Hard Piano Playing

As the hammer strikes the string the wire is set in motion in two primary directions: amplitude and longitude. The first creates string movement perpendicular to resting position, temporarily increasing tension, causing a forceful lifting of the string upwards. When this occurs pressure is released from the bridge surface lessoning over-all downbearing very slightly. In addition, tension is also increased along the longitude of the string. This force increases torsion against the plate, rim, and other areas of the super structure. In cases of poorly designed and made instruments flexing occurs in the rim, back frame, and pinblock. Whatever the case, the string will want to settle at a lower tension.

Piano Tuning Instability Caused by Climate

Pianos are composed primarily of organic materials susceptible to humidity changes. The soft wood of the soundboard, although very high in fibrous tensile strength, is very sensitive to changes in relative humidity. As relative humidity (amount of water vapor present in the air) changes it directly affects the internal moisture content present in the wood through equilibrium.  The higher the moisture content in the wood of the soundboard the more the board will expand upwards, and vice versa. This fluctuation causes changes in string position more than anything.

Pianos will adapt to any given environment over time. I tune pianos regularly in the northeastern US as well as the southwestern US. Each is in total contrast to the other. What I find is that once an instrument stabilizes it does fine as long as the variation remains within 25 – 30 percent. It doesn’t seem to matter much, to a point, if it is high or low. As long as it is stable. The effects of high and low humidity will be discussed in another article.

The very best way to stabilize humidity is from an external source away from the piano such as using both air conditioning and humidification during respective seasons. I will also discuss in another article the use of built-in climate control systems and why I do not recommend them.

I hope this article is helpful. I will add to it as time allows.