Piano acoustics is the set of physical properties of the piano that affect its sound. It is an area of study within musical acoustics.
String length, mass and tension
The strings of a piano vary in thickness, and therefore in mass per length, with bass strings thicker than treble. A typical range is from , string size 13) for the highest treble strings to for the lowest bass. These differences in string thickness follow from well-understood acoustic properties of strings.
Given two strings, equally taut and heavy, one twice as long as the other, the longer will vibrate with a pitch one octave lower than the shorter. However, if one were to use this principle to design a piano, i.e. if one began with the highest notes and then doubled the length of the strings again and again for each lower octave, it would be impossible to fit the bass strings onto a frame of any reasonable size. Furthermore, when strings vibrate, the width of the vibrations is related to the string length; in such a hypothetical ultra-long piano, the lowest strings would strike one another when played. Instead, piano makers take advantage of the fact that a heavy string vibrates more slowly than a light string of identical length and tension; thus, the bass strings on the piano are shorter than the "double with each octave" rule would predict, and are much thicker than the others.
The other factor that can affect pitch, other than length, density and mass, is tension. Strings in an upright piano usually have a tension of 750 to 900 N (75–90 kg weight) each.
Inharmonicity and piano size
Any vibrating thing produces vibrations at a number of frequencies above the fundamental pitch. These are called overtones. When the overtones are integer multiples (e.g., 2×, 3× ... 6× ... ) of the fundamental frequency (called harmonics), then - neglecting damping - the oscillation is periodic—i.e., it vibrates exactly the same way over and over. Humans seem to enjoy the sound of periodic oscillations. For this reason, many musical instruments, including pianos, are designed to produce nearly periodic oscillations, that is, to have overtones as close as possible to the harmonics of the fundamental tone.
In an ideal vibrating string, when the wavelength of a wave on a stretched string is much greater than the thickness of the string (the theoretical ideal being a string of zero thickness and zero resistance to bending), the wave velocity on the string is constant and the overtones are at the harmonics. That is why so many instruments are constructed of skinny strings or thin columns of air.
However, for high overtones with short wavelengths that approach the diameter of the string, the string behaves more like a thick metal bar: its mechanical resistance to bending becomes an additional force to the tension, which 'raises the pitch' of the overtones. Only when the bending force is much smaller than the tension of the string, are its wave-speed (and the overtones pitched as harmonics) unchanged. The frequency-raised overtones (above the harmonics), called 'partials', can produce an unpleasant effect called inharmonicity. Basic strategies to reduce inharmonicity include decreasing the thickness of the string or increasing its length, choosing a flexible material with a low bending force, and increasing the tension force so that it stays much bigger than the bending force.
Winding a string allows an effective decrease in the thickness of the string. In a wound string, only the inner core resists bending while the windings function only to increase the linear density of the string. The thickness of the inner core is limited by its strength and by its tension; stronger materials allow for thinner cores at higher tensions, reducing inharmonicity. Hence, piano designers choose high-quality steel for their strings, as its strength and durability help them minimize string diameters.
If string diameter, tension, mass, uniformity, and length compromises were the only factors—all pianos could be small, spinet-sized instruments. Piano builders, however, have found that longer strings increase instrument power, harmonicity, and reverberation, and help produce a properly tempered tuning scale.
With longer strings, larger pianos achieve the longer wavelengths and tonal characteristics desired. Piano designers strive to fit the longest strings possible within the case; moreover, all else being equal, the sensible piano buyer tries to obtain the largest instrument compatible with budget and space.
