The cast-iron frame of an upright piano
The amount of tension on each string of a (modern) piano is commonly the equivalent of the force exerted by a weight of around 75kg (this can vary significantly). An average piano might have 220 strings or thereabouts, so using these calculations the total force is around 16-17 tonnes for a typical instrument. In an article to follow I'll explain how it's possible to calculate the amount of tension on a given string.
However, an obvious question to ask is - why bother having so much tension? Why not have just enough to keep the string taut and allow the pitch to be adjusted, thereby avoiding the need for a heavy cast-iron frame in the first place?
Cranking it up a notch
Early pianos, which were wooden-framed, had much lower string tensions as they couldn't withstand as much force as a cast-iron frame, but despite this on occasion still used to buckle under the strain. The tone and power of these early pianos (which generally had lightweight leather-covered hammers) was fairly modest in comparison to a modern instrument. Over time, piano makers increased the tension on the strings, introduced metal bracings (later cast-iron frames) and replaced smaller leather hammers with heavier felt ones.
Having strings under higher tension confers a number of advantages:
(i) The amount of energy required to get the string vibrating is greater at a higher tension - this is beneficial since, once in motion, the string transfers more energy to the bridge and soundboard, giving improved volume and power (this is also the reason for the change from small leather-covered to more substantial felt hammers).
(ii) The string undergoes more complex vibrations at a higher tension, so there is a brighter and fuller sound; in fact, many early pianos sounded rather like harpsichords - the important difference was that the hammers allowed control over the volume through the strength of the blow on the piano. The timbre we associate with a piano today is a result of the increases in string tension through the 19th Century.
(iii) The pitch of the string is more thermally stable if the string is at a higher tension. Unfortunately, I'm not enough of a physicist to explain to you why this is the case, but it is. Samuel Wolfenden* (see reference below) says that the pitch of some older low-tension pianos from the earlier part of the 19th Century could vary by as much as a semitone(!) between summer and winter (and unevenly between notes, because string tensions weren't designed to be equal on these early insturments).
Generally speaking, when designing a piano, it is better to keep string tensions as similar as possible across all the strings. The main reason for this is that changes in temperature and moisture (the latter mainly affecting the wooden parts of a piano) will tend to make the piano change in pitch more evenly with similar tensions (i.e. the piano will stay in tune with itself), as well as giving a more even spread of tension on the pins and frame.
The 19th Century - The Piano Evolves
The results of these changes can be seen from a comparison of pianos at the beginning of the 19th Century with those at the end.
The picture shows a piano typical of one which might have been built around 1800 - two obvious differences from what would be seen today are the absence of a cast-iron frame and the arrangement of the strings (straight strung, as opposed to the overstrung arrangement used on all modern pianos). There are many other differences - this particular piano has 6¼ octaves as opposed to 7⅓ (88 notes) which is the standard compass today.
It should also be noted that actions in pianos around this period were of several different types, none of which were particularly similar to those used today; the Erard Double Escapement Action, which is the basis for the modern grand action, dates from 1821 whilst Robert Wornum patented the tape check action (the basis of the modern upright action) in 1842 (though it should be noted that similar concepts were employed on actions much earlier than this). A popular type of instrument in 1800 was the square piano, around the size of a large kitchen table (I have written about these in my previous post on the Broadwood Piano Festival); by 1900, the square had long since been eclipsed by the upright.
This picture shows a typical grand piano from just after 1900, which has practically all the features you would expect to see on a modern instrument - notably, the frame is cast-iron, allowing for much higher string tensions, and it is also overstrung (that is the bass strings cross over the tenor ones). It would also be remiss not to mention that the manufacture of wire also greatly improved throughout the century, which was also important in allowing the increased strain on the strings without frequent breakages.
In case you're interested in more technical information about this, there's an article here which has a graph showing changes in tension over time - the values on the left are in kilogrammes per note (I believe the notes shown are trichords on this graph - for the newer pianos at least it looks as if the bichords and monochords are not plotted), so should normally be divided by 3 for the tension on each string individually. These show that on Cristofori's piano of 1726, the tensions varied between 5 and 20kg per note; by 1808 (Streicher) this had increased to 40-80kg per note, and by 1914 (Ibach) was up to 220-260kg per note, where it has roughly remained ever since. Notably also, on the later pianos (Steinway M and Ibach) the tension is relatively much more even across the compass than on the earlier ones, reflecting a better understanding of scale design.
In my next post I'll explain how to calculate the tension of a particular string in any piano and a bit more about piano scale design.
* Reference: Samuel Wolfenden - A Treatise on the Art of Pianoforte Construction (1916)
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