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I haven't tuned my upright acoustic piano for one, maybe two, years. I'm inept, so I can't tell if it's out of tune, but a friend recommended me to get a tune-up at least once a year.

My question is: How exactly does a piano go out of tune? Do all the notes go out of tune at the same rate? I'd imagine that how often I use certain notes, especially the middle two octaves, would accelerate the out-of-tune process--Is that correct?

(Suppose I leave a piano untouched in an attic for say, five years--Would all of the notes go out of the tune at the same rate, that is, all modulate down a few semitones?)

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There are several reasons. When new strings are put into a piano, they slowly "stretch" or relax and go flat. In a day it will be out of tune. You have to tune it 2 or 3 times the first month. After a few months the strings will have settled in and will stay in tune better.

If the pinblock is very old, the pins can slip, making the notes go flat.

The total tension of the strings is 20 to 30 tons. That is, the pinblock is pulled toward the bridge with 20 to 30 tons of force. That is enough force to distort the frame a little. So when several notes go flat, reducing the tension, the frame goes back into shape, stretching the other strings and making them go sharp!

You asked what it sounds like when the strings of a unison start to go out of tune. if two vibrate at 440 Hz and one at 439 Hz, the two pitches will interfere with each other and you will hear a wavering "beat" at one cycle per second. If the third is at 438 Hz, you'll hear two beats per second. It gives a feeling of motion to the sound. This effect can be done deliberately in electronic organs and synthesizers and is called "chorus". It's like the difference between one singer singing alone and several singing in unison.

Pipe organs have a stop named "Voix Celeste" or "Unda Maris" which consists of two pipes for each note, tuned slightly apart so that they beat at about 3 to 5 beats per second. It's a heavenly effect, or at least whoever named it "Voix Celeste" thought so.

A "Honky Tonk" effect is when the strings of a piano are badly out of tune, making it sound like the piano player in the saloon in an old Western movie. Some electronic pianos have a "Honky Tonk" patch imitating this.

Cold weather makes pianos go sharp, and hot weather makes them go flat. But cold weather makes pipe organs go flat, and hot weather makes them go sharp, and that is why a church piano and pipe organ are rarely in tune with each other.

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@ Mark - that 'voix celeste' must be similar to the chorus effect used by guitarists and Rhodes players. –  Tim May 6 at 5:47
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+1 for the fascinating tidbits about "Voix Celeste" and "Honky Tonk" –  SEL May 6 at 11:50
    
The beats happen because the two notes go in and out of phase in a periodic fashion, causing a periodic variation in amplitude. Piano tuners use beats of overtones to calibrate the slight flatting of 5ths that is needed to achieve equal temperament. Here is a page with a diagram that shows this. Also, if you think about two strings vibrating at slightly different frequencies, you can imagine that at certain points they are each moving in the same direction and at others moving in opposite directions. This is what causes the amplitude to vary. –  BobRodes May 6 at 17:38
    
One of the other characteristics of a "honky tonk" piano was that they put thumbtacks in the hammers to make the sound louder. –  BobRodes May 6 at 17:39
    
A piano that has been played hard for years but never maintained will have very hard hammers, and thumbtacks create a very similar sound, as BobRodes says. –  Mark Lutton May 6 at 19:12

There are several reasons why a piano may go out of tune. No piano can be in perfect tune for more than 6 months, sometimes even less, even when it's not being played. Therefore, one must tune their piano at least every 6 months (the reason why is given below). The reasons for the piano going out of tune are:

1. Tension in the strings

The different strings in the piano are under different levels of tension. The low frequency strings are under lesser tension as compared to the high frequency strings. Also the number of strings per frequency varies, thus adding to the variation in tension across the cast iron plate.

Normally, when you don't play the piano, there is a lot of energy stored in the strings due to the tension, just waiting to be released. This tensile force may give way (a tiny little way, and very gradually) and the string becomes ever so slightly less taught (as the frictional force that keeps the string under tension in the pins gives way to the tensile force in the strings).

When you play the piano, as the hammers hit and vibrate the string, this loosening takes place a little faster, especially for the strings being struck.

2. Temperature change

With the main logic being change in tension of the strings and their loosening, this process can be accelerated by changes in temperature. During the summer season, there is linear expansion of the strings, and they become naturally loose even though they do not slip on the pins. During the winter, the temperature drop contracts the strings, thus making them more tight, and they experience more tensile force.

This temperature change is the main reason why pianos go out of tune, and you have to tune them every 6 months, when the seasons change. This is independent of how well the piano is made, and thus why all pianos go out of tune.

3. Humidity change

Though humidity doesn't affect the steel strings much where frequency is concerned (they do cause rusting over many years though, and strings can snap when they become weak), it affects the wood a lot. The steel string vibrating themselves produce a faint sound. The sound you hear is the sound of the wooden soundboard vibrating.

Humidity changes the density and elasticity of the soundboard, thus rendering the piano out of tune. As humidity increases, wood absorbs more water and increases in density and size, and vice versa.

As the moisture level in the soundboard increases during periods of high relative humidity, the crown expands and pushes the bridge harder against the strings. The strings are stretched tighter and the piano’s pitch rises. Because this increase in crown is greater in the center of the soundboard than at the edges, the pitch rises more in the middle octaves than in the bass or treble registers.

4. Other factors

I've also noticed people placing goods inside the piano, especially in grands. This may add extra forces on the string, and even causes the timbre to change (since more than the strings and soundboard are vibrating). Keeping stuff inside the piano is highly not recommended.

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It must be 5 years since mine were tuned. Two are still pretty much concert, the third in a very sunny room has moved a fraction, but is certainly not unplayable.They all get played every day/week. In Britain, we don't have extremes of temperature,or humidity, certainly not in the house!!I'd have thought that the tension on strings was fairly even, as the density and length vary to compensate. What sort of goods go inside pianos ? –  Tim May 6 at 15:38
    
How many pianos have wood as part of the structure maintaining string tension? I know that at least historically some did, but I thought that nearly all pianos had a cast iron frame which would hold the strings in tension even if all the wooden portions of the structure were removed. –  supercat May 6 at 18:21
    
Iron bars were added in the early 19th century, and the full cast-iron frame was invented in the 1830's. Except for historical reproductions, all pianos made after about 1860 have cast iron frames. Even so, string tension is so high that it distorts the frame a tiny bit. If you lower the pitch of most of the strings, the remaining strings will go sharp and may even break. –  Mark Lutton May 6 at 19:21
    
@supercat - there are still loads of pianos around (in Britain) that have wooden harps. Usually they're well out of tune, and have over-damped strings, making them not very good pianos.Lots will never tune properly for the reasons stated above, and are often found in sales, etc.,sadly neglected, - and useless. –  Tim May 10 at 9:11

No, the notes don't all go out of tune at the same rate, and the notes that get hit more often do go out of tune a bit more quickly.

Piano strings are strung at one end so as not to move, and on the other end they are held by "pins" (more like pegs) wedged very tightly into a "pinblock", which is a very hard piece of wood with holes in it. When a piano is in tune, there are actually several tons of pressure per square inch on the pins, and over time the pins twist a little bit and loosen the strings. This causes the piano to go out of tune, because the notes get lower as the strings get looser. If the strings take repeated blows from their hammer, they will tend to loosen a bit more quickly; a piano in a music student practice room will need more tunings than the one in your living room, unless you practice for many hours a day.

You can imagine that there is a slight difference in each pin's tension, and tendency to loosen. The better the piano, the more even this is, but nonetheless you are dealing with pins stuck in a block of wood. There are enough variables that the strings will loosen at different rates.

Now, most notes on the piano actually have three strings tuned in unison. (The lowest strings are thick and only have one string, then the not-so-low ones have two, and the rest have three.) There's another variable: a lower, thicker string will have a bigger and heavier vibration, so when played it will loosen a bit more than high strings that are played.

All this taken together means that the pitches of the notes drop a pretty much random amount over time.

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I never knew that some notes had multiple strings. Is it possible for those strings to go out of unison? What would the note sound like if they did? Would you hear two or three sounds even though you only struck one key? Also, if the strings get looser over time, would it be correct to say that the notes can only go out of tune downwards? –  SEL May 6 at 0:02
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@Sel, music.stackexchange.com/questions/ask –  msh210 May 6 at 3:26
    
@Bob - Would I be right to say that for the same drop in tension, a short, higher sounding string will show to be more out of tune than a long low sounding string ? –  Tim May 6 at 5:39
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@Sel: Absolutely. If you ever watch a piano tuner, you will see that he has these rubber wedges that he puts between various strings. This stops off all but one string on a pitch. He tunes that string to the others, and when finished will tune the unisons to the newly-tuned string. If the unisons are out of tune (and in an out-of-tune piano they certainly will be), you would hear more of a "muddiness" than discrete sounds, unless perhaps you had a very fine ear. This is because the overtones will mix with one another in complex ways. –  BobRodes May 6 at 14:14
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@Sel: finally, the only way to make the note go out of tune upwards is to tighten the string. This can happen very slightly if you increase the humidity in the room, causing all of the wooden parts to expand, but that aside, yes, notes go out of tune downwards. If you haven't tuned a piano for quite some time, a tuner will do a "pitch raising" and come back in a couple of weeks to retune. This is for two reasons. First, if you increase the tension too much the string will break, and second, when you make a big change the string will "relax" and need to be retuned. –  BobRodes May 6 at 14:23

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