QUOTE(petrat @ Aug 8 2007, 01:50 PM)

QUOTE(carol*piano @ Aug 8 2007, 10:36 AM)

QUOTE(petrat @ Aug 7 2007, 10:19 PM)

As I have said before it is these slight differences that give a non-uniform pattern to the tunings in what is called equal temperament, but is in fact not equal in the hands of a skilled tuner. Therefore each key will have a slightly different pattern of tones and semitones from every other key and it is this that gives the individual characteristic to each scale and key. I have now put this into what I feel are very simple terms and I hope that what I have written will be comprehended.
It is a balancing act - because the physics doesn't quite add up - there are slight differences in the intervals in the different keys - at least that is how I have always understood it.

Yes!! By jove, she's got it.
No, neither of you are right.
I'm going to try and gloss over as much as I can and explain the mathematics behind intervals. Each single note we hear has a
frequency, and the thing that makes different notes in different octaves sound higher or lower is that their frequencies are different. A frequency is a number of vibrations a second, measured in Hertz, and one can think of it in terms of vibrating strings: a big long string will vibrate slowly, producing a low note; a tiny string will vibrate quickly, producing a high note. The unit of frequency is Hertz, abbreviated to Hz.
So individual notes each have a frequency, and we can start talking about intervals. The sound of an interval is determined by the relationship between the frequencies of the two notes. Say we have two notes, one with frequency A and one with frequency B. As if by magic, the two notes will sound an octave apart if B is double A. For example, the A above middle C is 440Hz. The A above that will have
double its frequency - 880Hz. Or, to get the A an octave below 440, we halve it to get 220. The really important thing though is that the characteristic sound of an interval is solely based on how many times bigger one frequency is than the other: the octave from 440-880 sounds like an octave, as does the one from 220-440, or from 110-220 and so on.
So, now we can define an octave: play two notes with one frequency double that of the other, and we have an octave

. Also, if we fix A above middle C to 440Hz, we can tune all the As on a piano: double the frequency to go up octaves; halve the frequency to go down octaves. It'd be nice to know how to tune the notes in between the As though, too. In a given octave, there are 12 semitones; for instance:
A-A#, A#-B, B-C, C-C#, ... , G#-A'
How do we do semitones? We know for octaves, we use doubling: to move up an octave from a note, double its frequency. But how do we move up a semitone from a note? Furthermore, how can we ensure if we move up from a note by a semitone twelve times, we'll definitely 100% get to an octave above the original note?
The trick on a piano is to make all the semitones exactly the same size, and rig it so that moving up 12 semitones eventually results in doubling. The magic number is:
1.0594630943592952645618252949463
Or, more properly: the twelvth root of 2. For convenience, I'll call this number Larry.
To move up from a note by a semitone, multiply it's frequency by Larry. To move up a semitone from there, multiply its frequency by Larry. Eventually, we'll have moved up by twelve Larrys, and eureka - we're at double the frequency we started with

.
So now we know how to tune a piano: set A to 440, set the other As off that by doubling, and fill in the gaps by multiplying by Larry. But what does this have to do with keys? Well, a piece can only sound different if played in a different key if the frequency relationships between the notes in the scale change if we change key. But they don't. A fifth from C to G is will sound like the interval we make by going up 7 Larrys from a starting note. A fifth from C# to G# will also sound like that. We can do that for any pair of notes: if we transpose the piece up by a semitone, all the intervals are the same as there are the same number of semitones and hence the same number of Larrys between the notes in the piece.
I'm not getting into temperament here as it's not relevant, not with a modern piano; all I will say is that the method described above (using Larry for each semitone) is what is known as Equal Temperament. Any modern piano is tuned using this method, and have been for in excess of 100 years. All electronic instruments - keyboards, digital pianos - are tuned using the method described above. The last time I got a piano tuner in, he used an electronic tuner; again, set to follow the rules given above. Now and again, a piano will be tuned using a non-equal temperament (varying the Larrys used depending on which semitone on the keyboard we're looking at) but that is very much not the norm.
I hope that clears a few misconceptions up

.
Edit: quick mathmo summary, given Carol Piano has a maths degree

.
The frequency of a note on a piano is 440 * (Larry ^ number of semitones from A above middle C to the note in question). I.e., it's exponential, and thus we have the constant ratio property and the divisor for any two equally-spaced notes is the same.