Ratios Between Keys on a Piano
Date: 11/05/97 at 19:59:18 From: Michael M. Wiseman Subject: Music: Formula to figure mathematical relations between keys on a piano I know that there is a formula to describe a ratio from one key to another but I can't find it. I tried to calculate it using trial and error but I could only get it precise to 10 digits. My daughter wants to explain the formula for a science project. Thanks
Date: 11/06/97 at 14:00:57 From: Doctor Tom Subject: Re: Music: Formula to figure mathematical relations between keys on a piano Hi Michael, The answer, of course, is, "it's complicated." There is an easy answer that isn't quite right but is pretty close. Each time you go up an octave, the frequency has to double, and there are 12 half-steps to go up an octave: A -> A sharp -> B -> C -> C sharp, etc. Twelve equal multiplicative steps have to amount to a factor of 2, so the 12th root of 2 is the answer. My calculator gives it as approximately: 1.05946309435929526455 So if A is 440 cycles/second, A sharp should be 440*1.059... cycles per second, and so on. But musicians also like to talk about "thirds" and "fifths" and stuff like that. These correspond, physically, to the ratio of the frequencies of a stretched wire and of a wire 1/3 or 1/5 as long. These are close to the appropriate products of 1.059... by itself, but not exact. So pianos are usually tuned to a "well-tempered" scale that's a sort of compromise between the mathematical ratios and the Pythagorean ratios that would give exact thirds and fifths. The well-tempered scale is purported to sound "good," but I don't have a terribly good musical ear, so I'm not one to judge. In the old days, without high-tech tone generators that could make any frequency desired, piano tuners had a tuning fork (usually at A, or 440 cycles/second) from which they'd get an exact tuning of one key. Then they'd work back and forth, starting with pure thirds and fifths, but using an approximation procedure that gradually zeroed in on a well-tempered scale. It used to be quite an art. See if your daughter can dig up an old book on piano tuning - it's really eye-opening. Good luck. -Doctor Tom, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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