Measurement of Frequency Deviations in Equal Temperament Based on the Number of Humming Cycles
Problem Statement
Measurement of Frequency Deviations in Equal Temperament Based on the Number of Humming Cycles
Problem Statement
Let F be the frequency of the reference tone (the standard tone).
According to the tuning system known as “twelve-tone equal temperament,” in which the frequency ratios of all 12 semitones within an octave (the interval where the frequency exactly doubles) are set to be equal, the frequency increases uniformly by a factor of 2121 for each semitone higher than the reference tone.
Let f be the true natural frequency of a specific note that is three semitones higher than the reference tone. Based on the twelve-tone equal temperament, the true natural frequency f can be expressed in terms of the reference tone’s frequency F as follows:
f=F×(2121)3=F×241When a specific note was played on an actual instrument, the tuning was slightly off, so the note sounded at a frequency f′ that was slightly lower than the true natural frequency f. When this off-tune note was played simultaneously with the note at the true frequency f, n “beats” per second were observed. The number of beats per second is equal to the difference (in absolute value) between the frequencies of the two sound waves played simultaneously.
Here, if we let Ff′ be the ratio of the actual frequency f′ of the offset sound to the reference frequency F, the following equation is derived.
Ff′=241−FnWhen the two terms on the right-hand side are simplified into a single fraction using the common denominator F, the numerator takes the form a×241−b (where a and b are natural numbers). Determine the values of the coefficients a and b, and find the natural numbers that satisfy the following input format.
Constraints
- Frequency of the reference tone: F=440 Hz
- Number of beats per second: n=3 s−1
Input Format
Enter the product of the two coefficients, a×b, as a natural number.
Solution
Explanation
The Beat Equation and Derivation of Frequency
When two sound waves are played simultaneously, the difference in their frequencies results in n beats per second. Since the actual frequency f′ of the sound is lower than the true fundamental frequency f, the number of beats n is given by the following equation:
n=f−f′Rearrange this equation for f′ and substitute the fundamental frequency f=F×241.
f′=f−n=F×241−nCalculating the Ratio of Frequencies and Finding a Common Denominator
To calculate the ratio Ff′, divide both sides of the equation by F.
Ff′=241−FnSubstitute the values given in the constraints (F=440, n=3) into this expression.
Ff′=241−4403Unify the denominators on the right-hand side to 440 and simplify the fraction.
Ff′=440440×241−3Determining the Coefficients and Final Calculation
Compare the numerator in the form a×241−b specified in the problem statement with the numerator obtained after finding a common denominator.
a×241−b=440×241−3This determines the coefficients as the following natural numbers:
- a=440
- b=3
Following the input format, we calculate the product of these two coefficients, a×b.
a×b=440×3=1320Therefore, the natural number to be entered is 1320.