Try it here: https://sebastiansimon.github.io/ji-exponents/
In music theory, just intonation (JI) refers to musical intervals obtained from multiplying a base frequency by (simple) ratios. In contrast, equal temperament (ET) equally divides an interval (like the octave) into a fixed number of equal steps.
Given an interval of two notes, mathematically speaking, the frequency of one note can be obtained by multiplying it by some factor of the frequency of the other note. In ET these factors are almost all irrational; for example, an equal tempered fifth in 12 equal divisions of the octave (12-EDO) has a factor of 27/12 ≈ 1.4983070768766815; only octaves are exactly times 2. In JI, however, the perfect fifth has a factor of exactly 3/2 = 1.5.
Here’s a table of some common intervals on Wikipedia.
And here’s an extended table of intervals.
The twelve different notes used in western music can all be obtained by multiplying or dividing frequencies by positive integers up to 5. Only the prime numbers 2, 3, and 5 are necessary (since 4 = 2 · 2).
This audio player has powers with bases up to 29 (currently), so it’s 29-limit JI.
440 Hz is the note A4.
For example you can enter a different integer at 20, or increase it to make it 21, 22, or decrease it to make it 2-1, 2-2, etc.
Click the ▶ Play button to play the note.
The button also shows the resulting frequency.