I’ve been reading the book, Harmonic Experience by W.A. Mathieu. He starts the book by explaining how to build the notes in a musical scale by deriving them from the overtone series. The difference between Mathieu’s writing, and what I have read in other books about the topic, is that he encourages the reader to learn musical relationships by singing over a drone, and directly listening to and singing along with the partials in the overtone series.
I developed a Reaktor patch to help with ear training with the overtone series. Mathieu suggests using a guitar, plucking harmonics from the strings, and retuning the strings to different notes. But not everyone has a guitar and knows how to get those harmonics. Also the sound of a guitar decays quite fast, and isn’t very loud, which makes it hard to feel the resonance of the harmonies. Another problem is that retuning the strings leads to pitch instability. Using this Reaktor patch makes it easy.

This patch was made with reaktor 5.6.2.
link to OvertoneEars.ens
Here’s quick tutorial on how to use this patch. First, it helps to be familiar with the harmonic series. The wikipedia article is a great resource, particularly this illustration. Choose the overtone you want to hear, and play it along with a drone. I suggest a very low tone as the drone note, because the most musically useful partials are mostly between 2 and 3 octaves above the fundamental note.
To ear train on a major third, for instance, as shown in the above image, choose the fifth partial. First, play it loudly, and sing along to get a feeling for the pitch. Then lower the volume of the partial until it almost fuses with the fundamental tone, and sing along. Eventually, you can turn it off altogether. If you listen closely, you should be able to hear that exact same partial already contained in the original note.
I’ve included some snapshots in the ensemble of other important intervals, like a minor seventh, major seventh, perfect fifth, and some others. To hear how these notes would sound lower, in the same octave, or even lower, you can transpose the partials with the transposition setting. I suggest using them in the ROOT setting first, to feel the resonance with the overtone series.
To louden the partial that you are listening to in the drone sound, change the filter frequency the same frequency as displayed for the partial you have selected on the right. This way you can emphasize the frequency band with the same partial from the overtones of the sawtooth oscillator that is used for the drone.
One of the most wonderful things about music is the fact that ALL of music, all the materials, can be found in a single note played by a single instrument. And if you learn to hear these sounds inside the note, a whole new world of beauty and amazing interrelationships emerge.