Frequency is the term used by physicists to communicate the rate at which waves (in this case, sound waves) move through the air per second. Frequency is measured in units called "hertz." When a wave repeats itself, it is called a "cycle." One hertz is equal to one cycle per second. For example, middle C on a piano is tuned to a frequency of 261.63 hertz, or 261.63 completed sound wave cycles per second.
Every sound has a frequency that expresses the exact number of wave cycles completed per second; however, not all sounds at a particular frequency sound the same. This is because of other sonic aspects, such as overtones and harmonics, that help to make up certain qualities of a sound. "Timbre" is the term given to other sonic characteristics of a sound that aren't defined by frequency; a plucked violin string will sound differently than a bowed violin string, even when playing the same note.
A simple frequency recollection training mechanism is similar to the elementary flashcard method, except in auditory form. The key is repetition, and an iPod (or other MP3 player) is a perfect tool to train with. Take baby steps so that the student does not get overwhelmed or confused.
The first recollection attempts will be the most difficult because the student is only beginning to build a mental reference library; however, progress will become simpler and swifter as more time and concentration is invested in the study. The student should begin with two notes at a time in only a few octaves and should attempt to identify both the pitch (for example A4) and the frequency (440 hertz).
Extending beyond the single-note fundamentals, it is beneficial to practice with note intervals, first with pairs of notes. This is often taught and practiced with the first two notes of popular pieces of music such as "When the Saints Go Marching In" (major 3rd) and "Smoke on the Water" (minor 3rd). Moving on, the student should practice chords, starting with three notes, in all three inversions. This will develop the ability to hear the relationship between the three notes and how they harmonize.