A calculator for tone pitch, keys and intervals. The calculator produces equal scaled pure tones (sine waves) of one second length each, or any other duration and equal volume. But the human ear hears low tones much worse than high tones. So if you don't hear a low tone, first try a high one, before turning up your loudspeakers.
If you played a note, it will be marked by a red square and names and frequency of it will be displayed. If you choose a key (left column), the according notes will be marked blue on the upper keyboard, if you choose an interval (right column), they will be marked green. To form an interval, first play a tone to start it, then press an interval button. Play different notes on the upper and lower keyboard to show the ratio of their frequencies. Tuning determines the frequency of the standard pitch A4. The common value is 440 hertz. This value can be changed for each of the two keyboards individually.