Do an experiment: silent press the piano, and then swipe and immediately release the key an octave below (for example, hold up the second octave, and hit up first). Taken your tone quickly fade, but still long to hear a quiet but distinct sound down arrow.
You can silently press two octaves above waraenai. The corresponding sound will also be heard, albeit less clearly.
Let's see why this happens. If you have read that talks about the sound, you already know that it arises as a result of fluctuations of the elastic body, in this case strings. Pitch depends on the length of the string. You hit, for example, to the first octave. Shook, vibrating string, there was a sound. But fluctuates string not only the whole. Vibrate all its parts: half, third quarter, and so on. Thus, at the same time heard not one sound, but a polyphonic chord. Only the basic tone, the lowest, you can hear much better than others and is perceived by the ear as the only cymbal. The rest are formed by parts of a string and therefore higher overtones (Oberton German "top tone"), or harmonic overtones, complement the sound, affect the quality of the sound of his voice. All these harmonic overtones together with the main tone form the so-called natural scale or the scale of overtones, which are numbered from the bottom up in order: the first sound is the main, second octave, third - octave + pure fifth, fourth - octave + pure Quint + clean quart (that is 2 octaves above main). Further overtones are closer to each other.
This property is to publish not only the main sound, but the overtones - is sometimes used when playing on stringed instruments. If, at the time of sound generation bow lightly touch your finger to the string in the place where it is divided in half or third, fourth, etc. part of the swing large parts disappear, and will not hear the basic sound and higher (respectively, the remaining part of the string) overtone. String this sound is called the flageolet. He is very gentle, mild, cool voice. Composers are harmonics of the strings as a special paint.
Well, our experience with silently pressed key? When we did it, not hitting the string, piano, freed her from the muffler, and it began to vibrate in resonance half longer hurt us strings. That when the key is returned to the place, stopped, and fluctuations in the upper strings continued. The sound you heard.