Final answer:
The instrument described as both a percussion and a stringed instrument with a range of seven octaves is a piano. It uses resonance in its soundboard to amplify the sound from the strings, and its size allows for a broad range of sound frequencies.
Step-by-step explanation:
The instrument in question, which is both a percussion and a stringed instrument, capable of producing sound through small hammers hitting the strings across a range of seven octaves, is the piano. This instrument employs the principles of resonance in its sounding board, which amplifies the vibrations from the strings and contributes to its rich tonal quality. The sound energy of a piano varies with the string length and tension, as well as the size of its soundboard and the body of the piano itself. In general, larger musical instruments have the ability to produce lower frequency sounds with longer wavelengths, resulting in deeper tones. Conversely, smaller instruments typically produce higher-pitched sounds with higher frequencies and shorter wavelengths. The piano is unique in that its large size allows for a broad range of frequencies, from very low to very high pitches.