Other instruments sound different combinations of overtones, which give them their special tone quality. The human voice and stringed instruments such as the violin and piano are very rich in overtones. Overtones that harmonize better than others are notes of the same scale. Sounds thought of as harsh are combinations of tones that do not harmonize. If the raucous call of a bluejay were analyzed, it would be found to be a combination of extremely discordant notes.
All noises are miscellaneous combinations of tones, unpleasant because they are unrelated.
Like light waves, sound waves can be reflected and focused. A flat surface, like that of a cliff or wall, reflects sound better than an irregular surface, which tends to break up sound waves. Echoes are useful in many ways. In a fog, a ship's captain can often tell whether he is near a hilly shoreline by listening for echoes of the ship's whistle. Underwater sonar equipment uses echoes of a supersonic signal similarly to detect submarines.