Midway between these positions the displacement is zero. These positions are called nodes . The energy of a sound wave is directly proportional to both the square of its frequency and amplitude. The rate at which energy is radiated or received per second is called power . Sound intensity is the energy radiated or received per unit area.
Sound intensity decreases with distance from a sound source according to the square of the distance.
This is called the inverse square law . Resonance occurs when a large amplitude of vibration is cause in a vibrating system by small periodic stimuli at the same frequency. Energy is transfered from one system to another. Resonance amplifies very sound sources. The tubes of all wind instruments are resonators, for example, as is the enclosed air mass in the body of stringed instruments such as the violin, and the tubes beneath the bars in the marimba.