15.15 : Concept of Resonance and its Characteristics

If a driven oscillator needs to resonate at a specific frequency, then very light damping is required. An example of light damping includes playing piano strings and many other musical instruments. Conversely, to achieve small-amplitude oscillations as in a car's suspension system, heavy damping is required. Heavy damping reduces the amplitude, but the tradeoff is that the system responds at more frequencies. Speed bumps and gravel roads prove that even a car's suspension system is not immune to resonance. Despite highly engineered shock absorbers, which ordinarily convert mechanical energy to thermal energy almost as fast as it comes in, speed bumps still cause a large-amplitude oscillation. On gravel roads that are corrugated, the bumps are very noticeable if the car travels at a wrong speed, whereas at other speeds, the bumps are hardly felt.

These features of driven harmonic oscillators apply to a huge variety of systems. When a radio is tuned, for example, its resonant frequency is adjusted so that it only oscillates to the desired station's broadcast (driving) frequency. The more selective the radio is in discriminating between two stations, the smaller is its damping. In all of these cases, the efficiency of energy transfer from the driving force into the oscillator is best observed at resonance.