Resonance takes place when an excitation frequency approaches the self frequency of a system

Examples:


A kid is on a swing. The self frequency depends on the length of the swing chain.
We can make the amplitude of oscillation large only if we push at the correct time intervals. Ie at the same frequency as the self frequency.


We change the length of an antenna to match its self frequency to that of the radio station we are interested in listening or tuning in.


A soprano breaking a glass when she sings at the same frequency as the natural frequency of the glass.


Soldiers marching all at the same frequency and this frequency happens to be the resonant frequency of the bridge.


Singing near a guitar the note D and then hearing it vibrating.


Singing A4 for example near a piano where sustain pedal is pressed. We then can hear the piano repeating.


In fact we hear the piano repeat at the tone quality of our voice like reverb. This happens because our voice has many overtones. These overtones excite the right strings or overtones of the piano strings.
Ie the piano does Fourier analysis and synthesis in real time.


When we clap our hands the piano repeats the clap sound. Clapping our hands creates a board band frequency spectrum and all strings vibrate sympathetically.
Other way to look at it is that the wave by our clapping hands creates a push air presure pulse so short in time that the strings do not have the time to move back while this is happening. So all strings move. Even the high frequency ones.


This means that the shorter a pulse the greater its frequency spectrum or bandwidth.


This is the Heisenberg's (quantum mechanics) uncertainty principle in acoustics.


The less a tone lasts the less we are sure what note it is.


The shorter an event the grater bandwidth it creates. For example turning on or off a light switch would create a click sound to an AM radio no matter to which frequency it is tuned. The short breaking of circuit is such a short pulse that the electromagnetic waves emitted have content even at radio frequencies.


See also:


Waves - Berkeley Physics Course - F. S. Crawford Jr.


Musical Acoustics - Hall


The Feynman lectures on Physics - Feynman, Leighton, Sands