Acoustic resonance
= Acoustic resonance
<Acoustic resonance> is an enhanced response when an acoustic system is driven near one of its natural oscillation frequencies. Reflections and boundary conditions select <standing waves>, while <damping> limits the steady response and broadens its frequency peak. Resonance concerns the response to a drive; a large sound at one frequency alone does not identify the mechanism.
A classic measurement holds a vibrating tuning fork above an air column whose lower end is closed by water. Varying the water level changes the column length. In the ideal narrow-tube approximation, pressure has an antinode at the closed end and a node at the open end, giving
$$
f_m\simeq\frac{(2m+1)c}{4L_{\mathrm{eff}}},\qquad m=0,1,\ldots,
$$
where $c$ is <speed of sound> and $L_{\mathrm{eff}}$ includes an open-end correction. Successive resonant lengths at fixed frequency differ by approximately half a wavelength. If a 500 Hz fork gives successive peaks 0.34 m apart, $c\simeq2f\Delta L=340\,\mathrm{m/s}$. Using the difference reduces a common end-correction error.<Airspace: Speed of Sound Grades 9-12 by NASA>[Measuring the Speed of Sound Using Resonance, water-column/tuning-fork procedure]{ref}