Acoustics/formulas/Max efficiency

This formula is a template, for inclusion into pages about Acoustics. Use the following syntax to include it:

$$\eta_{\max} = 1 - \frac{2}{k_\mathrm{eff} \sqrt{Q_E Q_{m0}}} \qquad \left ( k_\mathrm{eff} \sqrt{Q_E Q_{m0}} \ll 1 \right )$$