什么是 Sound level？

Example 7.6 (How small a sound is)

At the threshold, $1\,\mathrm{kHz}$: $p_m = 2.8 \times 10^{-5}\,\mathrm{Pa}$, $v_m = p_m/Z = 7 \times 10^{-8}\,\mathrm{m}/\mathrm{s}$, and the displacement amplitude $\xi_m = v_m/\omega = 1 \times 10^{-11}\,\mathrm{m}$ — a tenth of an atomic diameter: the eardrum detects motions smaller than an atom. At $120\,\mathrm{dB}$, $\xi_m = 11\,\text{µ}\mathrm{m}$, still invisible, and $p_m/P_0 = 3 \times 10^{-4}$: even the loudest sounds are tiny perturbations, which is why the linear theory works so well.

Example 7.8 (A siren, a conversation)

A $10\,\mathrm{W}$ siren: $I = 10/4\pi r^2$, $0.8\,\mathrm{W}/\mathrm{m}^{2}$ at $1\,\mathrm{m}$ ($119\,\mathrm{dB}$, pain), $79\,\mathrm{dB}$ at $100\,\mathrm{m}$, $59\,\mathrm{dB}$ at $1\,\mathrm{km}$ — still audible over the city. A voice radiates about $10\,\text{µ}\mathrm{W}$: $60\,\mathrm{dB}$ at $1\,\mathrm{m}$, and $40\,\mathrm{dB}$ at $10\,\mathrm{m}$. Air absorbs sound too (more at high frequency, a few decibels per hundred metres at $10\,\mathrm{kHz}$), which is why distant thunder rumbles low.