The volume-integrated pH of seawater can be determined from the frequency and depth dependence of wind-generated ambient noise in the ocean. Over the 1 − 10 kHz frequency band, three main processes contribute to the acoustic attenuation in seawater: the chemical relaxation of boric acid and magnesium carbonate (< 3 kHz, related to pH), and magnesium sulfate (> 3 kHz, unrelated to pH). When local winds are strong (> 10 m/s), the ambient noise is dominated by locally generated surface noise, which exhibits a depth-independent directionality, and weak frequency and depth-dependent intensity. By measuring the depth-dependence of the spectral slope, the pH may be estimated from a comparison of the experimental data with an analytical model of ambient noise. Measurements of the depth-dependent ambient noise field were carried out in the Philippine Sea, Mariana Trench, and Tonga Trench from 2009 to 2021. The wideband (5 Hz - 30 kHz) acoustic data were recorded with untethered, free-falling, autonomous instrument platforms known as Deep Sound, equipped with two or four hydrophones. In all the data collected, the power spectral slopes became steeper with depth due to the stronger attenuation of high frequencies compared to low frequencies. Depth-averaged pH values, ranging from 7.68 to 8.35, were obtained from eight instrument drops. The noise spectral method, which has the potential for determining the depth-averaged value of pH, with the averaging depth being adjustable, could be suitable for the long-term passive acoustic monitoring of ocean acidity.