Assessing the low frequency acoustic characteristics of Macrocystis pyrifera, Egregia menziessi, and Laminaria solidungula

The acoustic properties of kelp forests are not well known, but are of interest for the development of environmental remote sensing applications. This study examined the low-frequency (0.2–4.5 kHz) acoustic properties of three species of kelp (Macrocystis pyrifera, Egregia menziessi, and Laminaria solidungula) using a one-dimensional acoustic resonator. Acoustic observations and measurements of kelp morphology were then used to test the validity of Wood's multi-phase medium model in describing the acoustic behavior of the kelp. For Macrocystis and Egregia, the two species of kelp possessing pneumatocysts, the change in sound speed was highly dependent on the volume of free air contained in the kelp. The volume of air alone, however, was unable to predict the effective sound speed of the multi-phase medium using a simple two-phase (air + water) form of Wood's model. A separate implementation of this model (frond + water) successfully yielded the acoustic compressibility of the frond structure for each species (Macrocystis = 1.39 ± 0.82 × 10⁻⁸ Pa⁻¹; Egregia = 2.59 ± 5.75 × 10⁻⁹ Pa⁻¹; Laminaria = 8.65 ± 8.22 × 10⁻⁹ Pa⁻¹). This investigation demonstrates that the acoustic characteristics of kelp are species-specific, biomass-dependent, and differ between species with and without pneumatocyst structures.