Effects of strong sound velocity gradients on propagation of low-frequency impulse sound: Comparison of fast field program predictions and experimental data

Abstract

As part of a large outdoor sound propagation experiment series, propagation of low-frequency impluse sound at ranges from 100 to 1400 m was studied at Haslemoen, Norway in February 1995. Sound sources were charges of 1 and 8 kg C4 explosives. Experiments were carried out in both a uniform forest and above a flat and uniform open field. Extensive meteorological measurements were carried out. Both automatic weather stations mounted in permanent towers and a tethered balloon were employed. The measurements resulted in a manifold set of sound velocity profiles. Occasionally, profiles were pure downwind or upwind, but mostly they were complex, often with large vertical gradients. One conclusion is that one should not uncritically use simple profile parameterizations, e.g., logarithmic, in sound propagation models. A fast field program (FFP), CAPROS, handles the complex sound-speed profiles well, predicting propagation of sound at 8, 16, 32.5, and 63 Hz. In this model, ground is considered a viscoelastic medium. At 63 Hz, the forest seems to cause an excess attenuation. At the lower frequencies, distinct effects of forest are not observed. There is no evidence that FFP predictions are less accurate in periods with high turbulence levels, which is here characterized with dynamical instability. However, variability of meteorological profiles causes predictions to be less accurate. This investigation suggests that it is sufficient to sample atmospheric variables up to 300 m above ground to predict sound propagation at the ranges studied here.

Acoustic estimation of wildlife abundance: methodology for vocal mammals in forested habitats

“…In acoustic surveys of elephants, because the low frequencies in elephant calls (from this study, Mean: 19 Hz, Range: 7–81 Hz, 95% below 32 Hz, n = 423) propagate through the forest efficiently, detection probabilities among habitats can reasonably be assumed constant. A study of the propagation of low‐frequency sound over distances up to 1.5 km revealed that frequencies below 63 Hz did not experience measurable excess attenuation in the forested environment (Hole, Lunde & Gjessing, 1997). Although studies from the savannah habitat have demonstrated that elephant call detection varies with daily temperature inversions (Larom et al.…”

Past Research on Sound Propagation through Forests

“…Studies have been published discussing the influence of atmospheric conditions on the measurements (Hole 1997. One study did detect higher attenuation at 63 Hz within a forest (Hole et al 1997), but this determination was based on comparison with a modeled prediction that used viscoelastic ground instead of a porous surface. A more straightforward analysis of these waveforms is recommended.…”

Blast noise characteristics as a function of distance for temperate and desert climates

“…This set of experiments examined blast noise propagation in summer and winter conditions near Haslemoen, Norway, recording over 550 blast events at distances up to 24 km. Hole et al (1997) presented variability in transmission loss for a subset of these data out to 1.4 km, and showed that the standard deviation of transmission loss increases with increasing distance and frequency, and the authors commented that meteorological variability contributed largely to this standard deviation and the errors in prediction. Researchers have also documented impacts of time-of-day on sound propagation.…”