The western North Atlantic was divided into 17 sediment provinces, and sonobuoy soundvelocities were compiled from each region. The regions were analyzed statistically to develop least‐squares regressions of the form V=V 0+K t, where t is one‐way vertical travel time. Eight regions provided velocity functions, four were inadequately sampled, and five provided mean values from single‐layer solutions in sediment bodies less than 1 km thick. The eight values of K are grouped very closely about a mean of 1.14±0.17 km/s2. By contrast, 12 regions from the Norwegian Sea yield 10 values of K that range from 0.6 to 2.5 km/s2. The three highest values (≳2.1 km/s2) are clearly separable from the rest and were measured on data from the Voring plateau, Jan Mayen ridge, and eastern Greenland margin. These three regions are uniquely floored by thick Mesozoic epicontinental sediments that predate the breakup of the Norwegian Sea. These older sediment bodies account for most of the regional heterogeneity in the velocity functions of the Norwegian Sea. Approximately one fifth the surface area of the North Atlantic is floored by sediments thinner than 200 m. Within these vast areas, principally near the midocean ridges, the soundvelocity of acoustic basement is a crucial factor in developing low‐frequency propagation models. A contour map of basement refraction velocities in the North Atlantic shows a rough increase from 3.5 km/s near the mid‐Atlantic ridge to 5.0 km/s at variable distance from the ridge. The region in which geoacoustic models are most sensitive to basement velocity is between 15 ° and 30 °N, where sediment cover thinner than 200 m account for about half the surface area. A significant portion of this area is topographically rough and does not yield reliable basement refraction data. Interval velocities from the thinner deposits of pelagic sediment seaward of the abyssal plains, are greater by about 200 m/s than those from the continental rise and abyssal plains at comparable depths of overburden. The difference suggests that soundvelocities in the predominantly pelagic sediments increase more rapidly with depth than they do in the silty hemipelagic clays and turbidites nearer the continents.