Ambient noise in the surface duct

On a number of occasions, and with a variety of instrumentation, the ambient noise level in an area north of St. Croix, VI, has been found to be higher, by amounts up to 8 dB, than at depths below the duct. The difference occurs at the frequencies greater than a few hundred hertz that are trapped in a 120–180‐ft duct. Other data in the form of time‐delay correlograms obtained with a vertical string of hydrophones indicate that the excess noise arrives at a receiver from near‐horizontal directions. It follows that a vertical array used in the surface duct would have a higher noise background and a lower array gain than when placed below the duct. A likely hypothesis is that the excess noise in the duct represents noise from distant near‐surface sources trapped by, and traveling within, the duct. This occurrence of duct noise may merely represent some peculiarity of the area where it was observed; on the other hand, it may well be a phenomen of widespread, though apparently hitherto unreported, occurrence wherever a well‐developed surface duct exists.