The present invention relates to a compound of the formula I: in which: A, B, R1, Z, n and R2 are as defined in claim 1. The array is operable to generate simultaneously a first acoustic beam signal at a first frequency and a second acoustic beam signal at a second frequency different than the first frequency. In some embodiments, the copy of the raw data is evaluated for malicious code, allowing a user or application to take protective actions, such as deleting the inaccessible source file. The receiver’s position has been linear-approximated by using the estimation results of the passed receiver’s positions. The devices installed can include clamshell-shaped strakes, shrouds, fairings, sleeves and flotation modules.